Coral Reef Microorganisms in a Changing Climate

Abstract

Coral reefs are one of the most diverse and productive ecosystems on the planet, yet they have suffered tremendous losses due to anthropogenic disturbances and are predicted to be one of the most adversely affected habitats under future climate change conditions. Coral reefs can be viewed as microbially driven ecosystems that rely on the efficient capture, retention, and recycling of nutrients in order to thrive in oligotrophic waters. Microorganisms play vital roles in maintaining holobiont health and ecosystem resilience under environmental stress; however, they are also key players in positive feedback loops that intensify coral reef decline, with cascading effects on biogeochemical cycles and marine food webs. There is an urgent need to develop a fundamental understanding of the complex microbial interactions within coral reefs and their role in ecosystem acclimatization, and it is important to include microorganisms in reef conservation in order to secure a future for these unique environments.

Keywords: Biogeoscience; Global Nutrient Cycle; Microbiology; Microbiome.

Graphical abstract

Figure 1

Impacts of Human Activities on the Important Services Provided by Coral Reefs Overview of ecosystems services provided by a healthy coral reef ecosystem (left) and anthropogenic disturbances that lead to coral reef decline (right). Top left, counter clockwise: Habitat and biodiversity, biogeochemical cycling, tourism, shoreline protection, and fisheries. Top right, clockwise: Climate change, nutrient and sediment run-off from agricultural practices and deforestation, reduced water quality due to waste(water) pollution, physical destruction (e.g., due to shipping and dredging), and overfishing. The photos represent the Great Barrier Reef (left, 2008) and the Scott Reef lagoon (right, 2016).

Figure 2

Microbial Processes and Climate Change Impacts within the Coral Holobiont Microbially mediated processes within the coral holobiont in a healthy state (left) compared with a state of dysbiosis (right) resulting from increased seawater temperature and/or ocean acidification. Microorganisms in healthy corals are involved in nutrient cycling, production of essential amino acids and vitamins, and maintaining pathogen control. Stress events can lead to a shift in the microbiome, with an increase in opportunistic and potentially pathogenic species that could result in coral disease. Furthermore, elevated temperatures may result in increased nitrogen fixation, which disrupts internal nutrient balances and may enhance coral bleaching.

Box 2 Figure. The microhabitats of the coral holobiont (adapted from Hernandez-Agreda et al. [2017]; van Oppen and Blackall [2019]).

Box 3 Figure. A simplified overview of DOM—consisting of dissolved organic carbon (DOC), nitrogen (DON), phosphorous (DOP)—cycling in marine environments via the microbial loop.

Figure 3

Cycling of Dissolved and Particulate Organic Matter through the Microbial and Sponge Loops As coral reef ecosystems shift toward an algae-dominated state after coral bleaching and/or disease, significant amounts of readily available DOM are released into the environment, which stimulates growth of copiotrophic and potentially pathogenic microorganisms. Although microbes in healthy reefs dominated by calcifying corals excel at capturing and recycling energy and nutrients within the reef—thereby supporting high primary production and a diverse marine food web—microbialization limits the flow of energy to higher trophic levels and can lead to an overall loss of energy from the ecosystem under future climate change conditions.