Algae: Planktonic Groups
EA-Plant Sciences
(R. Ross)
ALGAE: PLANKTONIC GROUPS
CONTENTS
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Page
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Bacillariophyceae
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1
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Structure
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1
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Reproduction
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4
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Colony Formation
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5
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Arctic Habitats
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5
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Composition of Arctic Diatom Flora
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9
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Dinophyceae
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10
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Arctic Occurrence
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14
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Chrysophyceae
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15
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Arctic Occurrence
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19
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Xanthophyceae
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20
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Bibliography
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23
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EA-Plant Sciences
(R. Ross)
ALGAE: PLANKTONIC GROUPS
BACILLARIOPHYCEAE
The Bacillariophyceae or diatoms (
Bacillariophyta
) are a group of
microscopic unicellular plants belonging to the Algae. Like almost all
plants, they manufacture their own food from mineral salts, carbon dioxide,
and water, using energy from sunlight absorbed by their green pigments.
They occur in all waters, both fresh and salt, throughout the world and,
except in tropical oceans, are among the predominant forms of plant life
in regard to quantity. They are less affected by adverse conditions than
higher plants, and some species can survive exposure to a temperature of
-80°C. for long periods. Being unicellular, they can multiply rapidly as
soon as the water they live in thaws. They are particularly abundant in
arctic and Antarctic waters and form the starting point of the food chains
which lead through the smaller marine animals — copepods, medusa, pteropods,
etc. — to fishes, whales, and other marine mammals. The rich diatom popula–
tion is the principal reason for the abundant life in arctic seas.
Structure
Individual diatoms are one-called and vary in size from about 0.005
millimeters long and about one-third as broad, to disks 2 millimeters in
diameter, or rod-shaped forms 5 millimeters long. In the majority, however,
the longest dimension is between 0.02 and 0.2 millimeters. Their chief
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Algae: Planktonic Groups
distinguishing feature is that each cell has a boxlike siliceous exoskeleton
known as the frustules. This consists of two halves fitting over each other
like the top and bottom of a pillbox. Each half is normally made up of two
parts, the valve, which forms the top or bottom of the box, and the girdle,
which is the side. Although there is usually only one girdle attached to
each valve, in some species the valves are separated by a number of complete
or partial loops. Other species have perforate septa across the interior of
the frustule in a plane parallel to the valve. There may be only two of
these, one attached to each valve, or many which fit into each other.
The frustules, especially its valves, is normally covered with regularly
disposed fine markings in the form of dots, bars, or a honeycomb. These are
cavities in the silica which are usually open on the inner side and closed
by a fine membrane on the other side. This membrane itself often shows
fine markings; whether or not it is perforate is still undecided in spite
of investigations with the electron microscope.
There is great variety in the shape of the frustule and in the dispose–
tion and nature of the markings, and a full survey of the variations is
beyond the scope of this account. Reference should be made to the works
quoted in the bibliography for more detailed information than is given
here. many diatoms have circular valves, flat, convex, or undulating, and
have their markings arranged in a radial or concentric pattern. These,
together with a number of elliptical, triangular, or polygonal forms with
processes at their corners, comprise the order Centrales. In this order
also is included the suborder Soleniineae whose long cylindrical frustules
are usually very feebly silicified and have a complex connective zone
consisting of many incomplete hoops; their valves are small and offen
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Algae: Planktonic Groups
markedly excentric. They are all planktonic and the cells unite to form
long chains, as they also do in another planktonic group, the family
Chaetopceraceae, which are illiptical or circular forms with long awnlike
processes.
In the order Pennales, which comprises the remainder of the diatoms,
the markings of the valve are disposed about a longitudinal line and are
thus more or less bilaterally symmetrical. This order, which is numerically
larger than the Centrales, exhibits an even greater variety of shape and
structure. The longitudinal axis may be straight, arcuate, biarcuate, or
sigmoid, more or less median or markedly excentric, and is often raised on
a keel. The outline of the valve varies from the linear to the arbicular
and is sometimes cuneate.
An organ known as the raphe occurs in some of the suborders of the
Pennales. This is a cleft, often oblique or folded, through the silica of
the valve, and normally lies in the longitudinal axis. It usually occurs in
both valves of the frustule, but in one family the Achnanthaceae, it is found on
only one valve. In two large families, the Bacillariaceae and the Surirellaceae,
the raphe is on a keel and is of a rather different structure. It consists of
a canal opening to the interior of the frustule by a series of pores and to
the exterior by a fine oblique slit. Only those diatoms which possess a
raphe can move, a fact which provides strong evidence for the view that their
movement, a comparatively slow creeping motion, is due to the streaming of
protoplasm along the raphe. Other theories, most notably the presence of
cilia, are still put forward but seem less well founded.
