Ecology of phytoplankton 2006
551 pág.

Ecology of phytoplankton 2006


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Peridinium
Order: PHYTODINIALES
Coccoid dinoflagellates with thick cell walls but lacking thecal plates. Many
epiphytic for part of life history. Some in plankton of humic fresh waters.
Includes: Hemidinium
CLASS: Adinophyceae
Order: PROROCENTRALES
Naked or cellulose-covered cells comprising two watchglass-shaped halves.
Marine and freshwater species.
Includes: Exuviella, Prorocentrum
pigments, called phycobilins, are associated with
these membranes, where they are carried in
granular phycobilisomes. Life forms among the
Cyanobacteria have diversi\ufb01ed from simple coc-
coids and rods into loose mucilaginous colonies,
called coenobia, into \ufb01lamentous and to pseu-
dotissued forms. Four main evolutionary lines
are recognised, three of which (the chroococ-
calean, the oscillatorialean and the nostocalean;
the stigonematalean line is the exception) have
major planktic representatives that have diversi-
\ufb01ed greatly among marine and freshwater sys-
tems. The most ancient group of the surviv-
ing groups of photosynthetic organisms is, in
THE DIVERSIFICATION OF PHYTOPLANKTON 11
terms of individuals, the most abundant on the
planet.
Links to eukaryotic protists, plants and ani-
mals from the Cyanobacteria had been sup-
posed explicitly and sought implicitly. The dis-
covery of a prokaryote containing chlorophyll a
and b but lacking phycobilins, thus resembling
the pigmentation of green plants, seemed to
\ufb01t the bill (Lewin, 1981). Prochloron, a symbiont
of salps, is not itself planktic but is recover-
able in collections of marine plankton. The \ufb01rst
description of Prochlorothrix from the freshwa-
ter phytoplankton in the Netherlands (Burger-
Wiersma et al., 1989) helped to consolidate the
impression of an evolutionary \u2018missing link\u2019 of
chlorophyll-a- and -b-containing bacteria. Then
came another remarkable \ufb01nding: the most
abundant picoplankter in the low-latitude ocean
was not a Synechococcus, as had been thitherto sup-
posed, but another oxyphototrophic prokaryote
containing divinyl chlorophyll-a and -b pigments
but no bilins (Chisholm et al., 1988, 1992); it was
named Prochlorococcus. The elucidation of a bio-
spheric role of a previously unrecognised organ-
ism is achievement enough by itself (Pinevich
et al., 2000); for the organisms apparently to
occupy this transitional position in the evolu-
tion of plant life doubles the sense of scienti\ufb01c
satisfaction. Nevertheless, subsequent investiga-
tions of the phylogenetic relationships of the
newly de\ufb01ned Prochlorobacteria, using immuno-
logical and molecular techniques, failed to group
Prochlorococcus with the other Prochlorales or even
to separate it distinctly from Synechococcus (Moore
et al., 1998; Urbach et al., 1998). The present view
is that it is expedient to regard the Prochlorales
as aberrent Cyanobacteria (Lewin, 2002).
The common root of all eukaryotic algae and
higher plants is now understood to be based
upon original primary endosymbioses involv-
ing early eukaryote protistans and Cyanobacteria
(Margulis, 1970, 1981). As more is learned about
the genomes and gene sequences of microorgan-
isms, so the role of \u2018lateral\u2019 gene transfers in
shaping them is increasingly appreciated (Doolit-
tle et al., 2003). For instance, in terms of ultra-
structure, the similarity of 16S rRNA sequences,
several common genes and the identical pho-
tosynthetic proteins, all point to cyanobacterial
origin of eukaruote plastids (Bhattacharya and
Medlin, 1998; Douglas and Raven, 2003). Prag-
matically, we may judge this to have been a
highly successful combination. There may well
have been others of which nothing is known,
apart from the small group of glaucophytes that
carry cyanelles rather than plastids. The cyanelles
are supposed to be an evolutionary interme-
diate between cyanobacterial cells and chloro-
plasts (admittedly, much closer to the latter).
