The Vital Question: Why is life the way it is?

by Nick Lane

You might think this is just playing trivial games with numbers, that it holds no real meaning. I must confess that worried me too – these numbers are quite literally incredible – but this theorising does at least make a clear prediction. Giant bacteria should have thousands of copies of their full genome. Well, that prediction is easily testable. There are some giant bacteria out there; they’re not common, but they do exist. Two species have been studied in detail. Epulopiscium is known only from the anaerobic hind gut of surgeonfish. It is a battleship of a cell – long and streamlined, about half a millimetre in length, just visible to the naked eye. That’s substantially larger than most eukaryotes, including paramecium (Figure 23). Why Epulopiscium is so big is unknown. Thiomargarita is even larger. These cells are spheres, nearly a millimetre in diameter and composed mostly of a huge vacuole. A single cell can be as big as the head of a fruit fly! Thiomargarita lives in ocean waters periodically enriched in nitrates by upwelling currents. The cells trap the nitrates in their vacuoles for use as electron acceptors in respiration, enabling them to keep respiring during days or weeks of nitrate deprivation. But that is not the point. The point is that both Epulopiscium and Thiomargarita exhibit ‘extreme polyploidy’. That means they have thousands of copies of their full genome – up to 200,000 copies in the case of Epulopiscium and 18,000 copies in the case of Thiomargarit...