PROTIST-PROKARYOTE AND ANIMAL-PROKARYOTE SYMBIOSES IN MODERN SULFIDIC SEDIMENTS

Although intimate associations between protists and prokaryotes as well as metazoans and prokaryotes are relatively common in oxygen-depleted, sulfide-enriched habitats, to date, very little is known about the identity of these putative symbionts and the role they play in the physiology of the host and biogeochemistry of their surrounding sediments. A synopsis of recent microscopic and genetic advances in identifying prokaryotic symbionts will be presented. For example, the flagellate Calkinsia aureus supports epsilon proteobacteria closely related to the sulfur oxidizing Arcobacter as ectobionts and an allogromiid foraminifer supports gamma proteobacteria, closely related to Pseudomonas, as endobionts. Many pseudomonads are known to dentrify, and we have recovered a key gene of denitrification from the allogromiid. Metazoan species (polychaete, nematode) are known to support ectobionts of, as yet, unknown identity. Many epibionts likely fulfill a role in detoxifying the immediate surrounding environment for their hosts and the hosts probably serve as a mobile substrate that delivers the ectobionts to the ideal locations along oxic/anoxic interfaces. Such activity can affect sediment fabric, and thus, impact the geologic record. Highlighted will be a case of multiple symbioses, where a ciliate supports at least 3 types of prokaryotes, including a sulfate reducing bacteria, a methanogen, a spirochete, and possibly a Type I methanotroph. The methanotroph and sulfate reducing bacteria belong to the two groups comprising the microbial consortia responsible for anaerobic oxidation of methane. Many spirochetes are fermenters that use carbon dioxide and hydrogen to produce acetate, which could serve as a potential energy source for the host. By identifying the symbionts systematically, the biogeochemical cycling between these symbionts and hosts is slowly becoming clear. In sum, depending on the host, it is possible that C, S and N cycling are affected by the eukaryote-symbiont associations. Findings will be discussed in light of the common agreement that protists lacking biomineralized structures evolved in the Proterozoic, when sulfidic habitats were likely more extensive than today. Supported by NSF grants MCB-0604084 to VPE and JMB and MIP-0702491 to JMB, K Casciotti, and VPE.