The cell content is much less variable than the frustules. On the
inside of these is a lining of protoplasm which encloses one or two large
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Algae: Planktonic Groups
vacuoles. The single nucleus occurs either in the bridge of protoplasm
separating the two vacuoles or else in that lining the frustule. Each cell
contains one to many olive-green bodies, the chromatophores. In
[:
]
general,
the Pennales posses large flat chromatophores, usually two in number, while
the Centrales normally have numerous, small, lens-shaped ones. The olive–
green color is due to the yellow pigments being present in greater propor–
tion to the green than is the case in higher plants. The food reserves are
normally stored as oil ad a few oil globules are usually
^
^
present in the cell.
Reproduction
This is normally by binary fission. When the cell divides, two nes
valves are formed in the old frustule before the two halves of the old girdle
separate. These new valves, with their girdles, form the inner halves of the
new frustules and, since they cannot grow after they are formed, one daughter
cell is the same size as the parent while the other is slightly smalle
d
^
r
^
.
Growth of the cell between divisions takes place by the two valves moving
farther apart. Repeated divisions accordingly reduce the average size of
the individuals in the population and if may fall to one-quarter of its
original figure.
The original size is restored by a different type of reproduction, known
as auxospore formation, in which the cell contents emerge from the frustule
and new large-size individuals are formed. This is a sexual form of repro–
duction. Typically two individuals come together and, after a reduction
division, one, two, or four gametes are formed in each cell. These gametes
fuse in pairs, one from each cell, and the resulting zygotes grow rapidly
and form the auxospores, which lay down new, larger frustules about themselves.
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Algae: Planktonic Groups
Apogamy, in which after the reduction division two nuclei from the same
cell fuse to form the zygote, occurs in some Pennales and is the rule in
the Centrales.
Some planktonic species of diatoms form resting spores. The cell
contents contract away from the valves at the onset of unfavorable conditions
and lay down a thick-walled siliceous endocyst around themselves. This does
not resemble the normal frustule of the species; it has no fine markings and
usually bears spines. These resting spores survive the unfavorable period
and germinate, producing cells with normal valves, when favorable conditions
return.
Colony Formation
Most diatoms live free and solitary lives but some attach themselves
to the substratum by secreting a gelatinous pad or stalk. In some such
forms this attachment forks when cells divide, and a colony on a much-branched
stipe results. Some raphe-bearing species form long, branched, gelatinous
tubes in which the frustules live. A more frequent type of colony is a
filament formed by the failure of the daughter cells to separate after cell
division. These are sometimes attached to a substratum by their terminal
frustule, or they may occur free. Many planktonic species form such chains;
others form colonies consisting of a number of cells embedded in a common
mucilaginous envelope.
Arctic Habitats
In both fresh waters and the sea, there are two ecological groups of
diatoms - those which live on substratum, and the free-floating plankton.
In fresh waters, where their growth begins again each summer as the ice and
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Algae: Planktonic Groups
and snow melt, diatoms form an olive-green to gray-green flocculent layer on
the bottom and on submerged plants, etc. in sunlight, photosynthesis is
often so active that bubbles of free oxygen are formed which, becoming en–
tangled in this layer, carry lumps of it to the surface. All except the
smallest bodies of water have their planktonic diatom flora, but little is
known of the plankton of any arctic lakes. The growth of diatoms, both
planktonic and bottom-living, is limited to depths shallow enough to permit
the penetration of enough light for photosynthesis. This limit depends on
the clearness of the water and varies from a few feet to more than fifty.
Diatoms not only occur in practically all permanent bodies of water but
also in puddles, in which they grow rapidly, and in any damp places, such as
tufts of moss where they are found in quantity. Soils in temperate regions
usually have a considerable population of diatoms, mostly very small forms.
The presence of a well-developed diatom flora in Icelandic soils indicates the
strong probability that one will be found also in the Arctic.
In general, arctic freshwater diatoms are smaller and have finer markings
than those of more temperate waters. Individuals from arctic localities are
not smaller
a
than those of the same species from farther south, but the
smaller and more finely marked species tend to extend farther north than the
larger and coarser ones. This appears to be a temperature effect, since
shallow waters well warmed by the sun contain larger and coarser forms than
are found in colde
d
^
r
^
waters in the same district. However, the chemical
content of the water, in particular the pH and salinity, is often the
principal factor determining the diatom flora, diatoms being scarce in
acid waters poor in mineral salts, and frequent in waters of higher pH and
salt content.
