Effects of nitrogen-fixing symbiotic cyanobacteria on the microbial ecology of the coral, Montastraea cavernosa

Corals form the physical structure of coral reefs, one of the most ecologically and economically important ecosystems in the world. The abundant and broadly distributed Caribbean coral Montastraea cavernosa forms a symbiosis with intracellular nitrogen-fixing cyanobacteria in some, but not all colonies, which make up approximately 30% of the population and display a characteristic orange fluorescence. Diverse and functionally important microbial communities of dinoflagellates, bacteria, Archaea, viruses, fungi, and other organisms are also associated with corals and together with the host compose what is termed the coral holobiont. Whether the cyanobacteria are mutualists, commensals, or parasites, and their effects on the coral holobiont, are unknown. The influence of the cyanobacteria on the microbial ecology of M. cavernosa and overall holobiont fitness were investigated using sequencing of ribosomal RNA PCR amplicons, metatranscriptomic sequencing of rRNA and mRNA from the holobiont, and experimental tests of various fitness metrics. The cyanobacterial symbionts appear to be diverse, but many are Pleurocapsa-like and related to nitrogen-fixing cyanobacterial symbionts of diatoms. Their presence does not affect the taxonomic composition of the diverse coral-associated prokaryotic community, but such communities do exhibit significant geographic differences. Metatranscriptomic analysis of mRNA indicates that coral-associated prokaryotes, including cyanobacteria, are transcriptionally active, although few transcripts related to nitrogen fixation were recovered. There were no significant fitness differences between colonies with and without cyanobacteria for any of the metrics tested, including coral growth, response to thermal stress, and the ability to deter predators and produce cyanobacterial toxins. Genotyping of coral hosts revealed that colonies with and without cyanobacteria form two genetically distinct populations at small spatial scales, providing evidence for selection. The cyanobacteria do not appear to be parasitic to the coral host, but any potential benefits they may convey remain unknown. Analysis of metatranscriptomic data to investigate differences in the functional activity of coral hosts and associated microbial communities is ongoing