This is the sixth installment of a seven-part series on food sources to corals and coral reefs. This article will address a very important food to corals and many other animals, particulate organic material (POM). This food source has many names, including detritus, floculant organic matter, reef snow, marine snow, suspended organic material (SOM), and more. While there may be some distinctions between the material described by these terms, they all basically refer to the same material.

Not so long ago, marine aquarists made every attempt to be assured that their water column was "polished." I never fully understood the term, but the premise was that a clean water column was a good water column. Various means were employed to accomplish this, including the use of various power filters, mechanical flosses and screens, sterilizers, ozonizers, canister filters, diatom filters, foam fractionators and many other devices. In this article, I will describe the composition and role of POM, and discuss why such "polished" water might not be in the best interest of reef tanks or corals.

What is Particulate Organic Material?

Particulate organic material has its origins in life, being composed by and large of the remains, secretions and excretions of living organisms. On coral reefs, it is composed mostly of dead algae, bacteria, mucus, and feces. To aquarists, detritus is any flocculent organic material originating as part of the excess secretions, productions, breakdown, additions or waste in the aquarium habitat. In aquariums as in the wild, it has its origin in fish feces, coral mucus, algal remnants, worm castings and burrowings, the molts of small crustaceans, uneaten food, and other debris. Another source of POM to reef environments, depending on the location, may be organic inputs derived, and potentially composed largely of terrigenous sediments. Strictly speaking, detritus tends to settle out of the water column, while suspended organic material is light enough, as it is nearly neutrally buoyant to remain afloat in the water column more easily. There is really no other difference, except that detrital material that is in suspension can be used by different organisms from those who could feed on it once settled or incorporated into sediments.

In aquaria, this material is typically removed from the water column by protein skimmers or other filtering devices. Larger heavier particles settle to the bottom where it forms a fluffy waste that aquarists once found objectionable, and siphoned it out manually in the days of bare-bottomed tanks. Live sand soon became a means to biologically manage detritus, but many still questioned its role as a desirable component of a captive system. Some early skeptics even proclaimed it necessary to vacuum, filter, or replace sand periodically to prevent accumulation of detrital material. Fortunately, the natural processing abilities of an adequately sized sand bed seem to be greater than the normal deposition rates of detritus in all but the most heavily stocked and fed aquaria.

A Paradox

When food, waste, or other particulate organic matter (POM) is trapped, especially in an aerobic environment, it is acted upon by several types of bacteria that break down the substances into more basic dissolved organic and inorganic components. Some of these breakdown components are organic acids and refractory compounds that can impart a yellow tint to the water column. This yellowing has been called "gelbstoff." Other components of concern to aquarists are nitrogen and phosphorous, either alone or complexed to other molecules. These substances typically act to degrade the water quality in closed systems like aquaria, and are also partly responsible for the change of many wild coral reef communities into algal dominated communities. Additionally, phosphorous (usually as phosphates) can decrease calcification rates and act as a "fertilizer" for marine flora. Nitrogen (usually as ammonium or nitrate ions) can increase the density of coral zooxanthellae, concurrently lowering amounts of translocated photosynthetic products to the polyp. Like phosphorous, nitrogen is also a nutrient source for other flora and fauna. On coral reefs, nitrogen and phosphorous levels are usually very low, but in aquaria levels can become problematically high to where the resultant community looks and functions more like that of a eutrophied reef dominated by algae.

It is a paradox that in an attempt to "cleanse" the water column of particulate material using flosses and other mechanical traps, the result can be poorer quality water as well as one which is visibly less "clear." To address the inherent problems of these microbial breakdown products, activated carbon -often an inseparable component of filter media - is frequently used in conjunction with some of these devices in order to remove the yellowing compounds; the very same compounds that were ironically the result of the filter media itself. Ozone is also occasionally employed for this purpose. Following the absorption of some of these refractory compounds, the result is a variable amount of dissolved organic matter (DOM) and inorganic nutrients that simply pass through the filter. However, both the substances remaining after filtration, as well as the substances removed by filtration, can be utilized by the life in the aquaria and are taken up by corals, sponges, some other invertebrates, phytoplankton, bacteria, and algae.

