The origin of modern phytoplankton.

A single valve of the diatom Thalassiosira pacifica, the coccolithophore Scyphospahaera apsteinii and a pair of phycomas of Pterosperma moebii. (From Falkowski 2004)

The term “phytoplankton” derives from Greek roots: φψτος (phytos) –related to plants – and πλαγκτον (plankton) meaning a “wanderer” or a “drifter”. It was coined by Christian Gottfried Ehrenberg in 1897 and describes a diverse, polyphyletic group of mostly single-celled photosynthetic organisms that drift with the currents in marine and fresh waters.

The evolutionary history of eukaryotic phytoplankton has been studied through morphological fossils and molecular biomarkers such as lipids or nucleic acids. Organic walled fossils  made by eukaryotic phytoplankton occur in rocks as old as 1.6 to 1.8 billion years, but their morphological diversity is low and their phylogenetic relationships obscure.

The acritarchs were dominant forms of eukaryotic phytoplankton during the NeoProterozoic and the Paleozoic. These forms diversified markedly, in parallel with the Cambrian and Ordovician radiations of marine invertebrates. The group began to decline in the Late Devonian. And of course, the End-Permian mass extinction marked a major transition in ocean ecosystem structure.

Geologic range and relative diversity of eukaryotic phytoplankton. From Martin 2012.

Diatoms, dinoflagellates and coccolithophores, appeared in the geologic record during the Mesozoic. The radiation of this modern eukaryotic phytoplankton is paralleled with a long-term increase in sea level with and expansion of flooded continental shelf area.

These taxa have been grouped under the informal heading of “red” algal lineages primarily on the basis of their chlorophyll-c plastids. The shift from green to red phytoplankton lineages may have actually begun during the late Paleozoic.

Trace elements were important in defining the evolutionary trajectory of these groups of phytoplankton. While green algae need higher concentrations of iron, zinc and copper, red forms need higher amounts of manganese, cobalt and cadmium. Geologic evidence indicates that oxygen levels in the Mesozoic were much higher and that helped the micronutrients used by the red phytoplankton to remain dissolved in the oceans and available for uptake.

Diatoms living between crystals of annual sea ice in McMurdo Sound, Antarctica. From Wikimedia Commons.

But, changes in the availability of macronutrients, such as phosphorus, also contributed significantly to the success of these groups. The patterns of diversity in the fossil records of dinoflagellates and coccolithophorids are roughly concordant, but differs with that of diatoms. This reflect different ecological strategies. In contrast to dinoflagellates and coccolithophores, diatoms have evolved a nutrient storage vacuole that can retain sufficiently high concentrations of nitrate and phosphate such that a cell can undergo several divisions without the need for external macronutrients.

Human activities are altering ocean conditions at a speed unsurpassed in our Earth’s history. In modern oceans, the coccolithophores and other calcifying phytoplankton that live in surface waters may be devastated by the acidification, which reduces the availability of minerals needed to make and maintain their shells.
As the oceans warm, they may also become increasingly stratified, impeding upwelling and circulation. In such conditions, dinoflagellates could increase the frequency and surface area covered by toxic blooms in coastal habitats.

References:

Martin, R. E. and Quigg, A. 2012 Evolving Phytoplankton Stoichiometry Fueled Diversification of the Marine Biosphere. Geosciences. Special Issue on Paleontology and Geo/Biological Evolution. 2:130-146.

Falkowski PG, Katz ME, Knoll AH, Quigg A, Raven JA, Schofield O, Taylor FJ., The evolution of modern eukaryotic phytoplankton, Science. 2004 Jul 16; 305(5682):354-60

 

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