For many of us, the term “coral reef” is synonymous with “coral bleaching.” When temperatures creep too high, corals expel their algal symbionts – commonly known as zooxanthellae – which are crucial to the survival of the coral animal. We’ve all seen evidence of this phenomenon: carpets of exposed coral skeleton, a ghostly boneyard where vibrant ecosystems once flourished. Sometimes recovery is possible but oftentimes bleaching spells out death for a reef. Global warming and coral bleaching are inextricably linked. In fact, the added thermal stress accompanied by the current El Niño weather pattern has led to the third worldwide coral bleaching event.
Without question coral bleaching is one of the leading causes of global coral mortality. However, there is another huge threat facing the world’s reefs, perhaps more insidious and certainly less understood than bleaching: coral disease. The frequency and severity of coral diseases have increased exponentially in the past few decades. However, unlike coral bleaching, coral disease is poorly characterized. This is because the organisms responsible for coral disease are more diverse and their relationship to coral is more complex than the zooxanthellae-coral partnership. These tiny beings are the simplest life forms on the planet but without them existence on earth would not be possible. I’m talking, of course, about bacteria.
Just like humans, corals have a vast and elaborate microbiome. Unfortunately, also like the human microbiome, very little is known about coral-associated bacteria. One established point is that breakdowns in the relationship between a coral and its microbes can lead to coral disease. Often these breakdowns are directly connected to anthropogenic stressors including climate change and deteriorating water quality near coral reefs. In this way, the drivers behind coral disease are similar to those that trigger bleaching. The destructive effects of coral disease are obvious, even if the disease mechanisms remain vague. Infection appears and advances along the surface of a coral colony, destroying tissue and exposing skeleton as it progresses. The process is distinct but the outcome is comparable to bleaching; unprotected coral skeleton cannot fend off opportunistic colonizers and the coral dies. Over 20 coral diseases have been described worldwide, but despite their devastating impacts, key details regarding their origin and ecology remain elusive.
There are two schools of thought when it comes to coral disease. The first is that novel pathogens are being introduced to reef communities and wreaking havoc on unsuspecting corals. The second is that opportunistic pathogens, namely bacteria already associated with corals, are taking advantage of worsening environmental conditions and the resulting increased vulnerability of their host. The latter theory is particularly dangerous because it means that corals already harbor the agents of their own demise. When abiotic factors are favorable, these bacteria are innocuous but when corals become stressed their own microbiota turns into a ticking time bomb, ready to overwhelm the coral’s already vulnerable defenses.
To make matters more confusing, many coral diseases are not associated with a discrete pathogen. Historically speaking, for a bacterium to be considered a pathogen it must be isolated from a diseased organism, grown in pure culture, re-infected onto a second organism, and re-isolated from the second organism upon the appearance of disease symptoms. This process (known as Koch’s postulates) has proven difficult to fulfill for most coral diseases for a number of reasons. First and foremost, recent research suggests that the vast majority of bacteria cannot be grown in the lab. In fact, environmental microbiologists estimate that less than 2% of bacteria can be cultured in the traditional sense. This phenomenon has enormous epidemiological ramifications, not just for corals. Another hitch in the study of coral disease is the growing evidence that many coral infections involve more than one pathogen. The classic example is black band disease, a highly virulent infection caused by a consortium of cyanobacteria, heterotrophic bacteria, and fungi.
Unfortunately, while the origins and organisms involved in coral disease are multifaceted and ambiguous, the spread of coral disease is simple and effective. Due to the literal fluidity of marine ecosystems, waterborne transmission is a common method of disease dissemination. As tissue from infected corals sloughs away, bacteria can easily ride the ocean currents to new hosts. Coral-coral contact is another method of disease transmission since corals generally live in close proximity and are constantly competing for space by growing over each other. Non-coral animal vectors that live and/or feed on coral also have the ability to pass disease agents from one colony to another. Reef fish, snails, and worms have all been shown to harbor a variety of coral ailments.
This is not to say that bacteria are all bad for corals. After all, corals and bacteria have evolved together over millions of years, far longer than humans and our microbial symbionts. Corals are very simple animals and as such they do not possess an adaptive immune system. When faced with the threat of disease, corals’ only defenses are nonspecific and they cannot adapt to recognize pathogens upon subsequent infections. Recent research suggests that corals may have a community of bacterial symbionts that safeguard their host from potential virulent microbes. These “good” bacteria probably produce a variety of chemicals designed to keep “bad” bacteria from colonizing the coral. These bacterial symbionts probably also provide their coral hosts with useful metabolites. Coral reefs live in nutrient-poor conditions (clear water allows sunlight to reach the zooxanthellae) where many crucial nutrients are scare. Bacteria are especially proficient at acquiring minerals – such as nitrogen and phosphorus – from the environment and converting them into usable compounds. It is likely that corals rely on their bacterial symbionts to provide them with the compounds essential to their survival.
Against the steady march of climate change, coral diseases will continue to spread and worsen. However, all is not lost. There is mounting evidence that marine reserves significantly reduce the incidence of disease. Human activities disrupt the millennia-old balance amongst the thousands of organisms within reef ecosystems. Left to their own devices, corals reefs have the ability to repair themselves. If there is anything we can learn from corals, it is that everything is connected, especially the relationship between corals and their rich microbiome.