Protistan-Prokaryotic Symbioses in Deep-Sea Sulfidic Sediments

Concluding remarks

The most abundant protistan groups (euglenoids, ciliates and foraminifera) from two deep-sea sulfidic sedimentary environments possess a variety of symbiotic associations with prokaryotes. These associations include both epibiotic and endobiotic relationships. To date, endobiotic symbioses in euglenoids are not well documented while numerous examples of endosymbioses exist in ciliates and foraminfera. In all protistan-prokaryote symbioses observed to date from these deep-sea environments the exact benefit to the protist as well as the prokaryote remains only speculative. Quite clearly the biomass enhancement of these three groups in these deep-sea sulfidic environments, compared to nearby aerated sites is likely related to the rich and diverse symbioses we observe.

In addition to the questions of the nature of the host-symbiont relationships, future research will likely focus on the trophic mechanisms of the host (e.g., raptorial vs. epibiont phagocytosis) and symbiont metabolism (e.g., sulfur oxidizing, sulfate reducing, methanogenic). Only sulfur oxidizing epibionts have positively been identified with euglenoids when many other possibilities have been shown for ciliates.

Exploration of other deep-sea sulfidic environments for their symbiotic communities and their symbiotic consortia and utilization of molecular techniques such as fluorescent in situ hybridization are two promising approaches to resolving some of these questions. We occasionally observe symbioses between protists and prokaryotes in more aerated or less reduced sites in both shallow and deep marine settings (Epstein et al. 1998, Bernhard et al 2000). Are these relationships fundamentally different from those we see at sulfidic sites or are the organisms refugees from sulfidic microhabitats not normally observed by classic benthic ecological approaches?