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Algae: Planktonic Groups
Marine
. Marine diatoms are plentiful along those arctic shores which
are free from ice in the summer months, growing on the larger algae, on
mollusk shells, stones, etc., and living free on the bottom in shallow
water, on the shore between tidemarks, and in rock pools — as in temperate
waters. In the Arctic, in addition, the ice floes support a large diatom
population containing many species not found elsewhere.
Diatoms begin to grow on the ice about March. They grow on the
undersurfaces and sides of the floes, particularly on the ice foot, and in
pools in the surface of the floes. There they form small, round patches and,
since these, being dark-colored, absorb more solar energy than the surrounding
ice, they raise the temperature locally and melt the ice on which they are
growing. In this way the ice becomes pitted with holes giving rise to the
condition known as “rotten ice” by the early whalers. Among diatoms from
ponds on the ice, some freshwater species, and also the spores of marine
planktonic species, are often found. The spores are formed in the autumn
and incorporated in the ice as it freezes. The freshwater forms are not
found in large numbers and have only been seen in acid-cleaned preparations.
It is accordingly not known whether they were living on the ice, and indeed
it has been generally assumed that they were dead frustules, either blown
there or present because the ice had formed in a river estuary. The water
in the pools is often almost fresh, however, and the frequency of their
occurrence suggests that they may be able to maintain themselves there for
a time.
In those parts of the Arctic Sea which are not ice-free during the
summer, there is little true phytoplankton. However, in the leads between
the floes, globular aggregations of littoral diatoms, often as large as a
EA-PS. Ross: Bacillariophycease
man’s fist, are found. These lumps, often present in large numbers, contain
species of other algal groups as well as
[:
]
most of the diatoms found on
the neighboring ice, and they apparently originate from the film of diatoms
on the ice. They congregate at the interface between the salt and fresher
water which is formed one or two feet below the surface as the ice melts.
The sudden change of salinity encountered there result
^
^
in the death of their
outer layers which become bleached in the later part of the summer.
With the onset of winter and littoral diatoms of the ice fields and most
of those growing along the coasts become embedded in the newly formed ice.
All the species are able to survive this, although the majority of individuals
succumb. The survivors produce the next season’s crop when light and warmth
return in the spring.
The planktonic diatoms fall into two groups. There are the truly oceanic
types which maintain themselves in open water throughout the year. Most of
these are discoid forms or species of the genus
Chaetoceros
. The Soleniineae,
which are prominent in the plankton of most seas, are comparatively rare in
arctic waters, although a few species are carried by Atlantic water into the
Greenland and Barents seas. The other group are the neritic types, which are
incapable of passing the whole year in the plankton and are only found in
coastal waters or near the ice fields. In some it is only the resting spores
which are not planktonic, but others, e.g.,
Melosire arctica
, probably the
commonest arctic diatom, can flourish either on the ice or in the plankton.
A fuller account of the arctic phytoplankton will be found in the article
“Phytoplankton.”
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Bacillariophycease
Algae: Planktonic Groups
Composition of Arctic Diatom Flora
The freshwater diatom flora of the Arctic contains few species
peculiar to the region but consists largely of forms also common in
temperate lands. Many found in warmer climates, however, are unable to
withstand arctic conditions and hence the arctic diatom flora is less varied
than that of countries farther south. It is nevertheless quite extensive,
and more than 500 species of freshwater diatoms have been recorded from north
of the Arctic Circle. The freshwater diatom flora of Ellesmere Island, our
present knowledge of which is based on only six gatherings, illustrates
these points. From these 72 species are known, of which 14 are confined
to the Arctic and Subarctic and 7 more elsewhere only occur in alpine habitats.
The remaining 70% of the freshwater diatoms from this island, which lies north
of latitude 75° N., are temperate species. Most diatoms found only in the
Arctic have a circumpolar distribution. Although many species have wide
ranges, the freshwater species peculiar to the Arctic
[:
]
^
have
^
not been found in
the Antarctic.
The marine diatoms present a different picture. The location and move–
ment of the principal water messes control the distribution of planktonic
forms; hence the plankton characteristic of the cold arctic water is not
normally found outside high latitudes, except when carried southward by
such surface currents as the Labrador Current. Similarly the plankton diatoms
of temperate waters are carried into the Greenland and Barents seas by the
Gulf Stream. Many arctic marine plankton species are also found in the
Antarctic.
While the diatoms found along the coasts of the Arctic Sea include many
temperate species, most of those found on the ice floes do not occur outside
EA-PS. Ross: Bacillariophycease
the Arctic. Again, while some of these forms occur throughout the Arctic
Sea, many have been reported only from the region of the great polar drift
from the Bering Strait westward to northeast Greenland, which accordingly has
a richer flora than is found elsewhere. Physical and chemical conditions,
particularly the latter, seem to be the main factors in determining the
distribution of diatoms, and the low and varying salinities of the surface
waters in the ice fields are not encountered in more temperate seas. It is,
therefore, not surprising that the diatoms found there are peculiar to that
habitat. Similar conditions are, however, encountered in the Antarctic but
the flora there is different, and very few species are common to the ice of
both polar regions.
