While I have spoken much about the bacterial component of the holobiont, there are many more players in a coral’s arsenal of symbionts. Something I have seen very few discuss in the hobby is marine fungi. While fungal populations exist in marine environments external to coral or even reefs, many unique coral-associated fungi also reside in the coral skeleton’s endolithic zone. Typically, these symbionts are partially associated with endolithic algae populations, where they attempt a process of “lichenization” (forming a lichen assemblage). Both the algae and the coral thwart this process. In response to rapid fungal growth, the coral produces “pearls” of calcium carbonate to contain regions of excess fungal growth – this effectively safeguards the coral’s soft tissue layer and partitions the fungal populations, isolating them to the skeletal system.
Many coral-associated fungi exist in a constant state of boom and bust, where they attempt to grow but are stopped by the coral’s immune responses. These fungal populations can quickly overtake the coral if the coral were to stop their trek into the tissue. Often, when localized immune responses persist for too long, the build-up of reactive oxygen species in the soft tissue of the coral will cause selective spots on the coral to bleach rather than the whole organism. This leads to excess light reaching the coral’s skeleton – a region typically devoid of light. This new light source causes an exponential increase in endolithic algae populations. Due to fungi’s close association with these algae, they also grow. When a coral is already dealing with localized bleaching and immune challenges, it typically is strained for resources, leading to a downregulation in the production of the protective skeletal pearls. Downregulation of skeletal isolation processes combined with the fast growth rate of the fungal populations typically causes an event where the fungi finally reach the coral’s soft tissue and begin to invade. Fungal hyphae colonize the coral, ripping their way through soft tissue layers and siphoning core nutrients from the primary system. If there is not already a dysbiotic event occurring at this point, this typically acts as the element of biotic stress needed to “turn the key” and start the typical dysbiotic cascade. This then presents the coral with a multi-faceted poly microbial immune challenge where it has to deal with its typical bacterial symbionts switching to a virulent state due to differential expression, external bacterial pathogens chemo locating to the coral due to a build-up of infochemicals, and an opportunistic fungal pathogen erupting from the skeleton and taking out the coral from the bottom up. This diversity of immune challenges quickly makes short work of the coral and manifests as a rapid form of tissue necrosis. Many disease states in the wild typically have a pathogenic fungal component in combination with widely known bacterial issues. On top of this, extracellular fungal pathogens may exist that drift through the water column in unicellular form. It has been shown that some unicellular marine fungi can chemotactically move towards sources of amino acids and carbohydrates, which mirrors some of the chemotactic behavior seen in external bacterial pathogens.
The only currently described primary fungal pathogen that can affect corals Aspergillus sydowii – it is known to infect some species of sponges and gorgonians https://www.nature.com/articles/ismej200918
The Good
While that may have painted a terrifying picture, fear not, as fungi can also be friends. When their growth is efficiently regulated, natural consequences of their development and metabolic function can significantly benefit the entire holobiont. For instance, since fungi are eukaryotic, they have much more complex machinery and a much larger genome than the common bacterial symbionts. This allows them to have a wide array of enzymes and some forms of rudimentary digestion. These enzymes enable coral-associated fungi to break down relatively complex environmental molecules that bacteria lack the tools to “eat.” In turn, the waste products from breaking down these primary molecules yield simpler secondary molecules, acting as carbon sources for the rest of the holobiont. Another benefit of endolithic fungi is their role in sulfur cycling. Many species have been shown to actively metabolize the info chemical DMSP, which decreases the chance a pathogenic event could occur due to its build-up. In turn, the breakdown of DMSP into molecules like DMS contributes to the global sulfur cycle. DMS and other gaseous byproducts can bubble up from the coral and enter the atmosphere, where they play an integral role in cloud formation, forming the crux of most weather systems.
Fungi also play an important role in cycling nitrogen, carbon, and phosphorus. There’s mounting evidence that endolithic fungi populations can help sustain corals during times of stress, such as bleaching events, by providing them with excess nutrients localized to the endolithic space. It is now believed that the high levels of inorganic nitrogen and phosphorus found in pockets in the coral skeleton are due to endolithic fungi. An additional function of coral-associated fungi is the product of mycosporine-like amino acids. These compounds act as sunblock and help protect the coral from excess UV light, shielding them from DNA damage and oxidative stress.
The endolithic community of a coral https://microbiomejournal.biomedcentral.com/articles/10.1186/s40168-019-0762-y/figures/1
The Future
Due to the limited information available about coral-associated fungi, the correct approach would be to wait before taking corrective action. After working for several years with diseased coral, I have only encountered one example where a fungus was a primary pathogen (this was obvious and not to be confused with other disease states commonly observed in the industry.) Thus, based on what we know, it would not be wise to prophylactically add fluconazole to your system, as many of these fungal populations are likely very important to coral health. My goal with this article is not to scare people or make them think they have pathogenic fungi waiting to strike in their systems they should seek to eliminate; instead, I aim to highlight the potential benefits of maintaining a healthy endolithic fungal population. As science progresses, we will learn more about fungi’s critical role in the holobiont, which will likely lead to future strategies to up or downregulate these organisms.
Main image: A coral-associated fungus I isolated
