Were Precambrian oceans devoid of planktonic cyanobacteria? Insights from metabolism

Studies on biospheric evolution often assume planktonic cyanobacteria existed in Precambrian oceans, but that their productivity was limited due to various factors. However, available evidence suggests that planktonic cyanobacteria only colonized the open ocean near the Neoproterozoic-Phanerozoic boundary, close to the when a period of atmospheric oxygenation triggered the rise of animals. It is an open question if earlier planktonic marine cyanobacteria went extinct, possibly due to snowball Earth glaciations, or if cyanobacteria colonized the open ocean for the first time during this period. If the latter is true, consensus explanations are lacking for why cyanobacteria did not colonize the oceans sooner. Here I review the reconstructed metabolic evolution of marine picocyanobacteria, which perform ~25% of oceanic CO2-fixation and which emerged in the early Phanerozoic. These reconstructions provide insights into how the marine biosphere overcame earlier constraints on productivity, and how the rise of arthropods may have been key to allowing cyanobacteria to make the evolutionary leap from the benthos to the open ocean, helping transform the biosphere and Earth as a whole. The idea that photosynthesis was largely limited to shallow continental shelf waters during the Precambrian has implications for interpreting large-scale patterns of biospheric productivity and Earth history.