Coral Reef Nutrition

Eric Borneman contributed an exceptional article on Reef Nutrition and how this correlates to our marine aquariums. The full article can be found in ReefCulture Magazine Edition #4, but we have an excerpt here from Eric’s work…

“A coral reef supports a tremendous variety of life, all of which are dependent on energy sources for their survival, growth and reproduction.”

The diversity on coral reef systems

There are two basic types of organisms in terms of their method of gaining energy: heterotrophs and autotrophs. Most autotrophs are photoautotrophs, also called primary producers; they use sunlight, converting its energy through photosynthesis into energy rich products (reduced forms of carbon, usually in the form of simple sugars) that are used by the organism. In this way, they form the beginning of the food chain, as they are the original or primary source of dietary energy for all other organisms. Photosynthetic bacteria or cyanobacteria, may also be considered to be primary producers, and their biomass on and near coral reefs, including in the water column, is enormous. There are also chemoautotrophs; bacteria that oxidize inorganic compounds such as hydrogen sulfide, ammonium or ferrous iron as an energy source are chemoautotrophs. Heterotrophs are those organisms that must attain at least some nutrition from feeding or absorption of organic materials to acquire a reduced source of carbon. Even primary producers need more than sunlight to survive, and this is part of a great misconception that autotrophs can exist with only sunlight. Consider the houseplant that dies without nutrients from soil or fertilizer; it obviously needs additional nutrients besides light and water. Consider, as well, that fertilizers and soils are commonly described by their nitrogen and phosphorus content; these are also among the most important nutrients required by heterotrophic organisms. The main difference between autotrophs and heterotrophs is not that one ‘eats” and the other “just needs sun,’ but that one can provide various amounts of required carbon by using light energy. All animals are heterotrophs, including the corals.

The nutrients available in water to coral reefs can be dissolved in the water, in the form of particulate material, or as living biomass. The word ‘nutrient’ is often misunderstood. The terms ‘high nutrient’ and ‘low nutrient” can be taken in many contexts. In general, nutrients are those organic and inorganic compounds necessary to sustain life. While this comprises a very large group of potential compounds, nutrients are often simplified in terms of those elements that are major ‘building blocks” for lipids, amino acids, and carbohydrates. Furthermore, they are frequently those elements that tend to limit further growth of an organism by their availability and ability to be procured. In general, carbon, nitrogen and phosphorus are often used to describe the ‘nutrient’ condition of coral reefs and reef organisms (and others, as well). Plants and animals with photosynthetic symbionts, such as corals, tend to be nitrogen and/or phosphorous limited under normal conditions, since photosynthesis usually provides non-limiting carbon source material. Coral reef waters are typically ‘nutrient poor” as they usually contain very low levels of nitrogen and phosphorus (they are both precious commodities and any excess is usually taken up quickly). In nearshore areas where there is significant organic loading from land runoff, waters tend to be rather “nutrient rich,” or higher in nitrogen and phosphorus. Both types of environments sustain their own flora and fauna with varying amounts of habitat overlap in terms of the organisms that can exploit the continuum of nutrient conditions.

The coral reef is a place of both high primary productivity and consumption of nutrients, with a great deal of nutrients being recycling within the community. For many years, coral reefs were thought to be “nutrient poor deserts.’ In fact, this is not the case. It would be a very poor assumption to imagine any species-rich community that was not highly dependent on nutrients. While measurement of the water column shows it to be relatively devoid of organic and inorganic dissolved nitrogen, carbon and phosphorous, and therefore ‘nutrient poor,” it is largely because of the efficiency of the reef community that such water conditions are attained. Waters around coral reefs are rich in nutrients in the form of various types of plankton; these are largely removed by coral reef organisms. It should be noted that most of the plankton on coral reefs is produced by and lives within the reef or nearby communities, and is not borne into it in great quantities by the open ocean. There are coral reef areas where nutrients are brought upward from the deep ocean, called upwelling, but they are not ubiquitous to coral reefs. There are also seasonal plankton blooms when oceanic plankton levels may be transiently high, but the open ocean is also relatively devoid of nutrients.”

For the full article by Eric Borneman, check out ReefCulture Magazine Edition #4