A speculative review of factors controlling the evolution of phytoplankton during Paleozoic time

Abstract

Compilations of acritarch taxon diversity through geologic time show a severe decline at the end of the Devonian Period. While this diversity drop may have been due to intrinsic (biological) factors, it seems more likely that acritarch diversity has changed in response to extrinsic changes in seawater chemistry. Since acritarchs largely serve as a fossil proxy for phytoplankton, that is, oxygenic photoautotrophes, the evolution of their physiology is more directly linked with the chemistry of the environment than are the physiologies of other groups of marine eukaryotic organisms. Two major shifts occurred in Earth systems chemistry during the end of the Devonian: (1) a decline in atmospheric pCO 2 from 12 to perhaps 20 times that of today down to within near present day levels, and, (2) a shift in seawater chemistry from low Mg calcite to high Mg calcite + aragonite seas. Additionally, the rise in standing carbon biomass on land due to the evolution of trees during the Frasnian–Tournasian interval, caused a marked increase in carbon flux to estuarine and marine habitats. In combination, the marine realm experienced a drop in Ca/Mg, Cl/SO 4, K/Na, and CO 2(aq) with an increase in particulate ( POM) and dissolved ( DOM) organic matter. This last change would have brought with it an increased flux of P, which is mobilized in biological systems primarily as complexed with organic matter. Throughout their evolutionary history, cyanobacteria and the algae have shown a progressive acquisition of carbon concentration mechanisms ( CCMs) that are required for inorganic carbon (C i) uptake by their anabolic physiology. Paleozoic phytoplankton lacking these CCMs could have been growth-limited by C i uptake. Given the possibility of a lack of such CCMs during the relatively high levels of CO 2 during the Lower Paleozoic, it seems possible that the acritarch decline was a combination of extinction due to inefficient C i assimilation plus loss of cyst formation as a principal mechanism for survival following a large-scale shift to heterotrophic nutrition within multiple lineages.