DINOPHYCEAE
The Dinophyceae, often referred to as the dinoflagellates or peridinians,
are a class of Algae which are generally unicellular, although a few filamen–
tous forms occur. The majority of them are found in the plankton, either of
the sea or of fresh waters, and they constitute one of the most important
elements of the phytoplankton. While typically they are pigmented forms with
holophytic nutrition, there are a considerable number which are colorless and
holozoic or parasitic.
The unicellular forms are characterized by the presence of two flagella,
one of which is directed transversely and often encircles the cell more or
less completely, while the other is directed longitudinally. In most genera
these flagella are inserted on the side of the cell and lie in furrows, one
of which, containing the transverse flagellum, encircles the cell and is often
termed the girdle. The longitudinal flagellum is normally directed backward,
EA-PS. Ross: Algae: Planktonic Groups
and hence the longitudinal furrow is found on the part of the cell behind the
girdle, although it sometimes continues forward toward the apex. Both flagella
are used for progression, their undulating movements propelling the organism
forward with a rotating motion.
The chromatophores of the colored forms are usually numerous and disk–
shaped, and dark yellow or brown owing to the presence of carotinoid pigments —
particularly the dark red peridinin, in addition to the green chlorophyll.
These chromatophores lie in the outer part of the cell where the protoplasm
is dense and granular. The inner part of the cell is occupied by the large
nucleus and by one or more vacuoles filled with a sap which is normally rose–
or calmon-colored. These vacuoles have a well-defined membran
c
e and definite,
often spherical, shape. Their function is probably excretory. The majority
of the genera possess of a cell wall consisting of cellulose, which often is
very complex and composed of many plates.
The simplest and most primitive members of the Dinophyceae belong to the
subclass Desmokontae. In many of these the flagella are inserted at the apex
of the cell and there is no transverse, and frequently no longitudinal, furrow.
These are the only members of the class in which the longitudinal flagellum
is directed forward: they include some without a cellulose wall. In others
of this subclass the two flagella emerge from a single pore on the side of
the cell and both lie in furrows, the margins of which are often expanded
into wings. When these wings are well developed, the cell wall can have a
very complex shape, but it never consists of more than two portions joined
by a longitudinal suture.
Reproduction is by longitudinal division and one half of the cell wall
goes to each daughter cell, which lays down a new second half. This subclass
EA-PS. Ross: Algae: Planktonic Groups
is represented in the marine phytoplankton of the Arctic by a number of species
of
Exuviaella
and
Dinophysis
, both genera with a cellulose cell wall. In
Exuviaella
the flagella are apical, but in
Dinophysis
they are inserted
ventrally and lie in transverse and longitudinal furrows, respectively.
The simples members of the remaining subclass, the Dinokontae, also
have no cellulose cell wall. In all of them, however, the flagella emerge
separately on the ventral side; and they all have well-marked transverse and
longitudinal furrows in which these flagella lie. Some of these forms have
chromatophores and are holophytic. Others are colorless and holozoic, ingest–
ing small particles of food by means of pseudopodia protruded from the antapical
part of the cell near the longitudinal furrow. Both holophytic and holozoic
species of this type are found in the phytoplankton of the Arctic, the genus
Gymnodinium
being the best represented.
Closely related to these are a number of forms which live as parasites,
mostly on various types of animals, of which planktonic copepods are the
favorites. In the parasitic stage they occur as unicellular cystlike
organisms either in the tissues of their host or externally, and show little
evidence of being Dinophyceae. Their affinities are revealed, however, by
the structure of the motile swarmers which they form from time to time. These
resemble the simpler members of the Dinokontae which have no cellulose cell
wall. One of this group occurs as a parasite on marine planktonic diatoms
of the genus
Chaetoceros
, and has been recorded from the waters around
Greenland.
Most members of this subclass are forms with a definite cellulose cell
wall. This consists of a girdle forming the furrow in which the transverse
flagellum lies, and a series of unequal polygonal plates, the number and
EA-PS. Ross: Algae: Planktonic Groups
and arrangement of which differs from species to species. These plates have
knifelike margins and are firmly cemented together. They are usually pierced
by pores, which frequently lie in the center of the areolations with which
the plates of many species are ornamented. The shape of these armored
Dinokontae varies greatly. Peridinium, one of the commonest genera in the
sea and fresh waters in the Arctic, is subspherical, with an
[:
]
indentation
in the region of the longitudinal furrow and sometimes two small horns or
spines at the antapical end.