The products of aerobic bacterial breakdown, unless accompanied by "the other half" of nutrient cycles (mineralization, denitrification, and other reductive and/or oxidative processes, etc. that "regenerate" nitrogen and phosphorous) can far exceed the amount of direct uptake by living organisms in many tanks. For this reason, the use of live sand beds (and other more questionably effective means such as resins, media, denitrators, powders, etc.) are often employed to address at least part of the remaining decomposition and recycling processes that would occur in nature. Protein skimmers seek to "short circuit" the process partly by removing particulate matter before it is broken down by microbial action. Nitrification also typically occurs much more quickly than denitrification. Unfortunately, most aquarists do not have a few millennia to spare to allow for complete remineralization of organic matter or other processes often measured in centuries, not months. This is especially true given the much larger area of low-conductivity and low-oxygen state regenerative spaces in the wild. Therefore, increasing uptake by various organisms in the aquaria, coupled with both aquarist aide (water changes, nutrient export devices) and biological aide (live rock, sand, etc.) are often the best we can do.

Particulate Organic Material as a Trophic Resource

Both in the aquarium and on coral reefs, detritus is mostly algal in origin, being produced in situ and is thus referred to as being autochthonous. It is composed mainly of dead filamentous algae and phytoplankton, and secondarily of fleshy macroalgae, coralline algae, zooxanthellae, cyanobacteria, phytoplankton and seagrasses. Non-algal detritus is mostly congealed coral mucus bound with other particulate material (Alongi, 1988). On reef slopes and crests, the material is mostly coral mucus while over reef flats and lagoons, the material is mostly algae and fecal matter. This material, by itself, has a high carbon content. However, it acts as a substrate for bacteria, ciliates, cyanobacteria, and other microorganisms that coat the particles. Bacteria can even convert dissolved organic material (DOM) into particulate organic material (POM) by aggregating it in the presence of carbon. This provides a substantially enriched particle replete with amino acids and valuably higher nitrogen content. As such, detritus becomes a very nutritious food source for many organisms. It is such a complex "dirt" that detritus has been described as a completely self-contained microhabitat of its own with plant, animal and microbial components and its own "built-in" nutrient source.

The role of detritus in the food chain is mostly determined by water velocity and exchange, and the benthic community (sand flora and fauna). In areas of strong water flow and exchange, less detritus is deposited and is flushed away. Aquarists may be familiar with the term "detritivore." This term encompasses certain animal species known to feed primarily on detritus and considered to provide a "janitorial" role in some aquariums. Among such animals commonly utilized are sea cucumbers, brittle stars, sand dwelling sea stars, and certain "sand-sifting" gobies such as Valencienna spp. More recently, some facilities have been providing "detritivore kits" that include smaller detritus consumers such as various polychaete worms (bristle worms), amphipods, and small mysid shrimps.

These are perhaps the more well-known detritivores to aquarists, but are only a few of the potential consumers of detritus on a reef or in an aquarium. In fact, depending on the composition of individual particles, fish and invertebrates may intentionally or unintentionally consume and process significant amounts of detritus in their grazing activities. One study showed that a third of planktivorous fish had from 1-88% of their gut contents composed of detritus, and herbivorous fish have similarly high amounts. Crabs and shrimp can be heavy consumers of detritus. Detritivory occurs both pelagically (in open water) and in the benthos (substrates such as sand and limestone framework). On coral reefs, the major detritus consumers are sea cucumbers (holothurians) and thallassinid shrimp. Sponges are capable of taking up small particles, as are many sessile invertebrates such as sedentary polychaete fanworms, feather duster worms, anemones, tunicates, crinoids, and other filter feeders. Some intentionally intercept this particulate organic material while others receive it by gravitational deposition onto their surfaces. Deposit and filter feeders typically get from 2-50% of their nutrition from detritus. Another major consumer group of detritus is the zooplankton. These small animals, themselves a very important food sources to reef consumers, have been found to have 90% of their gut contents composed of detritus. Mucus-producing animals, like corals, tend to trap detritus, and the material is either removed or consumed by ciliary action across the tissue surface. Many fish also consume coral mucus, and any attached particulate organic material. In some animals, such as basket stars and crinoids, flocculent material of a certain size may be a limiting resource to sustain their metabolic and nutritional needs, and may be at least partly responsible for their lack of survival in aquaria.

