History Of Phytoplankton

Evolutionary History of Phytoplankton

Dinoflagellates, coccolithophores, and diatoms are eukaryotic phytoplankton that came to dominate the seas in the Mesozoic era and do so still today. These three kinds of phytoplankton share a history that traces back to red algae, rhodophytes, in the ancient seas. To study the evolution of these organisms we must look back to the beginnings of oxygenic photosynthesis, about 2.4 billion years ago.

Cyanobacteria, prokaryotic blue-green algae, were the first to perform oxygenic photosynthesis. This ability was spread to eukaryotes via endosymbiosis. Once engulfed by the eukaryotic cell, the cyanobacterium can experience gene loss and become a plastid, an organelle. This process gave rise to two kinds of eukaryotic algae. These algae, green and red, contain differing plasmids and thus produce two lines of descendent organisms. Green algae is the ancestor of land plants, united by their use of chlorophyll b. Red algae plays ancestor to a line of seaweeds and phytoplankton whose plasmids use chlorophyll c in photosynthesis. It is this group of phytoplankton that includes dinoflagellates, coccolithophores, and diatoms.

Diatoms are typically non-motile phytoplankton known for their outer shell or test, made of silica. Lacking the ability to swim, diatoms have come to dominate nutrient-rich waters where currents or winds regularly mix the supply of food, typically in temperate waters. Dating the origin of diatoms is affected by the fact that the silica tests tend to disintegrate in the sediment, thus preventing the formation of fossil evidence. Using a technique based on the evolution of ribosomal genes, diatoms have been dated back to the early Triassic. However, fossil evidence places the rise of diatoms in the Jurassic or Cretaceous periods.

Dinoflagllates are more able to live in low-nutrient and tropical/subtropical waters than diatoms. This is due to their motility, usually provided by two flagella. This allows them to move within the water column to gather light at the surface during the day and move to deeper, more nutrient-dense water at night. Also notable is the diversity among dinoflagellates; not all are motile, and they can be autotrophic, heterotrophic, or mixotrophic. The dinoflaggelate shell is called a theca, made of cellulose. The creatures can be thecate or athecate. The fossil record dates dinoflagellates back to the Triassic period with an increase in diversity and abundance in the Jurassic.

Coccolithophores, the last of the three main types of phytoplankton, also provide fossil evidence of their origin in the late Triassic. They surround themselves with a hard shell composed of calcite plates called coccoliths.

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During the Mesozoic era, between 251 and 65 million years ago, the dominant green algae gave way to organisms of red plasmid lineage; diatoms, dinoflagellates, and coccolithophores outcompeted other eukaryotic algae. A combination of the ocean environment with the advantages of secondary symbiosis encouraged this to happen. First of all, the extinction event that took place between the Permian and Triassic gave these organisms an opportunity to proliferate in the seas. Along with this extinction came a long period of anaerobic ocean conditions. This environment would have let to a lack of useable nitrogen, favoring those organisms that could fix nitrogen. It is here that the uptake of a plasmid would be favorable to heterotrophes that would otherwise be limited by the lack of useable nitrogen. These heterotrophs would also benefit from the ability to obtain organic carbon from photosynthesis, and would have further competitive edge as mixotrophs able to survive unstable nutrient conditions.

There is also a question as to why these three types of phytoplankton all share a red plastid lineage. A major difference between red and green plastids is the kind of symbiosis involved in obtaining them. Plants and algae with green plastids acquired them through primary symbiosis, that is, the engulfment of a cyanobacterium. Red plastids on the other hand were acquired by phytopklanton via secondary symbiosis. The ancestral red algae had been produced by symbiosis of a eukaryote and a cyanobacterium; phytoplankton that engulf red algae are performing secondary symbiosis. This is important when considering that plastids lose genes when inside a host, and must use parts of the host cell to carry out genetic instructions. A more complete genome inside the plastid, inside the symbiont, would make it more likely that the genes would be passed on. For example, red plastids contain a more complete set of genes that code for photosynthesis than do green plastids; red plastids, inside a primary eukaryote, retain more of their genes and require less use of the host nucleus. This makes it more likely that the trait will be passed on to a secondary host.