by Forest Rohwer, head researcher on the Microbe Team
Most of the life on a coral reef is microbial. When we look at corals through a microscope, we see that each coral has about 10 million Bacteria on each square centimeter of its mucus-covered surface. There are another million Bacteria in each milliliter of seawater. The number of viruses here is even more impressive—about ten times as many as Bacteria, or at least 100 million per square centimeter and ten million per milliliter. The microbes are everywhere on the reef. The sand, the algae, the fish, and even the sharks are all heavily colonized by microbes.
So what do these numbers add up to for a place like Kingman Reef? The approximate area of Kingman Reef is 80 square kilometers (800,000,000,000 square centimeters). Doing the math, this calculates out to 8 × 1018 Bacteria on the reef surface. If we assume an average water depth of 10 meters, there are an additional 8 × 1020 Bacteria in the water at Kingman. While these number are unfathomably large, they are still a gross underestimate because we assumed that the reef is a flat plane, which it most definitely is not. The mounds, branches, fronds, crevices, and cracks make the total surface area much greater, and all of that surface area is covered in microbes.
And those microbes are a diverse lot. Each species of coral harbors close to 100 unique bacterial species. Since there are approximately 200 coral species on Kingman, there are about 20,000 different kinds of Bacteria just on the corals. Katie Barott, one of the grad students on the Microbe Team, has recently shown that there are probably ten times as many bacterial species on the algae as on the corals, which calculates out to ~200,000 different algae-associated Bacteria. We can also safely assume that every other macro-organism on the reef will be coated with microbes, both inside and out. Added to this, there are at least 5000 unique bacterial species in the reef water.
As astronomical as the bacterial diversity is, the viral diversity is even greater, about ten-fold greater. Most of the viruses prey on Bacteria, with each type of virus being specific for a particular type of Bacteria and at least ten different viruses that prey on each species of Bacteria. Coral reefs are famous for their rich biodiversity—pictured most often as colorful fish, corals, anemones, and other macro-organisms—but actually most of the diversity on a reef is in the microscopic microbes and viruses.
These microbes and viruses are essential components of a healthy reef. What sorts of things are they doing? Here is just a small sample of their important roles.
- Remineralization. Some Bacteria eat the detritus (mucus, feces, dead bodies) produced by the other organisms on the reef. The Bacteria that do this are called heterotrophs, a term that simply means they rely on other organisms to provide the complex organic carbon compounds they need. This feeding is a very important step in the recycling of nutrients such as phosphate and nitrogen in older material for reuse in making new biomass. Without this microbial remineralization, the various nutrients on the reefs would be depleted and nothing new could be made. The viruses are an essential part of this process because they kill the nutrient-rich Bacteria. This releases the nutrients in a form that can then be used by algae, corals, and other organisms in the food web.
- Primary Production. A group of photosynthetic Bacteria, the cyanobacteria, are major producers of new energy-rich sugars on the coral reef. These Bacteria are termed autotrophs since they can make their own organic carbon compounds, such as sugars, from CO2 and water. These autotrophs are found in the water column as well as on the corals and algae. Many fish (e.g., the bristle mouth surgeonfish) slurp up the nutritious slime layers produced by the cyanobacteria. On the first Line Island cruise in 2005, we found that the reefs with less human disturbance, such as Kingman, had more photosynthetic Bacteria and fewer heterotrophic ones.
Both heterotrophic and autotrophic microbes are food for some macro-organisms. During a 2007 expedition to Kingman, Linda Wegley found surprisingly few microbes in the lagoon water. Were the vast clam gardens in the lagoon responsible for this? Clams are filter feeders, and it appeared that they might be filtering out both the Bacteria and viruses from the lagoon water. When visiting the Southern Line Islands in 2009 we were able to directly show this in experiments using clams in aquaria on Millennium Island, and we will be continuing these studies by investigating clams in the Northern Line Islands on this cruise.
- Beneficial symbioses. Usable nitrogen is a precious resource on coral reefs that is needed by all the organisms here to make proteins and other compounds. Some Bacteria can convert the abundant atmospheric nitrogen gas into organic nitrogen compounds (e.g., protein). These beneficial Bacteria, called nitrogen fixers, are found associated with corals and algae, and also in the sediment. They are an important source of nitrogen for the reef community. Some of the Bacteria that live on coral surfaces provide another useful service: they produce antibiotics that may protect the coral against invading pathogenic Bacteria.
- Disease. Some Bacteria and viruses cause diseases. Even on a near-pristine reef such as Kingman, there is a low percentage of organisms, such as corals, that show signs of disease (usually about 5%). In contrast, on reefs with high levels of human disturbance such as those as Christmas Island, we observed that more than >20% were diseased.
So please, when you look at a coral reef, remember what you are not seeing—a bewildering number of unique microbes that are determining the health of the community. This is the invisible microbial landscape of the reef.