Goniaulax,
another frequent arctic genus, is
very similar and only differs in the detailed arrangement of the plates. In
Ceratium
, another common marine genus, there is one long apical horn an two
or three antapical horns of varying length, which may be bent sideways or
forward. The length of these horns in proportion to the size of the body
tends to be greater in tropical species of the genus than in those inhabiting
colder water, a fact shich is presumably correlated with the lesser density
of warmer water and the consequent need for more “form resistance” therein.
Almost all these armored forms are pigmented and holophytic.
Reproduction in many species of Dinokontae is by binary fission during
movement. The plane of division is oblique and one flagellum goes to each
daughter cell, the other being regenerated. In some armored forms, including
many species of
Ceratium
, the cell wall ruptures along a lone between definite
plates. The exposed protoplast assumes the characteristic shape of the species
and the remaining plates of the cell wall are gradually developed. In others,
including
Peridinium
spp., there is no split but the new envelopes grow over
the surface of the daughter cells as they divide. In many other species
division occurs in a sedentary phase. The protoplast contrasts away from
the cell wall and divides obliquely in its contracted state. This division
EA-PS. Ross: Algae: Planktonic Groups
may occur before or after liberation from the old cell wall, which ruptures
to allow its contents to escape. In some cases the contracted protoplast
develops a new membrane before liberation from the parental one. The thin–
walled spherical cysts so formed may act as a resting stage of some duration,
but usually under favorable conditions they divide at once. These cysts
differ considerably from the thick-walled resting spores with ample food
reserves which are known in a number of armored freshwater species. These
are the means by which they survive unfavorable climatic periods. When they
germinate, the young individual which emerges from the ruptured cyst resembles
the unarmored members of the subclass, but in time grows a typical cellulose
envelope. Sexual reproduction in this group has never been observed, although
it is suspected to occur in some forms where unusually small unarmored swarmers
have been seen and interpreted as gametes.
In addition to these flagellate unicellular forms, there are a number of
general of Dinophyceae which are normally nonmotile, existing either as single
cells or as short filaments. Most of these reproduce by the liberation of
biflagellate swarmers with one longitudinal and one transverse flagellum,
and with evident furrows. No such forms have, however, yet been recorded
from the Arctic.
For a more detailed description of this class Fritsch (4) should be
consulted. Lindemann (11) also gives a fairly full account, together with
a synopsis of the genera and their characteristics, while Schiller (14)
gives descriptions and keys of all the species.
Arctic Occurrence
The Dinophyceae are typically planktonic organisms. In the sea they
EA-PS. Ross: Algae: Planktonic Groups
play an especially important part in oceanic waters and in the warmer regions,
where they make up the bulk of the phytoplankton. In the Arctic they are
less important than the diatoms, but they nevertheless constitute an appre–
ciable part of the phytoplankton. In the spring, before the ice melts, some
of the naked forms have been found along with other flagellates constituting
a not very rich phytoplankton in the waters of the East Greenland current (2).
Such a community is probably widespread at that time of year, but further
observations are required before this can be established. This flagellate
community is replaced by diatoms when the ice melts, and these are often
found in great numbers. Their maximum is followed, when the water is warmer
but less rich in mineral salts, by a phytoplankton dominated by armored
Dinophyceae, members of the genera
Ceratium
and
Peridinium
being particularly
prominent (2; 7). The parasitic species
Paulsenella chaetoceratis
is also
found in the early summer plankton off Greenland on its host, the diatom
genus
Chaetoceros.
It is to be expected that the species parasitic on
zooplanktonic copepods, etc., will be found to be represented in the Arctic
also. The total number of marine species so far recorded from the Arctic
is about 200. More details of the part which the Dinophyceae play in
marine phytoplankton will be found in the article “phytoplankton.”
About 10 species of Dinophyceae have so far been recorded from arctic
fresh waters (1; 15; 16), where they are apparently not a large component
of the algal flora. Little is known of the conditions which favor their
occurrence.
CHRYSOPHYCEAE
This class of Algae consists mainly of unicellular and colonial forms.
EA-PS. Ross: Algae: Planktonic Groups
There are a few filamentous types known, but none of these have so far
been recorded from the Arctic. The chromatophore o
s
^
f
^
this class are brown
or orange in color, owing to the presence of one or more accessory pigments.
They are normally large, few in number, and parietal in position. The
food reserves are stored as fat and as a substance known as “Leucosin”
which occurs as opaque rounded masses of a whitish color and is thought to
be a carbohydrate; starch does not occur. The protoplasm is always clear and
free from granules. Another characteristic feature of this class is the
formation of silicified cysts by its members. These cysts, which are formed
within the cell, are approximately spherical in shape, with various ornamenta–
tions on the surface, and with a pore closed by a cone-shaped, unsillicified
plug.
