A questions about those ancient bacterial fossils…

Both Jerry Coyne and Larry Moran have good write-ups on the recent discovery of what are purportedly the oldest fossil cells, at 3.4 billion years old. I just have to add one little comment: a small, niggling doubt and something that bugs me about them. All the smart guys are impressed with this paper, but this one little thing gives me pause.

I'm a microscopist — I look at micrographs all the time, and one of the things I always mentally do is place the size of things in context. And I was looking at the micrographs of these fossils, and what jumped out at me is how large they are. They're not impossibly large, they're just well out of the range I expect for prokaryotes.

Most prokaryotes have diameters in the range of 1-10µm, while typical eukaryotes are about 10 times that size. There are exceptions: Thiomargarita gets up to 500µm across, so like I say, there's nothing impossible about these cells, it's just that the micrographs show lots of cells with 10-30µm diameters. And the authors come right out and report that:

The size range is also typical of such assemblages, with small spheres and ellipsoids 5-25âµm in diameter, rare examples (<10) of larger cellular envelopes up to 80âµm in diameter, and tubes 7-20âµm across (see ref. 24).

How odd. When I poke into the nervous system of an embryonic insect or fish, those are the sizes of cells I often see (well, except there aren't many tubes of that size!). When I poke into a culture or embryo contaminated with bacteria, they're much, much smaller. So maybe paleoarchaean bacteria tended to be larger? And they do cite a source for that size range of prokaryotes…

Then here's a new problem: the source cited, ref. 24, is the Schopf paper, the older paper that claimed to have found ancient bacterial fossils, a claim that has since been discredited! Uh-oh. What they're calling "typical of such assemblages" is a data set that's widely considered artifactual now. Furthermore, that's a simplified version of what Schopf said — he actually broke the sizes down into categories, and the range was more like 1-30 µm.

• Very small solitary, paired or clustered rods (ca 0.75âµm broad, ca 1.5âµm long), inferred to be prokaryotic (bacterial) unicells: one unit (ca 2600âMyr old), one morphotype.
• Small, solitary, paired or clustered coccoids (average diameter ca 3âµm, range ca 2-5âµm), inferred to be prokaryotic (bacterial, perhaps cyanobacterial) unicells: three units (range 3320-2600âMyr old), three morphotypes.
• Large solitary or colonial coccoids (average diameter ca 13âµm, range ca 5-23âµm), inferred to be prokaryotic (bacterial, perhaps cyanobacterial) unicells: three units (range 3388-2560âMyr old), four morphotypes.
• Narrow unbranched sinuous filaments (average diameter ca 1.25âµm, range ca 0.2-3âµm), with or without discernable septations, inferred to be prokaryotic (bacterial, perhaps cyanobacterial) cellular trichomes and/or trichome-encompassing sheaths: 10 units (range 3496-2560âMyr old), 17 morphotypes.
• Broad unbranched septate filaments (average diameter ca 8âµm, range ca 2-19.5âµm), inferred to be prokaryotic (perhaps cyanobacterial) cellular trichomes: four units (range 3496-2723âMyr old), 10 morphotypes.
• Broad unbranched tubular or partially flattened cylinders (average diameter ca 13âµm, range ca 3-28âµm), inferred to be prokaryotic (perhaps cyanobacterial) trichome-encompassing sheaths: five units (range 3496-2516âMyr old), five morphotypes (e.g. figures 3a-e and 4l).

So Schopf was reporting larger cells in his older samples, and now Wacey et al. are describing what look like very large cells to me in their 3.4 billion year old rocks. I'm not a microbiologist so I could be way off on this, but…isn't this just a little bit strange? Maybe there are some micro people out there who can reassure me that this isn't a surprising result.

Wacey D, Kilburn MR, Saudners M, Cliff J, and Brasier MD (2011) Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old rocks of Western Australia. Nature Geoscience Published online Aug. 21, 20110 [doi:10.1038/ngeo1238]

(Also on FtB)