Resumo

Corals live in symbiotic relationship with microorganisms including bacteria capable of fixing nitrogen (diazotrophs). Several studies have shown that diazotrophs are not passive members in the coral microbial community and seem to interact in close physiological relationships with both the coral and its microbial community. Coral reefs are ecosystems with a significant role in keeping marine biodiversity and in biogeochemical cycles. However, these ecosystems are under a high risk due to events associated to global warm, such as temperature elevation. The effect of temperature increase on diazotrophs and how this key effect may affect corals remains unclear. This study investigated the effects of increase in 1°C, 2°C and 4.5°C on seawater temperature on diazotrophs associated with Mussismilia harttii coral species. To evaluate the impact were performed: quantification of nifH gene copies by qPCR, denaturing gradient gel electrophoresis (DGGE) analysis of the nifH gene and nifH gene clone libraries. We observed increase in the abundance and diversity upon temperature elevation. Moreover, a shift in phyla dominance occurred, showing increased numbers of Alphaproteobacteria, Cyanobacteria, and Betaproteobacteria and decrease of Gammaproteobacteria, revealing a gradual change in dominance as temperature increases and indicating the dominant groups in a future scenario of climate change. Increases in abundance and diversity of diazotrophs can be explained by the symbiotic relationship between diazotrophs and zooxanthellae and by alterations in photosynthetic rates under thermal stress. This event may be related to the resilience capacity of M. hartii coral species to thermal stress reported and may help understand the survival capacity of corals throughout evolution. The increase of diazotrophs population may be used as a bioindicator of coral reef health and may help measuring the impact of thermal anomalies. Additionally, strategies of diazotrophs bioaumentation may be used to improve the fitness of coral during thermal stress.