ocean ecologist caps and put on marine lab T-shirts to masquerade as marine
biologists. We’ll do that now.
Evolution of Phytoplankton
(^) Globally, cyanobacteria and microalgae dominate marine photosynthesis.
Cyanobacteria evolved about 2.85 billion years ago (Falkowski et al. 2004) and are
simple prokaryotic cells without a membrane-bound nucleus or other cellular
organelles. The microalgae are also single-celled organisms but have a more complex
cell structure. They were actually formed as symbioses of photosynthetic prokaryotes
or eukaryotes with heterotrophic eukaryotic hosts (Plate 2.1; Parker et al. 2008).
Microalgae comprising the various taxonomic groups were formed by three types of
these endosymbioses. In a primary endosymbiosis, a cyanobacterium was acquired by
a heterotrophic eukaryote. In a secondary endosymbiosis, a eukaryotic heterotroph
acquired a photosynthetic eukaryote. In a tertiary symbiosis, a dinoflagellate host
engulfed a secondary endosymbiont, that provided its chloroplast to the dinoflagellate.
The clue to this is that pigments and genes of the transferred chloroplasts match those
of extant eukaryotic algae (Keeling 2010). In all of these symbioses, some or all genes
from the chloroplasts were eventually transferred to the host nucleus.
Main Constituents of Marine Phytoplankton
Picoplankton – Both Prokaryotic and Eukaryotic
Cyanobacteria
(^) Bacteria are prokaryotes, organisms in which the macromolecules carrying genetic
information, deoxyribonucleic acid (DNA), are not held in a nucleus (“karyon”), a
specialized organelle surrounded by a membrane. The relatively simple DNA-bearing
chromosomes disperse in the central region of the cell, which is the basis of the term
“prokaryotic”. Bacteria are more fully characterized in Chapter 5. One group of
bacteria, called cyanobacteria, produce organic matter by photosynthesis. In that
sense, they are algae as well as bacteria, so botanists classify them as the division
Cyanophyta, the blue-green algae. Their photosynthetic pigments are arrayed in
layers, thylakoids, around the cell periphery. Streams and ponds often support sizeable
stocks of filamentous blue-green algae (Nostoc, Anabaena, and others), macroscopic
forms familiar to many from introductory biology classes. It was not until surprisingly
late that we realized cyanobacteria are important in marine, pelagic habitats.