SIZE MATTERED IN THE ARCHEAN

One of the many striking features that has emerged with reports on Archean microfossils is the large size of some of them. These include solitary, unicellular spindle or flanged-shaped microfossils from the ~3.4 Ga Strelley Pool Formation are up to 105 um in size (major axis) and similar fossils in the ~3.3 Ga Kromberg Formation are 135 um in size. The ~3.2 Ga Moodies Group contains spheroids up to 298 um in diameter. Large solitary microfossils are likely plankton, presumably phytoplankton. Such large sizes in pre-Phanerozoic microfossils are usually attributed to eukaryotes. It is generally assumed that eukaryotes did not arise until after the great oxidation event, 2.4 Ga. Thus, it appears that large size had selective advantages in prokaryotes early in their history. But, were these vegetative cells, reproductive cells, or preserved envelopes containing many cells?

The Archean Earth system was substantially different from younger times. It was characterized by elevated surface temperatures (40^oC to as much as 80^oC), reduced solar luminosity, no or trivial amounts of atmospheric oxygen, harmful UV radiation striking the surface, and an Earth spin rate almost double that of today. With the high spin rate, Hadley cells were more compact and severe, producing significant turbulence in the upper part of the water column. High temperatures could have resulted in increased salinity through evaporation and thus a higher density of seawater.

Large size seems disadvantageous in phytoplanktonic prokaryotes, yet it appears to have been achieved in the Archean. Large size has increased mass and thus sinking rate, lower surface area to volume ratio thus impacting rate of nutrient uptake and excretion, could require more rigid cell walls to retain shape, and likely need a longer cell division time. There are some solutions to these problems. To counteract increased mass, vacuoles or low density extracellular excretions would provide buoyancy. But, what were the selective advantages for large size at this time in Earth history? Aside from a reproductive strategy of multiple fission within a large mother cell as opposed to binary fission (producing many versus a few cells), other advantages for large size in prokaryotes remain elusive.