Diazotrophic Bacteria in the Reef Aquarium (The Most Important Article You'll Ever Read About Coral)

Updated: Oct 28, 2020

Tropical coral reef ecosystems are oligotrophic (i.e. nutrient-poor) environments with nitrogenous macronutrients such as ammonia in very short supply. This explains the relative scarcity of algae there well enough. 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 has 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, convert nitrogen gas into ammonia.