Neither cyanelles nor plastids can grow inde-
pendently of the eukaryote host and they are
apportioned among daughters when the host cell
divides. There is no evidence that the handful
of genera ascribed to this phylum are closely
related to each other, so it may well be an arti-
\ufb01cial grouping. Cyanophora is known from the
plankton of shallow, productive calcareous lakes
(Whitton in John et al., 2002).
Molecular investigation has revealed that the
seemingly disparate algal phyla conform to one
or other of two main lineages. The \u2018green line\u2019
of eukaryotes with endosymbiotic Cyanobacteria
re\ufb02ects the development of the chlorophyte and
euglenophyte phyla and to the important off-
shoots to the bryophytes and the vascular plant
phyla. The \u2018red line\u2019, with its secondary and even
tertiary endosymbioses, embraces the evolution
of the rhodophytes, the chrysophytes and the
haptophytes, is of equal or perhaps greater fas-
cination to the plankton ecologist interested in
diversity.
A key distinguishing feature of the algae of
the green line is the inclusion of chlorophyll
b among the photosynthetic pigments and, typ-
ically, the accumulation of glucose polymers
(such as starch, paramylon) as the main prod-
uct of carbon assimilation. The subdivision of
the green algae between the prasinophyte and
the chlorophyte phyla re\ufb02ects the evolutionary
development and anatomic diversi\ufb01cation within
the line, although both are believed to have
a long history on the planet (\u223c1.5 Ga). Both
are also well represented by modern genera, in
water generally and in the freshwater phyto-
plankton in particular. Of the modern prasino-
phyte orders, the Pedinomonadales, the Chloro-
dendrales and the Pyramimonadales each have
signi\ufb01cant planktic representation, in the sense
12 PHYTOPLANKTON
of producing populations of common occurrence
and forming \u2018blooms\u2019 on occasions. Several mod-
ern chlorophyte orders (including Oedogoniales,
Chaetophorales, Cladophorales, Coleochaetales,
Prasiolales, Charales, Ulvales a.o.) are without
modern planktic representation. In contrast,
there are large numbers of volvocalean, chloro-
coccalean and zygnematalean species in lakes
and ponds and the Tetrasporales and Ulotrichales
are also well represented. These show a very wide
span of cell size and organisation, with \ufb02agel-
lated and non-motile cells, unicells and \ufb01lamen-
tous or ball-like coenobia, with varying degrees of
mucilaginous investment and of varying consis-
tency. The highest level of colonial development
is arguably in Volvox, in which hundreds of net-
worked bi\ufb02agellate cells are coordinated to bring
about the controlled movement of the whole.
Colonies also reproduce by the budding off and
release of near-fully formed daughter colonies.
The desmid members of the Zygnematales are
amongst the best-studied green plankters. Mostly
unicellular, the often elaborate and beautiful
architecture of the semi-cells invite the gaze and
curiosity of the microscopist.
The euglenoids are unicellular \ufb02agellates.
A majority of the 800 or so known species
are colourless heterotrophs or phagotrophs and
are placed by zoologists in the protist order
Euglenida. Molecular investigations reveal them
to be a single, if disparate group, some of which
acquired the phototrophic capability through
secondary symbioses. It appears that even the
phototrophic euglenoids are capable of absorb-
ing and assimilating particular simple organic
solutes. Many of the extant species are associ-
ated with organically rich habitats (ponds and
lagoons, lake margins, sediments).
The \u2018red line\u2019 of eukaryotic evolution is based
on rhodophyte plastids that contain phycobilins
and chlorophyll a, and whose single thylakoids
lie separately and regularly spaced in the plastid
stroma (see, e.g., Kirk, 1994). The modern phy-
lum Rhodophyta is well represented in marine
(especially; mainly as red seaweeds) and fresh-
water habitats but no modern or extinct plank-
tic forms are known. However, among the