Abstract
Coral reefs are ecologically significant habitats. Coral-algal symbiosis confers ecological success on coral reefs and coral-microbial symbiosis is also vital to coral reefs. However, current understanding of coral-microbial symbiosis on a genomic scale is largely unknown. Here we report a potential microbial symbiont in corals revealed by metagenomics-based genomic study. Microbial cells in coral were enriched for metagenomic analysis and a high-quality draft genome of “Candidatus Prosthecochloris korallensis” was recovered by metagenome assembly and genome binning. Phylogenetic analysis shows “Ca. P. korallensis” belongs to the Prosthecochloris clade and is clustered with two Prosthecochloris clones derived from Caribbean corals. Genomic analysis reveals “Ca. P. korallensis” has potentially important ecological functions including anoxygenic photosynthesis, carbon fixation via the reductive tricarboxylic acid (rTCA) cycle, nitrogen fixation, and sulfur oxidization. Core metabolic pathway analysis suggests “Ca. P. korallensis” is a green sulfur bacterium capable of photoautotrophy or mixotrophy. Potential host-microbial interaction reveals a symbiotic relationship: “Ca. P. korallensis” might provide organic and nitrogenous nutrients to its host and detoxify sulfide for the host; the host might provide “Ca. P. korallensis” with an anaerobic environment for survival, carbon dioxide and acetate for growth, and hydrogen sulfide as an electron donor for photosynthesis.
Highlights
Coral reefs are significant marine ecosystems, but they are declining at an alarming rate, primarily due to climate change, ocean acidification, and anthropogenic disturbances[1, 2]
They are mostly found in anoxic aquatic environments with both light and sulfide, performing anoxygenic photosynthesis using electrons generated by sulfur oxidation[10]
To explore ecologically important bacterial species in local corals, we studied samples of Porites lutea, Platygyra carnosa and Montipora venosa from Lamma Island and Porites lutea, Galaxea fascicularis and Montipora peltiformis from Crescent Bay collected in both winter and summer, 2014 (Fig. 1 and Table S1)
Summary
Coral reefs are significant marine ecosystems, but they are declining at an alarming rate, primarily due to climate change, ocean acidification, and anthropogenic disturbances[1, 2]. Green sulfur bacteria fix carbon dioxide via the reductive tricarboxylic acid (rTCA) cycle but some organic compounds like acetate and pyruvate can be assimilated during mixotrophic growth[9, 11]. Chlorobium chlorochromatii can form a prokaryotic symbiosis with another bacterium “Candidatus Symbiobacter mobilis” in the phototrophic consortium “Chlorochromatium aggregatum”[14] These two species can benefit from each other through metabolic exchange, sensing and moving towards light and sulfide, and electron cycling[15]. We enriched microbial cells for coral metagenomic analysis, successfully identified a green sulfur bacterium “Candidatus Prosthecochloris korallensis” as a potential microbial symbiont in corals, and analysed the metabolic pathways of the bacterium to gain insights into the mechanisms of coral-microbial symbiosis. This study will provide a new understanding of coral-microbial symbiosis on a genomic scale
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