Filter feeders such as these polychaete worms (feather duster worms) become very prolific with the availability of detritus. They reproduce readily in aquariums with a proper food source. Photo by Eric Borneman.

Detritus as an Ecosystem Component

Detritus plays a critical role in the integrity of coral reefs, and it plays a similarly important role in the aquarium. There are five fates for detritus in an ecosystem (captive or wild): 1) Use by the microbial community 2) consumption by macroconsumers like fish, crustaceans and other detritivores, 3) incorporation and permanent burial in sediments, 4) export, and 5) regeneration. At least in the wild, and probably in aquaria, very little detritus reaches the sediment to be buried and incorporated into it permanently. In coarse sands or rubble areas, microalgae and cyanobacteria may be the primary site for uptake of microbial decomposition processing of detritus, while in finer sands bacterial uptake predominates.

Detritus forms the basis of several food webs that are part of a balanced autotrophic/heterotrophic community. It also plays a role in establishing various levels of nutrient production and decomposition. It is this material that is the principal food source for the many bacterial species that work in various nitrification and denitrification activities. Before reaching the microbial community, however, it acts as a food source for the smaller consumers such as amphipods, copepods, errant polychaetes, protozoans, flagellates, ciliates and other animals whose activities contribute to the stability and productivity of a coral reef and a coral reef aquarium.

It is the microbial community, though, that is most important in the detrital processes. On the reef, the productivity of bacteria (both aerobic and anaerobic oxidation and reduction, including important sulfate reduction) depends heavily on detritus. Without this microbial community, coral reefs would cease to exist. For aquarists, this translates more simply; without detritus, the denitrification of live sand ceases to exist. Microbial productivity is greatest where the levels of organic material are highest. Thus, disturbing or vacuuming stable sandbeds in aquaria is likely to be counterproductive.

A healthy sand bed is stratified with numerous burrows present from worms and small crustaceans. The coloration can vary greatly within the bed. Here, green colored cyanobacteria forms a middle layer below the highly oxic surface and the more anoxic levels below. Photo by Eric Borneman.

Coral reef benthos is thought to be nutrient limited. The microbial community is not overburdened under normal circumstances, and lagoons, seagrass beds and the water column have vast areas that can process the sum excess production of the reef and terrestrial inputs. A balance is reached under ordinary circumstances. If the amount of particulate matter becomes overwhelming, the communities that tend to respond first are the bacterial communities, followed by the algal communities. The same is true in the aquarium.

Under ordinary aquarium conditions, the detritus produced within a tank is easily managed by the various consumers and microbial flora. In the process, detritus provides an important food source or food web link for the tank's inhabitants. Under ordinary circumstances, the microbial community and various consumers will respond in classic cyclical fashion to fluxes in detritus present. If detritivory and/or microbial productivity is limited or reduced, or if there are enough devices enabled to intentionally remove this particulate matter from the water and substrate, then there may be a limitation of detritus available to sustain a productive community. The most notable signs of such a paucity of organic material are listed in Table 1. It should be noted that phosphorous and nitrogen levels in the sand bed are normally extremely high (200-400 ppm), but these pools are normally maintained and used by the benthic community with released nutrients being taken up by life in the water column (Entsch et al., 1983). This is, however, a point of interest for aquarists; sand beds, as stated before, should not be disturbed!

