1. Department of Environmental Health Sciences, School of Public Health, University of South Carolina, Columbia, South Carolina 29208, USA
2. Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, California 95039, USA
3. Department of Cellular Biology, University of Georgia, Athens, Georgia 30602, USA
4. Wadsworth Center, New York State Department of Health, Albany, New York 12201-0509, USA

Correspondence to: Joan M. Bernhard¹ Correspondence and requests for materials should be addressed to J.M.B. (e-mail: Email: jmbernha@sph.sc.edu).

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 anaerobes^{1, 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.