Climate change affects key nitrogen-fixing bacterial populations on coral reefs

Abstract

Coral reefs are at serious risk due to events associated with global climate change. Elevated ocean temperatures have unpredictable consequences for the ocean's biogeochemical cycles. The nitrogen cycle is driven by complex microbial transformations, including nitrogen fixation. This study investigated the effects of increased seawater temperature on bacteria able to fix nitrogen (diazotrophs) that live in association with the mussid coral Mussismilia harttii. Consistent increases in diazotroph abundances and diversities were found at increased temperatures. Moreover, gradual shifts in the dominance of particular diazotroph populations occurred as temperature increased, indicating a potential future scenario of climate change. The temperature-sensitive diazotrophs may provide useful bioindicators of the effects of thermal stress on coral reef health, allowing the impact of thermal anomalies to be monitored. In addition, our findings support the development of research on different strategies to improve the fitness of corals during events of thermal stress, such as augmentation with specific diazotrophs.

Diazotrophs: Overlooked Key Players within the Coral Symbiosis and Tropical Reef Ecosystems?

“…Nevertheless, the dominant associated diazotrophs may also vary depending on the health of the coral (Yang et al, 2015). Bacterial diversity decreases as corals grow, and the dominant groups also change with each life stage, likely reflecting a fine tuning of the hosted microbial community upon changes in metabolic needs in different life stages and/or when experiencing environmental stress or disease (Nissimov et al, 2009;Ceh et al, 2013b;Lema et al, 2014b;Santos et al, 2014 see Section Does Diazotrophy Enhance Coral Resilience? ).…”

“…In such events, corals' resilience could be enhanced by alternative fixed nitrogen sources such as that provided by endosymbiont diazotrophs. For example, Santos et al (2014) reported that the abundance and diversity of diazotrophs associated with the coral Mussismilia harttii increase in response to +2 to +4.5 • C above in situ background seawater temperature. In the Red Sea coral-associated N 2 fixation rates have been reported to increase 3-to 10-fold when corals are artificially exposed to higher temperatures (Cardini et al, 2016b) as well as in response to natural rising temperatures in the summer, suggesting that DDN could provide alternative nitrogen sources when environmental nutrient concentrations are scarce .…”

Ecology and biogeochemistry of cyanobacteria in soils, permafrost, aquatic and cryptic polar habitats

“…Recently, it has been shown that climate change is likely to affect nitrogen-fixing microorganisms in general and cyanobacteria in particular (Singh et al 2010;Santos et al 2014). Through the application of modern molecular phylogenetic approaches, we are beginning to better understand microbial functionality in Polar Regions.…”

Dominance of Endozoicomonas bacteria throughout coral bleaching and mortality suggests structural inflexibility of the Pocillopora verrucosa microbiome

“…In these cases, changes in the coral‐associated bacterial community composition were interpreted as detrimental. Accordingly, changes in bacterial community structure or activity may be either beneficial or deleterious, depending on the environmental context (Ainsworth & Gates, 2016; Pogoreutz, Rädecker, Cárdenas, Gärdes, Voolstra, et al., 2017; Pogoreutz, Rädecker, Cárdenas, Gärdes, Wild, et al., 2017; Rädecker et al., 2015, 2017; Santos et al., 2014). …”