Coral: A Microbial Metropolis

Written by Gus Fordyce
A coral is a city; microbes its inhabitants. The industrious citizens of this microscopic world are the engineers behind the success of corals, and agents of their demise.

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Fig. 1 Towering structures of the Great Barrier Reef made possible by coral microbes. Photo by Alexander Fordyce

Relationships between coral microbial populations are diverse and, in many cases, poorly understood. A quick census would reveal rich communities comprising fungi, algae, protists, viruses, bacteria and archaea. Together, they play an important part in defining corals’ health, growth and even the success of their offspring. This microbial metropolis is a dynamic, competitive and ever-changing world, where friend can quickly turn into foe and previously rare specialists can bloom into abundance.

Coral’s most popular microbe

Most of us are familiar with the ever popular, microscopic algae that are corals’ primary symbiont, zooxanthellae. The power stations of this microcosm, these algae are the corals’ main source of energy, and have been central in allowing reefs to thrive in warm tropical seas. They are diverse in their own right, with different types providing their own benefits to the coral organism. Research into these microbes has been intense and extensive as they are at the centre of the coral bleaching phenomenon. But in considering the coral’s microbial metropolis, they are only one piece of the puzzle.

Bacteria are literally everywhere, including corals

Bacteria and archaea, similar yet unrelated, are abundant in the various tissues of corals. Some play important roles in cycling nitrogen, sulphur and phosphorus, essential nutrients for coral growth and reproduction. If corals were to rely upon acquiring these nutrients by themselves, they would be severely limited in size and so too would the structures they build.

Others collectively comprise what may be termed a bacterial immune system. Within this system, there are those who prey upon harmful bacteria and others who produce antibacterial compounds. Simply occupying available space is enough to prevent the invasion of harmful bacteria, such as Vibrio spp. But if they do manage to infiltrate the coral’s microbial defence system, they can cause a range of diseases that at best leave a scar; at worst can wipe out swathes of reef. Our understanding of this bacterial immunity is still in its infancy, but already we are beginning to reveal the importance of these microbes in maintaining the health of the coral reef ecosystem

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Fig. 2 Resource: 2017. DOI: 10.3389/fmicb.2017.00341.

Endolithic Microbes

Lurking in the shaded pore space of the coral’s skeleton is a group of microbes known as endoliths. They are ubiquitous, mysterious and have a direct influence on the physical structure of reefs. As with other coral tissues, bacteria and archaea live abundant and happy in the skeleton, fulfilling much the same roles they do elsewhere within the animal. Particular to this microhabitat however are fungal and algal inhabitants specialised to this way of life.

Traditionally, fungal associates of corals have been viewed as pathogens that take advantage of weakened coral to infect and spread at the cost of the unfortunate host. Yet only the Aspergillus fungus has been convincingly vindicated as an agent of widespread degradation. We are now starting to understand a more complex relationship between fungi and coral. Many produce sunscreen-like compounds that may absorb UV light and protect their host from damage. Others produce compounds such as fatty acids that could benefit coral resilience to environmental stress.

They even affect the structure and strength of the coral’s skeleton. As they vie for space and erode skeletons before them, the coral responds by covering the holes it makes with especially dense calcium carbonate. This creates variation in strength throughout the coral skeleton which, through the process of erosion, can shape its very structure.

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Fig. 3 A thick green band of endolithic algae within the skeleton of Montipora digitata. Photo by Alexander Fordyce

In constant competition with these fungi are a group of charismatic algae called Ostreobium, who have been often referred to as corals’ secondary symbiont. These filamentous green algae form thick green bands within the skeleton, subsisting on the remnants of light outside zooxanthellae’s preferred spectrum They are specialised to this shaded world yet can rapidly adapt to high light conditions in ways we do not yet understand. Behaving as another symbiont, they pass carbon to their coral host which, in times of stress, may be critical in ensuring survival when the zooxanthellae have been expelled. Within days of settling, these algae have already formed a bond with their host, hinting at a long process of potential co-evolution between them, zooxanthellae and corals. As some of the most important microborers on the reef, they dissolve the skeleton from within in keeping up with the ever growing colony. They can rapidly collapse the internal structure in this way during severe marine heatwaves, potentially causing widespread loss of a reef’s physical structure. Research is quickly revealing them to be some of the most influential microbes in a coral’s arsenal and within the reef environment.

Research into coral microbes is advancing our understanding of coral life histories and how they are responding to a changing world. The advent of newer, more powerful technologies and the opportunities they afford is allowing us to see the world of coral reefs in a new light, where each coral is a city.

Further reading…

Ainsworth TD, Fordyce AJ, Camp EF. 2017. The Other Microeukaryotes of the Coral Reef Microbiome.Trends in Microbiology, 2017 Volume 25: 980 – 991

David G. Bourne, Kathleen M. Morrow, Nicole S. Webster. Insights into the Coral Microbiome: Underpinning the Health and Resilience of Reef Ecosystems.

Annual Review of Microbiology 2016 70:1, 317-340

Krediet CJ, Ritchie KB, Paul VJ, Teplitski M. Coral-associated micro-organisms and their roles in promoting coral health and thwarting diseases. Proceedings of the Royal Society B: Biological Sciences. 2013; 280: 20122328. doi:10.1098/rspb.2012.2328.

Marcelino VR, Morrow KM, van Oppen MJH, Bourne DG, Verbruggen H. Diversity and stability of coral endolithic microbial communities at a naturally high pCO2 reef. Mol Ecol. 2017;26:5344–5357. https://doi.org/10.1111/mec.14268

Peixoto RS, Rosado PM, Leite DC de A, Rosado AS, Bourne DG. Beneficial Microorganisms for Corals (BMC): Proposed Mechanisms for Coral Health and Resilience. Frontiers in Microbiology. 2017; 8: 341. doi:10.3389/fmicb.2017.00341.

Wegley L, Edwards R, Rodriguez‐Brito B, Liu H, Rohwer, F. Metagenomic analysis of the microbial community associated with the coral Porites astreoides. Environmental Microbiology, 2007; 9: 2707-2719. doi:10.1111/j.1462-2920.2007.01383.x

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