By Josh Burns, Bobby Murphy, Karissa Sciacca and Ben Walker
Where Microbes Are Found?
Microbes live in virtually every environment on the planet. From terrestrial to marine habitats and extreme to ideal conditions, microbes are most everywhere. Microbial communities are present in ambient seawater as well as many marine organisms, including corals and their predators. These microbes are usually harmless and sometimes even beneficial, but some can cause disease in corals . Coral mucus constantly traps microbes of all varieties from seawater and develops a community of bacteria and other microbes living at a much higher concentration than that of ambient seawater. Many of the problematic microbes that are introduced to the marine environment come from sources like human wastes or dust storms blowing in from dry coastal areas .
Human waste associated bacteria as well as more common marine microbes, like those of the genus Vibrio can be accidentally captured and infect a coral. In addition to random mucus capture, bacteria of this genus is often transferred by vector organisms like corallivorous fireworms. These worms envelop and digest coral tissues allowing the population of these microbes in their gut to transfer to the coral mucus .
The collection of microbes residing in the body of the coral is known as the holobiont. Rosenberg (2007) proposes that in lieu of an adaptive immune system corals take advantage of beneficial microbial symbionts to complement their “kill on sight” innate immune system. They act as a preliminary defense and even help to develop lasting immunities against invading pathogens.
Identifying Coral Microbes
Microbes can be identified through culture-dependent and culture-independent techniques. Culture-dependent techniques involve actually growing the microbes. Often a mucus sample will be spread on an agar plate and incubated over a period of a few days. The resulting lawn or colonies of bacteria present are a sample of the coral microbes. One problem associated with this technique is the Plate Paradox. This refers to the fact that not all coral microbes can be cultured on a plate . Even if a mucus sample contains all the microbes present in the coral, there is always the possibility that not all of them will successfully grow on an agar plate. This flaw produces an uneven representation of the coral’s microbe community.
Culture-independent techniques involve genetic analyses to separate microbe identities. Usually PCR amplification is used to observe specific DNA sequences to separate the identities of microbes. Libraries of DNA sequences, like GenBank, have been created, cataloging the various microbes present in corals . However, there are still many unknown microbes and the library is constantly expanding.
Beneficial and Harmful Microbes
Various microbes can live symbiotically in the mucus layer on just one coral. Scientists have suggested there are several different roles of these microbes including mutualistic and parasitic relationships. Some microbes, such as those in Porites astreoides, have been shown to play a role in nutrient cycling within coral colonies creating an important nitrogen source for the coral . Additionally, some microbes have also been shown to be a beneficial trophic resource to certain corals . In contrast, several microbes have been associated with diseases in corals. Specifically, Vibrio sp. bacteria has been highly associated with coral diseases such as White Band disease [2,5]. These microbes are thought to invade the coral and take advantage of it, depleting it of nutrients, and triggering the cells to release vital symbiotic Symbiodinium. Some microbes can also be categorized as opportunistic microbes in that they will only attack a coral colony once it is already weakened and therefore would not be able to reject the microbe.
Microbes and Coral Immunity
Currently, scientists are interested in investigating the possible role of microbes in a coral immune system. Corals are known to have an innate immune system and can rid themselves of harmful microbes successfully in various ways . However, corals do not have an adaptive immune system meaning that if a coral colony is not innately immune to a microbe already, it will not be able to reject the harmful microbe. It is thought that some of the microbes living mutualistically within the coral also offer another role of behaving like an adaptive immune system. Culture dependent lab techniques are being used to explore whether the beneficial microbes present in corals can develop a resistance to harmful microbes, such as Vibrio sp.. This idea that beneficial microbes within the coral can become or are resistant to harmful microbes acting as another immune system is called the Probiotic Hypothesis .
Coral diseases have been observed spreading through various microbes. Due to a global rise in marine disease epidemics, research efforts have increased in order to identify and understand those microbes responsible for various diseases. Many coral diseases have been described, but only a handful of pathogens have been verified. These pathogens can consist of any virus, bacteria, fungus, or other parasite.
In order to establish a causal relationship between a microbial pathogen and a disease, Koch’s postulates must be satisfied. In order to satisfy Koch’s postulates, regarding coral diseases, the following must be successfully completed: initial culturing of responsible pathogens, transmission of these pathogens to another individual of the same coral species, culturing the same pathogens on the newly diseased coral, transmission to a third coral colony, and then culturing pathogens from the third diseased coral .
Possible pathogens have been difficult to verify for many reasons. Koch’s postulates demand repeated transmission and verified plating. Infecting other corals has been hard to accomplish in the field since there are several possible means of transmission, but not many can be controlled by humans. Also, during these extensive infections and culturing, individual strains can mutate and prevent pathogen confirmation. These diseases can also be the result of multiple pathogens, which complicates identifying which are responsible. Especially since some microbes help protect the coral host from pathogens. So these are often abundant during attempted infection by the pathogen. Furthermore, successful plating is not possible with every microbe, explained by the Plate Paradox.
Despite identification difficulties, several coral pathogens have been identified according to Koch’s postulates. Some of the identified pathogens are responsible for the following diseases: Black-Band, White-Band, Yellow-Blotch, White-Plague, Red-Band, and Dark-Spot.
White Band Disease is one of the most common diseases infecting mostly Acroporoid corals throughout the Caribbean. It has decimated populations of Acropora sp. thus allowing for the growth of algae  on dead coral tissue. The coral tissue begins to decay at the base of the coral and spreads at a rate of approximately 5mm per day . Due to the difficulties of fulfilling Koch’s Postulate, little progress has been made on identifying the cause of White-band Disease. Studies suggest that a Vibrio bacteria may be a possible pathogen , however this has not been verified.
Similar to White-band Disease, White-Plague has affected Acroporoid coral but also many others, such as Mycetophyllia sp, Colpophyllia natans, and many more . The disease also appears to begin at the base of coral mounds and is capable of spreading at rates of 2cm per day. Suggested pathogens for White Plague include Aurantimonas coralicida  and Sphingomonas sp. .
Affecting mainly corals of the genus Montastrea in the Caribbean, Yellow-blotch disease is distinguished by its characteristic yellow patches on the coral tissue. This disease infected a large majority of coral reefs in the San Blas Islands in Panama during the mid 1990’s  while a similar in appearance yet distinct disease, Yellow-band disease, has infected many Acropora sp. and Porites sp. throughout the Arabian Gulf .
Other types of coral diseases in the Caribbean include Red-Band Disease and Black-Band Disease. Both of these diseases infect a wide range of coral, from Montastrea sp., Acropora sp., Porites sp., and many more . They appear as a red or black band, respectively, slowly killing the coral head.
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