Description
Coral reef ecosystems are a diverse but declining habitat. Most reefs around the world have transitioned from a coral to algal dominated benthos with the exact cause for this shift remaining elusive. There is recent evidence that microbes associated with algae either directly from the algae surface or via dissolved organic carbon (DOC) stimulation are detrimental to coral health, inducing mortality and providing space for algae recruitment and growth. To address this issue I investigated how multiple reef organisms influence microbial communities in the surrounding water column. I predicted the effluence from various reef macro-organisms would alter the water column microbial community in contrasting ways. This study consisted of an experiment that looked at water column microbes induced by DOC from various reef macro-organisms and a field assessment of microbial communities, comprised of both shed and induced microbes, above replicate patches dominated by a single macro-organism. Both were conducted on the reefs in the Abrolhos Archipelago of Eastern Brazil. The controlled experiment had replicate seawater microbial communities exposed for 30 hours to the presence of the DOC from different benthic organisms including 1) coral (Mussismilia harttii), 2) crustose coralline algae, 3) algae (Stypopodium zonale), and 4) a seawater control. The microbial communities were semi-cultured and sequenced. Metagenomes showed significant changes in microbial taxa and function when grown in the presence of different macro-organisms. In the second experiment, water above replicate macro-organisms including 1) coral (Mussismilia braziliensis), 2)algae (Stypopodium, Dictota and Canistrocarpus), 3) cyanobacterial mats, and 4) zoanthids (Palythoa caribaeorum) were collected in the field and were compared to the water microbes collected 2 m above the reef. Microbial genera were identified in the water column above each macro-organism. At both a broad and specific level, metabolic pathways of microbes influenced by reef macro-organisms were distinctive. These two studies show that dominant benthic macro-organisms influence the microbes in the water column surrounding them, developing a specific "aura-biome". The aura-biomes may reduce recruitment potential or growth of non-self macro-organisms. Therefore, as a reef changes to non-coral dominated, the microbial environment may transition to be unsuitable for coral growth, establishing a feedback loop.
