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Deep-sea corals provide many of the same habitat functions as shallow-water, tropical coral reefs like providing essential habitat for numerous marine species. Since they inhabit great depths (sometimes exceeding 1,000 meters underwater), it is much more difficult to sample and study these corals—including their microbiomes. These symbiotic microbial communities that thrive within coral tissues are critical for how these species survive in their environment.

Understanding the diversity, function, and differences of these microbial communities is vital for conserving these essential habitats—in both shallow and deep water. Most studies of coral microbiomes, at any depth, look at the taxonomy of the bacteria that live in their tissues. This new study is unique because it examines representatives of the whole microbiome (including bacteria, archaea, fungi, protists) as well as functional genes, which determine what they are doing and how they affect the coral.

One difference the team found between cold-water and tropical corals was in how these animals got their nutrition. Deep-water coral microbiomes had a higher capacity to assist with heterotrophy (eating other organisms like plankton) compared to shallow, tropical coral microbiomes that rely more on autotrophy (photosynthesis) for nutrition. Other differences included tropical coral microbiomes having higher amounts of antibiotic resistance genes—nearly twice as many compared to deep-water corals. This could be due to shallower corals having higher exposure to pollutants that often contain pharmaceutical remnants, including antibiotics.

Although there were several differences, the team identified a surprising amount of functional similarity between deep-sea and shallow-water coral microbiomes. This study provided the first comparison of microbial functional genes across species of cold-water corals from the deep ocean, including both octocoral and scleractinian representatives.

The article, “Functional gene composition and metabolic potential of deep-sea coral-associated microbial communities” was published in the journal Coral Reefs on August 9, 2023, by Dr. Christina Kellogg and colleagues from Montana State University.