Microbial indicators as a diagnostic tool for assessing water quality and climate stress in coral reef ecosystems

Microorganisms play a fundamental role in the functioning and stability of coral reef ecosystems. However, environmental disturbances can trigger alterations to the natural microbial community composition and their functional traits with potentially detrimental consequences for host organisms, such as corals, sponges and algae and concomitant implications for the entire coral reef ecosystem. Coral reefs are increasingly affected by localized impacts such as declining water quality and global pressures derived from human-induced climate change, which severely alters the natural conditions on reefs and can push dominating benthic life forms towards the limit of their resistance and resilience. Microorganisms can respond very rapidly to these altered environmental conditions so defining their natural variability over spatial and temporal gradients is critical for early and accurate identification of environmental disturbances. The rapid response of microbes to environmental change is likely to confer significant advantages over traditional reef monitoring methods, which are based on visual signs of health deterioration in benthic coral reef macroorganisms. This review discusses the potential of microbes as early warning indicators for environmental stress and coral reef health and proposes priorities for future research.