What's a holobiont? The term was recently coined by a coral researcher to describe his favorite organism. Holo implies something whole but including numerous parts, as in a holistic approach. Biont sounds like something alive. The coral holobiont is the entire community of living organisms that make up a healthy coral head. There is the coral animal itself, an invertebrate with a simple body plan called a polyp. A polyp is a sack, only a few millimeters in diameter. The bottom of the sack is attached to the substrate, and the coral's tentacles extend from the open end. A coral "head" is made up of thousands of these polyps.
Another familiar part of the coral holobiont is the internal farm—the algae that live as symbionts inside the cells of the coral (the zooxanthellae). The sugars that they produce by photosynthesis are an essential part of the diet of the coral. (link to the earlier post?) Coral reefs have made headline news in recent years as climate change has brought warmer sea surface temperatures. An increase of only a degree or two above the normal summer maximum can cause the coral to lose its symbionts. This is referred to as "bleaching" because, without the pigmented algae, the white skeleton shows through the transparent coral polyps. Bleaching is bad news for corals. Without their farm, they don't get enough nourishment to reproduce or to build their skeleton fast enough to stay ahead of erosion.
But there is more to the holobiont. There are bacteria, and lots of them. Some live inside the coral tissue, like the zooxanthellae algae do. These include cyanobacteria that, like the zooxanthellae, carry out photosynthesis. Their total sugar production is much, much less, but it may provide survival food for a bleached coral. More importantly, some of these cyanobacteria can "fix" nitrogen. Usable nitrogen is a precious, limited nutrient on the reef. Most organisms cannot use the abundant nitrogen present in the atmosphere as N2. Cyanobacteria convert N2 into nitrate that can then be used by both the coral and the zooxanthellae.
Many more bacteria live in the mucus that covers the surface of a coral head, and they are a diverse lot—perhaps several thousand different kinds. These are not the same bacteria that one finds in the water column drifting past the coral. A few of them seem to have a very specific association with particular coral species. Just as the normal bacteria on our own body surfaces protect us from infection by pathogenic types, the bacteria in the mucus appear to protect the coral from disease-causing organisms.
Even the coral skeleton itself is home to numerous living organisms. These include endolithic algae, eking out a meager existence in the darkness. (Yes, endolithic means living within rocks, but the term is also applied to organisms living within the shells or skeletons of marine animals.) The zooxanthellae take 98% of the incoming light, and much of what gets past them is absorbed by the skeleton itself. And as if that weren’t bad enough, the endolithic algae are parasitized by fungi that also live in the skeleton. These fungi would feed on the polyps, too, if they could. They bore into the skeleton, trying to penetrate into the chambers that house the polyps. The coral, in response, deposits extra material to thicken the skeleton at that site. The fungi keep boring, the coral keeps adding to its defenses. The result are nodules on the skeleton, reminiscent of pearls made by oysters.
In a healthy coral, all of these partners are in balance. In recent decades, there has been a dramatic increase in coral disease on reefs worldwide. Human activities, including overfishing and anthropogenic climate change, are often blamed. With so many players in the holobiont, you can imagine that it isn’t easy to figure out just what went wrong, or why, let alone what might be done about it.
Merry Youle is a collaborator with Small Things Considered, a microbe blog that showcases an appreciation for the width and depth of microbiology. Besides writing for The Reef Tank, she spends her time reading, writing, researching, freelance editing, and constructing her owner-built house in a lava field on Mauna Loa, Hawaii.