There are a large number of motile unicellular flagellate members of this
class, but very few have been reported from the Arctic. In view of the fact
that in temperate regions the freshwater forms favor cold conditions, occurring
most abundantly in the winter and in cold mountain waters, this might seem
surprising were it not for the fact that most of them are so delicate as to
lose all their distinguishing features on preservation. They
[:
]
therefore
need to be studied alive. Only two unicellular forms have so far been recorded
from the Arctic. One is a species of the often colonial genus
Dinobryon
and
will be described later. The other is
chrysococcus rufescens
, found by
Shirshov (15) in Novaya Zemlya. In this latter the cell is enclosed within
a rigid spherical envelope, leaving a pore through which the single flagellum
passes. In age, the envelope becomes colored brown by iron salts. Repro–
duction is by binary fission, one of the products escaping from the envelope
as a naked swarmer and forming a new individual, the other being retained in
the original envelope.
EA-PS. Ross: Algae: Planktonic Groups
Two groups of unicellular members of this class are recorded from the
marine plankton of the Arctic. The Coccolithophoridadeae are represented
by about 10 species. In this family the cells have an outer membrane,
gelatinous at first if not later, in which a number of calcified inclusions,
the coccoliths, are deposited. These vary in shape from genus to genus but
are most frequently circular. They are often perforated in the center and
may bear processes on their outer sides which stick out from the cell like
stout spines. The gelatinous membrance of some species becomes calcified with
age, so that the coccoliths become embedded in a rigid envelope. In this
family there are normally two flagella and two thromatophores. In reproduction
the flagella are withdrawn, the protoplasm contracts somewhat, and then divides.
In some species the division in equal and both daughter protoplasts emerge from
the parental envelope as naked swarmers; in others it is unequal, the larger
protoplast remains within, and only the smaller one escapes.
The Silicoflagellatae are only doubtfully referred to the class Chrysophyceae.
They possess an internal skeleton made of siliceous rods. Their chromatophores
are yellow or brownish-yellow, numerous, and discoid. Fat and leucosin are
both stated to occur as assimilatory storage products. There is a single apical
flagellum. Little is known of their life history, but reproduction appears to
occur normally by binary fission. Resting stages with no flagellum and a
distinct external membrane have been reported.
Many of the flagellate Chrysophyceae are colonial, the cells being grouped
in various ways. Three general reported from Arctic fresh waters,
Synura,
Chrysosphaerella
, and
Uroglena
, have spherical colonies. In
Synura
and
Chrysosphaerella
the cells are closely packed. In the latter of these they
are uniflagellate and on either side of the flagellum there is a small cup
EA-PS. Ross: Algae: Planktonic Groups
from which a long, hollow, siliceous rod protrudes. There are also small
siliceous scales in the mucilaginous envelope of the colony. In
Synura
the cells, each of which has two equal flagella, are united by the bases of
their membranes and there is no mucilage envelope for the whole colony. The
envelopes of the individual cells are tough and abovoid or ellipsoid in
shape.
Uroglena
has the cells at the surface of a large, roughly spherical
mass of mucus, and they are usually well separated. In all of these genera
the cells have two chromatophores. In none of them is the number of cells
in the colony definite, division of individual cells going on continuously.
Reproduction may occur by division of the whole colony or by the liberation
of swarmers.
Dendroid as well as spherical colonies are found in this class of Algae.
In the genus
Dinobryon
the individual cells each possess an envelope with a
wide mouth and a pointed base. The two flagella of each cell are unequal.
Some species are solitary and one such has been recorded from the fresh waters
of Greenland. This reproduces by longitudinal fission, one of the daughter
cells escaping from the envelope and forming a new individual. In other
species, after longitudinal division one daughter cell migrates until it
is attached by its base close to the opening of the parental envelope. It
there lays down a new cellular envelope. Branched colonies are thus formed
which differ in shape according to the frequency and sequence of the divisions.
New colonies are formed by the liberation of swarmers, either after longitu–
dinal division of a cell or by the escape of its whole protoplast. In one
case fusion of swarmers formed by longitudinal division has been observed.
Spherical cysts with a short projection on one side are formed just outside
the cellular envelope in many species.
The species concerned pass through
EA-PS. Ross: Algae: Planktonic Groups
By means of these, the species concerned pass through unfavorable climatic
periods.
Some members of this class are normally amoeboid in structure and holo–
zoic in nutrition, revealing their relationships only when they produce
swarmers of typical chrysophycean structure. None of these is recorded from
the Arctic. In other forms the normal state is a palmelloid one, concisting
of large numbers of rounded cells embedded in mucilage. The marine planktonic
Phaeocystis
has large mucilaginous colonies with spherical lobes, in the surface
of which are embedded rounded cells with two yellowish
[:
]
chromatophores.
