6 Genomics of Marine Algae
187
6.4.1 Diversification of the Phytoplankton During the Evolution
of the Earth
The term plankton refers to the free-floating organisms found in diverse marine
and fresh-water environments. The photosynthetic organisms in these ecosystems,
referred to collectively as the phytoplankton, represent only a small percentage of
the total global primary producer biomass (0.2%) and yet they make a very significant contribution to the planet’s primary production (estimated at 45% of the global
primary production; Field et al. 1998). Many eukaryotic algae are found in the phytoplankton but the most abundant phytoplankton organisms are cyanobacteria. From
an evolutionary point of view cyanobacteria are also the oldest component of the
phytoplankton. These organisms were responsible for the “invention” of oxygenic
photosynthesis and hence for the transition from the ancient anaerobic world to the
aerobic world of the modern Earth. As described above, the endosymbiotic enslavement of these cyanobacteria resulted in the emergence of photosynthetic eukaryotes
and this led to an increase in the complexity of the phytoplankton. Fossil records
indicate that red algae have existed for at least 1.2 billion years ago and there is
some evidence that eukaryotic phytoplankton may have been present as long ago
as 1.6–1.8 billion years (in Falkowski et al. 2004). Green algae are likely to have
been prominent in the marine phytoplankton during the mid-Paleozoic area but the
abundance and diversity of this group started to decline during the Triassic and they
were replaced to a large extent by the three dominant algal groups of modern phytoplankton ecosystems, the dinoflagellates, the coccolithophores and the diatoms
(Falkowski et al. 2004). All three of these latter groups are derived from secondary
endosymbioses, in most cases involving the capture of a red alga. This suggests
that the possession of a red-alga-derived plastid may have conferred an advantage,
perhaps related to the requirements for trace elements (Falkowski et al. 2004). In
more recent geological time, there have also been fluctuations in the relative abundances of dinoflagellates, coccolithophores and diatoms and these are thought to be
related to niche preference, with dinoflagellates and coccolithophores being better
adapted to stable environments, whilst diatoms tend to thrive in rapidly changing
environments where nutrients are supplied with high pulse frequencies.
6.4.2 Algae Are Important Components of the Phytoplankton
It has been known for many years that phytoplankton communities are complex
in terms of the phylogenetic range of the organisms present. This has been seen
as a paradox, conflicting with models in which these organisms were thought to
be competing for limited resources in fairly uniform ecological niches (Hutchinson
1961). Several hypotheses have been put forward to explain this situation, including some propositions by Hutchinson himself, but efforts to resolve this paradox
are handicapped by the fact that the ecosystems themselves remain very poorly
characterised.
187
6.4.1 Diversification of the Phytoplankton During the Evolution
of the Earth
The term plankton refers to the free-floating organisms found in diverse marine
and fresh-water environments. The photosynthetic organisms in these ecosystems,
referred to collectively as the phytoplankton, represent only a small percentage of
the total global primary producer biomass (0.2%) and yet they make a very significant contribution to the planet’s primary production (estimated at 45% of the global
primary production; Field et al. 1998). Many eukaryotic algae are found in the phytoplankton but the most abundant phytoplankton organisms are cyanobacteria. From
an evolutionary point of view cyanobacteria are also the oldest component of the
phytoplankton. These organisms were responsible for the “invention” of oxygenic
photosynthesis and hence for the transition from the ancient anaerobic world to the
aerobic world of the modern Earth. As described above, the endosymbiotic enslavement of these cyanobacteria resulted in the emergence of photosynthetic eukaryotes
and this led to an increase in the complexity of the phytoplankton. Fossil records
indicate that red algae have existed for at least 1.2 billion years ago and there is
some evidence that eukaryotic phytoplankton may have been present as long ago
as 1.6–1.8 billion years (in Falkowski et al. 2004). Green algae are likely to have
been prominent in the marine phytoplankton during the mid-Paleozoic area but the
abundance and diversity of this group started to decline during the Triassic and they
were replaced to a large extent by the three dominant algal groups of modern phytoplankton ecosystems, the dinoflagellates, the coccolithophores and the diatoms
(Falkowski et al. 2004). All three of these latter groups are derived from secondary
endosymbioses, in most cases involving the capture of a red alga. This suggests
that the possession of a red-alga-derived plastid may have conferred an advantage,
perhaps related to the requirements for trace elements (Falkowski et al. 2004). In
more recent geological time, there have also been fluctuations in the relative abundances of dinoflagellates, coccolithophores and diatoms and these are thought to be
related to niche preference, with dinoflagellates and coccolithophores being better
adapted to stable environments, whilst diatoms tend to thrive in rapidly changing
environments where nutrients are supplied with high pulse frequencies.
6.4.2 Algae Are Important Components of the Phytoplankton
It has been known for many years that phytoplankton communities are complex
in terms of the phylogenetic range of the organisms present. This has been seen
as a paradox, conflicting with models in which these organisms were thought to
be competing for limited resources in fairly uniform ecological niches (Hutchinson
1961). Several hypotheses have been put forward to explain this situation, including some propositions by Hutchinson himself, but efforts to resolve this paradox
are handicapped by the fact that the ecosystems themselves remain very poorly
characterised.