Coral reefs are vibrant underwater cities, teeming with life and supporting an immense array of marine biodiversity. These complex ecosystems, often called the “rainforests of the sea,” are built by stony corals, which are animals that form the structural foundation of the reef. Beyond the visible creatures, a vast and often unseen world of bacteria plays an integral role in the health and function of these remarkable environments.
The Microbial World of Coral Reefs
Coral reefs host an astonishing diversity of bacteria, forming intricate communities both within the corals themselves and in the surrounding marine environment. Corals, similar to humans, possess a “microbiome” — a collection of microorganisms that reside in and on the host. These bacterial communities colonize various microhabitats within the coral, including the surface mucus layer, different tissue layers, and even the internal skeleton made of calcium carbonate.
The bacterial communities associated with corals are complex and vary significantly depending on the coral species, geographic location, and environmental conditions. Researchers have identified that most cultivable bacteria belong to phyla such as Proteobacteria, Firmicutes, Bacteroidetes, and Actinobacteria. This diversity also extends to the overlying seawater and sediments, where bacteria contribute to broader reef ecosystem processes.
How Bacteria Support Coral Health
Bacteria perform many beneficial functions that support both individual coral health and the overall reef ecosystem. They are involved in nutrient cycling, which is particularly important in the nutrient-poor waters where coral reefs thrive. For example, some bacteria can fix atmospheric nitrogen, converting it into forms that corals and their symbiotic algae can utilize for growth.
Certain bacterial associates also provide corals with essential compounds, including vitamins and amino acids, which the coral host may not be able to produce on its own. These microbial partners can also assist in detoxification processes, helping corals neutralize harmful substances they encounter in their environment. Bacteria also contribute to the coral’s immune defense mechanisms, by producing secondary metabolites with antibacterial or antiviral properties that help protect the coral from pathogens.
When Bacteria Threaten Coral Reefs
While many bacteria are beneficial, certain species can become detrimental, leading to coral diseases and degradation. Some bacteria are opportunistic pathogens, meaning they can cause disease when coral hosts are weakened or environmental conditions become unfavorable. Specific bacterial species are associated with well-known coral diseases, such as white band disease and black band disease, which can cause significant coral tissue loss.
In some cases, bacteria can play a secondary role in coral bleaching events. While bleaching is primarily triggered by stressors like elevated ocean temperatures causing corals to expel their symbiotic algae, opportunistic bacteria can proliferate on the compromised coral. This proliferation can exacerbate the stress and hinder the coral’s recovery. The shift from a beneficial or neutral bacterial community to a harmful one occurs when the delicate balance of the coral microbiome is disrupted.
Environmental Stressors and Coral Microbiomes
External environmental factors can significantly disrupt the delicate balance of the coral microbiome, leading to negative health outcomes for corals. Rising ocean temperatures are a primary stressor that can alter the composition and function of coral-associated bacterial communities. Even temperatures below the threshold for full coral bleaching can lead to changes in the microbiome, potentially making corals more susceptible to disease.
Ocean acidification, caused by increased absorption of carbon dioxide by seawater, also poses a threat by altering the chemical environment that corals and their microbes inhabit. Pollution from land-based activities, such as nutrient runoff and sedimentation, can contribute to microbial imbalances, or “dysbiosis.” This disruption in the bacterial community can reduce a coral’s resilience and predispose it to disease.