Updated: Oct 20, 2019
Ultimately, all life on Earth depends upon the presence of certain forms of bacteria. The coral holobiont—composed of a group of symbiotic partners—is certainly no exception. In a paper recently published by the American Society for Microbiology, researchers reveal the surprisingly large extent to which corals (which necessarily live in nutrient-poor environments) rely on nitrogen-fixing microbes. Amazingly, these bacteria (a diverse group known as the diazotrophs) were found to account for up to half of the microbiota associated with the reef-building corals they analyzed.
Marine ecologists have for long considered the importance of diazotrophic nitrogen fixation in coral reef habitats as beyond doubt. These beneficial bacteria seem to directly promote the health and abundance of both symbiotic Symbiodinium and their host. This makes sense, as corals and zooxanthellae alike produce ammonium assimilation enzymes. While the presence of nitrogen-fixers was reported as continuous (i.e. nonseasonal) in common Great Barrier Reef Acropora spp., composition of these beneficial microbial communities can change as the coral host’s dietary needs change between seasons.
Moreover, investigators discovered that the diversity of coral-associated diazotrophic assemblages differed considerably between coral tissues and coral mucus. Not only was diversity found to be much greater in mucus microhabitats, but also that many of these same bacteria could be found abundantly in the surrounding seawater. These nitrogen-fixers do not appear to be specific to coral species or locale per mucus content. On the other hand, the predominant diazotrophs found in tissue samples do suggest fairly strong species-specific associations.
It is quite interesting that the dominant diazotrophs for all coral species studied were closely related to alphaproteobacteria affiliated with the order Rhizobiales (71% of the total sequences retrieved from tissue samples). Most likely, the rhizobia have developed a mutualistic relationship with corals (as they have with terrestrial legume plants) by providing fixed nitrogen to zooxanthellae. For example, the predominant rhizobial group from summer Porites lutea samples (representing 14.51% of the total recovered sequences) was affiliated with the purple nonsulfur bacterium Rhodopseudomonas palustris (92% similarity).
Rhodopseudomonas palustris is well-known and well-studied for its ability to utilize all four basic modes of metabolism: photoautotrophy, photoheterotrophy, chemoautotrophy and chemoheterotrophy. Biologists suggest that this not only allows the bacterium to freely inhabit open waters or seafloor but also provides critical protection from the copious amounts of oxygen generated by dinoflagellate photosynthesis in brightly illuminated coral tissues. These ubiquitous diazotrophs certainly would thrive in the anoxic conditions of the coral’s gastrodermis and gastrovascular cavity. As they inhabit stagnant areas over the coral’s tissue or mucus, they quite conceivably confer additional benefits to the coral in resisting certain pathogens/conditions.
Aquacultural applications for purple nonsulfur bacteria (especially R. palustris) have already gained favor on fish and shrimp farms (particularly in Asia). This owes primarily to the microbe’s probiotic properties as well as its ability to remediate waste water through nitrification, denitrification and consumption of dissolved organics. However, given its prominence amongst the diazotrophic species in this study, it seems R. palustris will almost certainly have significant value in the burgeoning coral farming industry.