PROTEROZOIC EVOLUTION OF PHOTOSYNTHETIC EUKARYOTES: A PRIMER FOR PROTEROZOIC EVOLUTION OF OCEAN AND ATMOSPHERIC REDOX STATE

Although numerically outnumbered by photosynthetic bacteria in the modern ocean, photosynthetic eukaryotes are responsible for the majority of organic carbon export from the ocean-atmospheric system to the sediments. Thus, they play an important role in controlling global organic carbon burial and ocean-atmospheric redox state. Available molecular and body fossils indicate that eukaryotes diverged in the Paleoproterozoic or earlier. The Proterozoic fossil record of photosynthetic eukaryotes is represented by microscopic acritarchs and macroscopic carbonaceous compressions. Most acritarchs can be plausibly interpreted as fossils of eukaryotic phytoplankton and many carbonaceous compressions are probably benthic macroalgae, although their exact phylogenetic positions remain uncertain. Quantitative analysis suggests that the morphological complexity of both eukaryotic phytoplankton and benthic macroalgae increased markedly in the Ediacaran Period. In addition, modeled surface/volume ratio and maximum canopy height of benthic macroalgae also increased significantly in the Ediacaran Period. As the surface/volume ratio of living photosynthetic organisms is strongly correlated with their mass-specific productivity, the Ediacaran increase in surface/volume ratio and morphological complexity (which in many cases are directly related to surface/volume ratio) of both eukaryotic phytoplankton and benthic macroalgae indicates greater bioproductivity. Thus, Ediacaran phytoplankton and macroalgal diversification (together with other geological factors such as sedimentation rate and absorption of organic carbon by clay minerals) may have contributed to the oxygenation of the ocean-atmospheric system and may have set the stage for the evolution of oxygen-requiring macroscopic bilaterian animals in the late Ediacaran and early Cambrian Period.