PROKARYOTE – EUKARYOTE ASSOCIATIONS IN SULFIDIC ENVIRONMENTS AND THEIR POSSIBLE EFFECTS ON S-CYCLING AND THE GEOLOGIC RECORD

Sulfide-enriched marine sediments not only support prokaryotic communities, they also support dense populations of eukaryotes. The majority of these eukaryotes have prokaryotic associates, or symbionts, many of which are sulfide-oxidizing bacteria (SOB). While SOB-eukaryote symbioses were first described from hydrothermal vent taxa (e.g., tube worms, mussels), a plethora of additional prokaryotic-eukaryote symbioses have subsequently been described from other sulfide-enriched environments. In these habitats, diverse protistan communities occur (foraminifera, flagellates, ciliates) as do less diverse assemblages of metazoans (e.g., nematodes, polychaetes). While some anaerobic protists are known to have sulfate reducing symbionts and metazoans typically have sulfide-oxidizers, notable exceptions to this generality exist (e.g., sulfate reducer on metazoan). This talk will review occurrences of eukaryotes inhabiting certain marine oxygen-depleted, sulfide-enriched environments (e.g., Santa Barbara Basin, Cariaco Basin), illustrate selected prokaryote-eukaryote associations, and discuss possible effects that these symbiont-bearing organisms have on sulfur cycling and the geologic record. Specific examples include the endosymbiont-bearing foraminifer Virgulinella fragilis, which occurs in extremely high abundance in Beggiatoa laden sediments at the periphery of Caricao Basin. Another example to be discussed is the Santa Barbara Basin polychaete Xenonerilla bactericola, which has rod-shaped ectobionts; sequence data for their 16S rRNA indicate identification as sulfate reducing bacteria. It is enigmatic that an animal has an anaerobic ectobiont that produces hydrogen sulfide, which is a metabolic byproduct supposedly toxic to aerobes. A further topic of discussion will be the effect of eukaryotic mobility on the survival capacity of symbionts. Associations with eukaryotes could provide a distinct advantage to symbionts compared to free-living sulfur bacteria, especially in habitats with dynamic redox boundaries (e.g., seasonal hypoxia, hydrocarbon seeps).