The Santa Barbara Basin is a symbiosis oasis

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Abstract
It is generally agreed that the origin and initial diversification of Eucarya occurred in the late Archaean or Proterozoic Eons when atmospheric oxygen levels were low1 and the risk of DNA damage due to ultraviolet radiation was high2. Because deep water provides refuge against ultraviolet radiation3 and early eukaryotes may have been aerotolerant anaerobes1,4,5, deep-water dysoxic environments are likely settings for primeval eukaryotic diversification. Fossil evidence shows that deep-sea microbial mats, possibly of sulphur bacteria similar to Beggiatoa, existed during that time6. Here we report on the eukaryotic community of a modern analogue, the Santa Barbara Basin (California, USA). The Beggiatoa mats of these severely dysoxic and sulphidic sediments support a surprisingly abundant protistan and metazoan meiofaunal community, most members of which harbour prokaryotic symbionts. Many of these taxa are new to science, and both microaerophilic and anaerobic taxa appear to be represented. Compared with nearby aerated sites, the Santa Barbara Basin is a ‘symbiosis oasis’ offering a new source of organisms for testing symbiosis hypotheses of eukaryogenesis.