Corals: diverse, yet declining
If you’ve seen any movie or documentary on marine life, you are probably aware that coral reefs are diverse habitats, but they are declining worldwide due to myriad anthropogenic stressors, including higher water temperatures. Although we often see coral reefs and their degrading state from a large scale, there is a very important component to coral reefs that is not easily viewed by the naked eye.
Microbial communities are diverse assemblages of bacteria, eukaryotes, cyanobacteria, and pathogens, and they are integral to coral reef health and stability. Early investigations of microbial associations with corals showed a strong relationship between the host macro-organism: most corals contain photosynthetic algae called zooxanthellae that live in their tissues. The coral provides the algae with a secure home, and the zooxanthellae, through photosynthesis, provide food for the growing coral. In this mix is also microbial communities associated with the coral host and symbiotic zooxanthellae. Research has shown that coral organism coverage is important in determining the types of microbial communities living directly on the coral and in the surrounding water.
Macro-organisms living on a coral reef, such as coral and different growth forms of algae, influence the amount of dissolved organic carbon (DOC), dissolved oxygen, and bacterial abundance in the surrounding water. Different levels of DOC that are sloughed off coral surfaces into the overlying water can influence the types of bacteria, and ultimately the major metabolic processes, occurring there. DOC is important because it is a readily available nutrient source for many microbial organisms.
Why care about microbes?
Even on a single reef ecosystem, the dominant coral organisms can be diverse; we can find observe a “patchwork mosaic” of microbial communities in the overlying water. Recent research has also discovered that microbial communities associated with the overlying waters of pristine corals consist of autotrophs and heterotrophs, while communities above degraded reefs are dominated by heterotrophs, including some pathogenic bacteria. As climate change and pollution continue to stress ocean life, including coral reefs, it is important to understand how changing coral coverage influences DOC dynamics and microbial communities. These changes may affect the ability of corals to recruit algae and grow.
The authors in the present study hypothesized that four different coral organisms, fleshy macroalgae, turf algae, zoanthids (an animal related to corals and jellyfish that colonizes corals), and coral, influences the microbial community (or “microbiome”) in the overlying water column. The authors refer to this microbiome that is directly above a particular coral cover type as the “aura-biome.” To test their hypothesis, the researchers collected water samples directly above different patches of coral cover present at the reefs of Ilha Santa Bárbara, an island off the coast of Brazil.
Then, the authors extracted all DNA present in the water samples, and used the DNA for shotgun metagenomics. Shotgun metagenomics is a relatively recent method in which all genes in all organisms from various environments are sampled. This method, unlike microscopy or cell cultures, enables researchers to evaluate bacterial diversity and detect abundances of microbes in various environments, and provides a means to study unculturable organisms that are difficult to analyze.
Using metagenomics, the authors were interested in not only identifying the microbial species in the water, but they also wanted to identify the active metabolisms, such as cell division and respiration, that were present in the coral-specific aura-biome.
Results and why they matter
As hypothesized, the authors found that each coral cover type was associated with a distinct microbial community in overlying water. The authors found higher representation of eukaryotic microorganisms in the aura-biome above the zoanthids. Across all of the aura-biomes sampled, bacteria comprised the majority of the metagenomes.
The researchers also found distinct groupings of microbial metabolisms in the aura-biomes. Respiration pathways were important in the fleshy algae aura-biome, and these pathways suggested anoxic growth, such as methanogenesis by anaerobic bacteria. Stress response, respiration, and membrane transport mechanisms dominated the zoanthid aura-biome. The zoanthid aura-biomes had high counts of potential coral pathogens and bacteria that are associated with coral yellow band disease. Zoanthids also contain a potent toxin, palytoxin, which may create a stressful environment that favors toxin-producing bacteria in the aura-biome.
Virulence, disease, and defense were overrepresented in the turf algae aura-biome. Turf algae often contains high abundances of cyanobacteria, the “blue-green” algae, and can release high amounts of DOC into overlying aura-biomes. Interestingly, in the turf algae aura-biome, the heterotrophic bacteria Vibrio and Flavobacterium comprised a significant portion of the microbial community. These bacteria were most likely feeding on the high levels of DOC. Vibrio are well-known pathogens associated with declines in coral health, coral bleaching, and diseases. Flavobacterium includes bacterial species that can cause disease in trout.
Because a single reef can contain a mosaic of cover types, the aura-biomes of these coral types may be detrimental to the health of adjacent organisms.
Bringing it all together
Just as climate change is exerting detrimental impacts on coral reefs, reefs are also influencing the microbial community in the water column surrounding the reefs. As this research illustrated, each aura-biome possesses distinct microbial assemblages and functions, which may interact with neighboring corals. The main driver in these changes may be the levels of DOC that are shed into the coral aura-biomes, which can fuel bacterial species that are detrimental to coral health. It will be interesting to see if future studies on other coral reef ecosystems around the world find similar trends in aura-biome composition. And, sampling aura-biomes using high-throughput metagenomics could be a useful tool to monitor and predict the health of coral ecosystems.
Kate received her Ph.D. in Aquatic Ecology from the University of Notre Dame and she holds a Masters in Environmental Science & Biology from SUNY Brockport. She currently teaches at a small college in Indiana and is starting out her neophyte research career in aquatic community monitoring. Outside of lab and fieldwork, she enjoys running and kickboxing.