HABITAT EVOLUTION: DOES FRESHWATER ANCESTRY OF LIVING PHOTOTROPHS SIGNIFY A LOST PRE-CRYOGENIAN MARINE BIOSPHERE?

Genomic trait evolution studies purportedly indicate that many extant marine bacterial and eukaryotic phototrophs have freshwater ancestry. This is surprising in view of the fossil record of putative crown-group cyanobacteria and eukaryotic algae from undoubted marine successions as old as 2.02 and 1.04 Ga, respectively. But what if we accept mounting radiometric evidence that hard snowball events occurred in tandem in early (717−661 Ma) and late (646±5−635 Ma) Cryogenian time, implying that oceans were everywhere covered by sea-glaciers≥100s of m thick for 67±5 Myr combined? Extant flora can only have evolved from survivors of those events. Modern polar and alpine ecosystems are informative regarding prospective phototrophy on hard snowballs. Hydrothermal vents are ephemeral on snowball time scales and permit phototrophy only where ice cover was exceptionally thin already. Ice-covered lakes would have dotted coastal and equatorial dry valleys. Ablation drives such lakes hypersaline, allowing light to penetrate thin clear ice. Polar ice-covered lakes host ≤15M cells mL^-1of planktonic picocyanobacteria in summer. When snowballs melted, most such lakes would have been inundated by marine waters. Supraglacial meltwater-hosted ecosystems greatly expand on hard snowballs. In the sea-glacier ablation zone, ca 12% of global surface area, meltwater arose from surface accumulation of solar energy-absorbing dust and volcanic ash, amplified by darkly-pigmented extracellular cyanobacterial product. The result was perpetual freshwater cycling through vast networks of semi-isolated sunlit oligotrophic ecosystems. In addition to cyanobacteria, these ecosystems typically include green algae, fungae, protists and select metazoans. It would be surprising if modern flora did not spring from cryoconite-based ecosystems of Cryogenian age, supplemented by dry-valley lakes. Mindful of convergence, caution is called for in assigning crown-group status to pre-Cryogenian marine fossils.