Tropical coral reef ecosystems are oligotrophic (i.e., nutrient-poor) environments with nitrogenous macronutrients such as ammonia in very short supply. This explains—at least in part—the relative scarcity of algae there. However, it makes the characteristically high prolificity of zooxanthellate corals seem a bit uncanny. In point of fact, reef waters are so infertile that they remain crystal-clear year-round; yet, reefs are among the most productive biomes on the planet. This paradox once puzzled generations of marine biologists and oceanographers. How can these corals grow so quickly, powered almost entirely by a symbiotic dinoflagellate (an alga), where a biologically available source of nitrogen is so deficient?
The relationship between corals and zooxanthellae is famously efficient. The duo very tightly recycles nitrogen back and forth. Specifically, the dinoflagellate is specially equipped to recover and reuse nitrogenous waste products expelled by its host. Even so, the contribution of extraneous nutrition obtained by the coral (through planktivory and/or absorbing free amino acids from the water column) is impossibly small. In other words, considering a coral’s paltry food intake, the efficient nutrient recycling with their symbiont isn’t enough to fully explain their impressive growth much less their very survival. Not in such infertile waters!
To be clear, zooxanthellae do not (nor do any plants/algae) require nitrogen to synthesize sugars and other carbohydrates. They harvest energy from sunlight, and indeed use that energy to fix carbon—that is, convert carbon dioxide into a stored source of energy (e.g., sugars) as well as tissue. But they still need inorganic 'fertilizer' in the form of ammonia in order to grow and reproduce. This is because all tissues (plant, animal, whatever) are made of protein, which is made of amino acids, which are derived from amines (ammonia with one or more hydrogen atoms replaced by organic groups). Zooxanthellae cannot (nor can any plant/algae) fix nitrogen—that is, they cannot convert nitrogen gas into ammonia. Yet, zooxanthellae require 'fixed' inorganic nitrogen in the form of ammonia (which they very strongly prefer over nitrate) for survival and growth.
Diazotrophy (i.e., nitrogen fixation) is a biological process that introduces new, biologically available nitrogen into the environment. This process is vital in many marine ecosystems (especially coral reef ecosystems) where nitrogen chronically limits primary production. Diazotrophy is carried out exclusively by a relatively small, though diverse, assemblage of bacteria and archaea.
Unsurprisingly, nitrogen-fixers have a distinct ecological advantage in nitrogen-poor habitats such as coral reefs. But then, why not everywhere? Nitrogen is, by far, the most abundant component of the Earth’s atmosphere, right? One big problem for most diazotrophs is that nitrogen fixation can only occur in anaerobic environments.
While diazotrophic purple non-sulfur bacteria such as Rhodopseudomonas palustris can survive in aerobic environments, they prefer (especially in oligotrophic waters) anaerobic conditions. On coral reefs, R. palustris finds excellent anaerobic habitat in two places: Within deeper, oxygen-depleted microzones of the sediments and also within the bodies of the corals themselves. It is the latter case that is, by far, of most interest to reef aquarists.
Corals take up a whole lot of bacteria (of all sorts, including diazotrophs) from the water as a food source. They are indeed capable of selectively feeding and appear to strongly prefer diazotrophs. They even prefer certain types of diazotroph; it indeed seems that corals really go after rhizobial bacteria such as R. palustris. These are the same sort of microbe (as their shared name implies) as those that symbiotically colonize root nodules of legume plants. Their association with corals is surprisingly similar.
A significant portion of the bacterial cells that corals ingest are used (much like zooxanthellae) as endosymbionts. However, rather than for fixing carbon, the primary purpose of the diazotrophs is of course to fix nitrogen which is supplied to the zooxanthellae. Both the coral and zooxanthellae, in turn, exude lots of organic compounds (i.e., fixed carbon) for these heterotrophic bacteria to eat. The relationship is so tight and so advantageous for each member that many biologists now refer to the trio as the “coral holobiont.”
These bacteria may live anywhere in the coral, including its gut, but seem to have a special affinity for its mucus-coated epidermis. Coral mucus is shown to contain about four hundred times more diazotroph cells than the surrounding seawater.
Starting to see how important these microbes are to zooxanthellate corals?
Marine biologists knew little of this complex symbiotic association until the 1970s. And most subsequent research has only been conducted over the last ten years! So it’s not all that surprising that most reef aquarists are only now beginning to catch on.
If anything, the least of reef aquarists’ worries is supplying nutrients. In the typical reef tank (a closed system with way too many fish that get fed way too much), nitrate accumulation is the norm. Fortunately, thanks to new techniques and new technologies, we’ve seen improvements in water quality management. And the corals we see today (compared to those from just a few years ago) really show it. As we get closer and closer to mimicking the oligotrophic conditions of natural coral reef ecosystems (perfecting our so-called low-nutrient systems), we just may find that our corals actually need a wee boost of nitrogen fertilizer.
Here’s where we can finally have our cake and eat it. By providing a regular influx of diazotrophic bacteria (rhizobial phototrophs in particular), we can simultaneously provide a supply of fixed nitrogen for the coral and its zooxanthellae without suffering the consequences of unnaturally high ammonia/nitrate concentrations in the water (nuisance algae growth, among other things). As if this benefit isn’t enough, these bacteria also serve to control dissolved organics, provide color-enhancing carotenoids and behave as highly effective probiotics.
Don't worry that these bacteria will generate too much ammonia. Nitrogen fixation is an energetically expensive process. Thus, they only rely on it in times of scarcity. And, when they do so, whatever nitrogen they fix is mostly delivered directly to the zooxanthellae. When fixed nitrogen such as ammonia, nitrite or nitrate is readily available in their environment, they simply use that instead. You heard right: They actually remove excess nitrogenous compounds from the water column. As such, they essentially balance the nitrogen cycle for the entire system.
With a new focus on the coral-zooxanthellae-diazotroph relationship, on the coral holobiont, we can build better reef aquaria with less effort and expense. It seems pretty safe to say that purple non-sulfur bacteria in general, and R. palustris in particular, have a pretty bright future in the reef aquarium industry!