The Role of Particulate Organic Material in Corals

Corals, in particular, are notable for their consumption of detritus. All corals studied feed to some degree on POM and coral communities have been found to remove half of the POM present on some reefs. So prevalent is this material that it is termed "reef snow" in the wild. In some cases, detritus may be the primary heterotrophic resource of corals, depending on the availability of other types of living plankton. The soft corals (Octocorallia), zoanthids (Zoantharia), stony corals (Scleractinia), and mushrooms (Corallimorpharia), all accept detritus as food and can, in some cases, be provided with over 100% of their carbon and nitrogen requirements by this resource alone. In particular, the gorgonians are well studied in this regard. In fact, some gorgonians seem to rely exclusively on detritus as a captured food source; rejecting zooplankton and phytoplankton in many cases - or at least having these other foods comprise an incidental part of their diet. Part of the reason that the Octocorallia are so "fond" of detritus is that their tentacles are well adapted to "sieving," and their nematocysts are less adept at capturing large or particularly motile prey. Corals are also notable in that they not only consume detritus, but also produce it by the bacterial and algal conglomerations that are trapped and grow on the very mucus they release.

Sponge growth is a good indication of small particulate foods in the water column. Here, foraminiferans exist even within the sponge growth that is not confined to small areas in crevices on the undersides of rock. Photo by Eric Borneman.

Often in turbid conditions resulting from the suspension of organic material, light availability becomes limited. In such cases, corals may need to acquire more energy from feeding, and this particulate material alone can act as a food source that can result in up to 50% of the tissue growth of such corals (Anthony, 1999). Of the many food sources available to corals and already discussed in this series of articles, particulate organic material, dissolved organic material and bacteria are the most universally accepted food sources across taxa. Often, bacterial ingestion is included as part of the POM intake because of the degree to which bacteria coat and increase the nutrition of organic flocs. In other words, they are often inseparable from each other. Given the ability of so many corals to consume and utilize this material, along with its relatively high abundance and ability to provide up to 100% of corals' carbon and nitrogen requirements, it may now, hopefully) seem rather foolish to attempt to remove this material from aquaria.

Because sediments collect detrital material and are high in nutrients, they foster the growth of grasses and algae. These plants can then also uptake nutrients from the water column, keeping the water nutrient level low and fostering a healthy reef community. Photo by Eric Borneman.

The photos above also illustrate variations in a healthy sand bed community, fostered by the settling of detritus. The top photo again illustrates the stratification present in an established sand bed. The bottom photo shows the variation in communities, with dark anaerobic areas near the bottom and various microbial, algal and cyanobacterial mats and areas displaying various colors. Both are riddled with the burrows of excavating benthic organisms. Photos by Eric Borneman.

Summary

The use of detrital material, or particulate organic material, as food source is a cornerstone of coral reef ecology and forms what is well accepted to be the base of the entire food chain. Coral reefs, being generally nutrient-limited from outside sources, depend highly on recycling of nutrients in order to maintain high diversity and productivity. Detritus forms the foundation of this recycling community. Additionally, when suspended in the water column as a "reef snow," POM becomes an abundant and easily utilized food source to all manner of filter-feeding invertebrates and is a nearly universally utilized, nutritious, and important food source amongst corals.

In aquariums, detritus is produced and consumed at a considerable rate, and yet the suspended components are actively removed by all manner of aquarium filtration devices, including protein skimmers. If suspended material removal is required in individual cases to maintain water quality conducive to the survival of aquarium inhabitants, it is suggested that a facsimile of this material is regularly provided to the aquarium. Many particulate food sources commercially available, such as Golden Pearls, VibraGro, crushed flake foods and Cyclop-Eze can provide a reasonable substitution for particulate organic material as they tend to remain in suspension easily. This characteristic is important in providing access to the food by filter feeding animals, including corals. As stated in a very early installment of this series, corals are not plants and must eat to survive. Light alone cannot and does not provide enough of the substances to maintain and grow animal tissue. Feeding is essential, and if not by naturally produced "reef snow" within the aquarium, by some method involving aquarist effort.

The final part of this series, concerning dissolved organic material, and a summary of the series, will be provided here, at Reefkeeping magazine, in an upcoming issue.

Links to Part 1, Part 2 , Part 3, Part 4, Part 5, Part 7

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