chroococcaceans, the most primitive (Giovannoni et al., 1988) and commonly occurring (Schopf, 1992d) Precambrian cyanobacterial families. Similar tolerance is also exhibited by members of other cyanobacterial families. For example, a nostocacean was revived after more than a century of storage in a dried state (Davis, 1972) and a scytonematacean is reported to have maintained growth at pH 13 (Vallentyne, 1963). Thus, cyanobacteria exhibit notable ecologic flexibility, and even though no single oscillatoriacean or chroococcacean species is known to be capable of tolerating the total range of observed growth conditions (for example, thermophiles dominant in 70°C waters rarely grow below 50°C, and species adapted to highly alkaline lakes do not occur in acid hot springs), both groups include impressive ecologic generalists, able to thrive in virtually all present-day widespread environments (Table 2). Moreover, many of the oscillatoriacean and chroococcacean genera for which wide ecologic tolerance has been demonstrated (Table 2) are the same as those having species-level Precambrian-extant counterparts (Figure 4). Finally, numerous cyanobacteria, including both oscillatoriaceans and chroococcaceans (Stewart, 1980), are capable of fixing atmospheric nitrogen; provided with light, CO2, a source of electrons (H2, H2S, H2O), and a few trace elements, such cyanobacteria are highly effective colonizers, able to invade and flourish in a wide range of habitats.

The wide ecologic tolerance of cyanobacteria is almost certainly a product of their early evolutionary history. Fossil evidence suggests that oscillatoriaceans (Schopf, 1993) and chroococcaceans (Schopf and Packer, 1987) were extant as early as 3.5 Ga ago. If so, they must have originated and initially diversified in an oxygen-deficient environment, one lacking an effective UV-absorbing ozone layer. In such an environment, the ability to photosynthesize at low light intensities (Table 2) coupled with the presence of gas vesicles to control buoyancy (Jensen, 1993) would have permitted planktonic cyanobacteria to avoid deleterious UV by inhabiting the deep oceanic photic zone, just as Synechococcus does today. Similarly, numerous characteristics of living benthic mat-building cyanobacteria—effective DNA repair mechanisms, synthesis of UV-absorbing scytonemin, secretion of copious extracellular mucilage, phototactic motility, adherence to substrates, stromatolitic mat formation—initially may have been adaptations to cope with a high UV flux in near-shore shallow water settings. Adaptive radiation in an early oxygen-deficient environment is also suggested by the ability of cyanobacteria to live in either the presence or absence of oxygen (Table 2), their capability to switch between oxygenic and anoxygenic photosynthesis (Olson and Pierson, 1987), the occurrence of oxygen-sensitive nitrogenase in many taxa (Stewart, 1980), and the restriction of nitroge-

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