This type reproduces both by detachment of lobes and by the liberation of
swarmers with two unequal flagella. Another palmelloid dorm is
Hydrurus foetidus,
which occurs in running water and has been reported from Greenland. It forms
tufts of touch mucilaginous strands in which the cells are embedded. In each
branch there is an apical cell which divides repeatedly. The cells are
initially spherical in shape but become drawn out and pear-shaped with age.
Reproduction is normally by swarmers which are formed on short side branches
by the longitudinal division of a series of cells. In summer some cells are
protruded in mucilaginous projections from the surface of the thallus and these
form silicified cysts with a broad, delicate wing extending around half the
periphery.
Fuller accounts of the morphology and life-history of this class of Algae
will be found in Pascher (12) and Fritsch (4).
Arctic Occurrence
Both Coccolithophoridaceae and Silicoflagellatae are characteristic of
temperate and tropical oceanic water, and they are only found within the
Arctic where the currents bring in such waters. One silicoflagellate,
EA-PS. Ross; Algae: Planktonic Groups
Distephanus speculum,
is, however, comparatively abundant along the coast of
Greenland in the summer (1; 7).
Phaeocystis pouchetii
is found associated
with diatoms in their spring maximum towards the southern limits of the Arctic
area in the Atlantic (7), although it also is typically a temperate form.
Dinobryon pellucidum
, on the other hand, is widely distributed in the Greenland
sea, Davis Strait, and Baffin Bay.
The freshwater
Dinobryon bavaricum
var.
vanhoeffenii
was the dominant
member of the phytoplankton of the lake near Karajak Fjord in West Greenland
investigated by Vanhoeffen (16). Other freshwater members of this class have
been listed as occurring in various localities in Greenland (1), Novaya Zemlya,
and Franz Josef Land (15), and, though few in number of species, are apparently
often prominent members of the algal population.
XANTHOPHYCEAE
This class of Algae, which is also known as the
Heterokontae
, is small
in numbers and sparsely represented in the Arctic by about 10 species, of
which one,
Halosphaera viridis
, is a marine planktonic form; the others
inhabit fresh water. Within the class there are to be found motile flagellate
forms, unicellular and colonial nonmotile forms, filamentous forms, and
siphoneous types. Within each type of group of forms, however, there is
little range of variation. The chromatophores are usually numerous and dis–
coid, and are of a yellowish-green color owing to the presence of an excess
of xanthophyll. Starch is absent, and oil is the usual food reserve. The
cell wall, when present, is rich in pectic substances and often consists of
two overlapping parts. The motile cells have two apically inserted flagella
of very unequal lengths. The longer of these is complex in structure, with
EA-PS. Ross: Algae: Planktonic Groups
many fine side branches.
In all the forms of this class so far reported from the Arctic, the
motile phase occurs only temporarily in reproduction.
Halosphaera viridis
has large spherical cells, most of the interior of each being occupied by a
central vacuole. The chromatophores are numerous, flattened, and somewhat
angular. The pectic cell wall is slightly silicified and consists of two
equal halves joined at their margins. As the cells grows, the membran
c
e is
burst open and a new one is laid down. Reproduction is by swarmers with
two chloroplasts, and an oval or spherical shape. This species is common
in Gulf Stream waters and is found wherever these penetrate into the Arctic,
spreading farther as the summer advances. There is some indication that it
maintains itself throughout the year in the open water of the Barents Sea (18).
Characiopsis
is a unicellular epiphytic form from fresh water and has
been found in Franz Josef Land (15). The cell membrane is in two unequal
parts and is attached by a mucilaginous cushion at the foot of a stalk. The
mature cells are often multinucleate, and zoospores are formed by the proto–
plasm rounding off the smaller, upper part of the cell membrane.
Ophiocytium
recorded from Greenland (1) and Baffin Island (17), is
another genus which is primarily epiphytic. It is invariably multinucleate
and has the form of an elongate cylinder. The cell wall is of two unequal
parts of which the larger consists of a series of long thimble-like strata
with expanded margins. The other part fits over this like a lid, and becomes
detached when the reproductive bodies, which may be flagellate zoospores or
nonmotile aplanospores, are liberated. The three species recorded from the
Arctic are not normally epiphytic.
Mischococcus confervicola
, which has also been reported from Franz Josef
EA-PS. Ross: Algae: Planktonic Groups
Land by Shirshov (15), is another epiphytic form. It consists of colonies
of rounded cells borne at the tips of more or less regularly forked mucilage
stalks. The cells contain two chromatophores. Reproduction is by unflagellate
motile spores which are formed singly or in pairs within the cells. Nonmotile
aplanospores are also frequently formed.
In the genus
Tribonema
, which has been reported from Greenland (1), Novaya
Zemlya, and Franz Josef Land (15), the alga consists of unbranched filaments.
The cells are usually uninucleate and have a number of discoid parietal chromato–
phores. A sexual reproduction is by zoospores with two unequal flagella. These
are formed singly or in pairs in the cells. Nonmotile thick-walled aplanospores
are also known. These may form new filaments which they germinate, or they may
liberate one or two flagellate spores. Isogamous sexual reproduction has been
observed once. Occasionally
Tribonema
secretes irregular masses of mucilage
around its filaments and these become impregnated by ferric carbonate deposited
by bacteria which are thought to be symbiotic with the alga.
Fuller accounts of the morphology and life history of this class will be
found in Fritsch (4) and Pascher (12).
EA-PS. Ross: Algae: Planktonic Groups
BIBLIOGRAPHY
1. Bachmann, H. “Beiträge zur Algenflora des Süsswassers von Westgrönland.”
Mitt.Naturf.Ges.Luzern
, vol.8, pp.1-181, 1921.
2. Braarud, T. “The ‘Øst’ expedition to the Denmark Strait, 1929.” II. The
phytoplankton and its conditions of growth,”
Hvalråd.Skr.
vol.10, pp.1-173, 1935.
3. Brown, R. “On the nature of the discolouration of the Arctic Seas,”
Bot.Soc.Edinb.
Trans.
vol.9, pp.244-52, 1868.
4. Fritsch, F.E.
The Structure and Reproduction of the Algae
. Cambridge,
England. The University Press, 1935. Vol.1.
5. Gran, H.H.
Diatomaceae from the Ice-Floes and Plankton of the Arctic
Ocean.
London, N.Y., Longmans, Green, 1904. Norwegian North
Polar Expedition, 1893-1896.
Scientific Results
no.11.
6. ----. “Die Diatomeen der Arktischen Meere,” Römer, and Schaudinn.
Fauna Arctica.
Jena, 1904. Vol.3.
7. Grontved, J., and Seidenfaden, G. “The Phytoplankton of the waters west
of Greenland.”
Medd.Grønland
, vol.82, no.5, pp.1-380, 1938.
8. Heurck, Henri van.
A Treatise on the Diatomaceae
. Trans. by W.E. Baxter.
London, Wesley, 1896.
9. Hustedt, F. “Diatomeen aus der Umbegung von Abisko in Schwedisch-Lappland,”
Archiv für Hydrobiol. Vol.39, pp.82-174, 1942.
10. Karsten, Gustav. “Diatomeae,” Engler, Adolf, and Prantl, K.
Die
Natürlichen Pflanzenfamilien
. Leipzig, 1928. Vol.2.
11. Lindemann, E. “Peridineae (Dinoflagellatae),” Naturl. pflFam., 2 aufl.,
vol.2, pp.3-104, 1928.
12. Pascher, A. “Heterokonten,”
Rabenhorst’s Krypt. -Flora Deutschl
. Vol.11,
pp.1-1092, 1937-9.
13. Ross, Robert. “Freshwater Diatomeae,” Polunin, Nicholas, ed.
Botany
of the Canadian Eastern Arctic, Pt.II, Thallophyta and
Bryophyta
. Ottawa, 1947. Nat.Mus.Can.
Bull.
no.97.
Biological Ser.
no.26.
14. Schiller, J. “Dinoflagellatae (Peridineae) in monographischer Behandlung”
Ibid.
vol.10, no.3, pp.1-617; 1-589, 1931-7.
EA-PS. Ross: Algae: Planktoni
g
^
c
^
Groups
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Bibliography
15. Shirshov, P.P. “Ecologic-geographical essay on the fresh-water algae
of Novaya Zemlya and Franz Joseph Land,” Leningrad.
Arkticheskii Nauchn. - Issled. Inst.
Trudy
, no.14, pp.73-162,
1935.
16. Fanhöffen, E. “Die Fauna and Flora Grönlands,”
Grönland-Exped. Gas.E
r
^
R
^
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Berlin
, vol.2, pp.1-383, 1897.
17. Whelden, R.M. “Algae,” Polunin, N.
Botany of the Canadian Eastern Arctic
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Part II,
Thallophyta and Bryphyta
, Ottawa, 1947. Nat.Mus.Can.
Bull
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Biological Series
no.26.
18. Wulff, A. “Ergebnisse der Untersuchung des Oberflächenplanktons,”
Ber.Wiss.
Komm.Meeresforsch.
n.f., vol.4, pp.313-37, 1929.
R. Ross