The Food of Reefs, Part 5: Bacteria

Fewer topics have given me as much consternation over the years than the subject of bacteria. The microbes are undoubtedly the largest biomass of life in virtually all ecosystems, and this can be extrapolated to include the reef aquarium. To most aquarists, bacteria comprise the “vast unknown,” and perhaps for good reason. They are vastly unknown, even in nature. However, aquarists may have some inkling that bacteria are a fairly large and diverse group of organisms, but generally think of bacteria in terms of “filtering” water (such as in nitrification and denitrification) and in terms of the mysterious “bacterial infections” that seem to rise up to slay fishes and invertebrates in their tanks.

In this article, I will very briefly use some examples and analogies to introduce some of the roles of bacteria in marine environments, and then proceed to their role as a trophic resource, or food, to corals (and other reef organisms).

Introduction: Microbial Ecology 101

Bacteria are the oldest living things on Earth, first appearing 3.5 to 4 billion years ago and were the only forms of life for about at least a billion years. Present day life would not exist without bacteria, and these early bacteria caused global geochemical cycles that made the Earth habitable and led to the evolution of eukaryotic organisms. In fact, eukaryotes were able to expand their diversity by acquiring bacteria as endosymbionts. Interestingly, some simple protozoa, which consume bacteria and can harbor them as endosymbionts, resemble the phagocytic cells of higher organisms, and the bacteria in many ways resemble mitochondria, the so-called power plants of cells.

To date, no environment on the Earth has been found to be free of bacteria. They are tiny, and only a very few species are visible to the unaided eye. Because of their diversity, relatively few bacteria are well known. Most of those that are well known either have some relation to human health (acting as pathogens, or agents of disease), or are bacterial “guinea pigs” used as models for other systems. Probably the most common, and often excruciatingly one-sided, way people are exposed to bacteria is from the public press or lay media. Stories of “killer” bacteria, flesh-eating bacteria, infection-causing bacteria, and others prompt the production of antibiotic drugs, antibiotic soaps, lotions, and all manner of weaponry to “eliminate” bacteria.

Relatively few bacteria cause human diseases, and of those, many are opportunistic pathogens. These species have not evolved to act as pathogens, but rather can cause diseases when conditions allow. The vast majority of bacteria are innocuous and/or beneficial. I guess the way I would like aquarists to begin to see bacteria are as things that are always present, everywhere, covering every surface, and are generally not problematic. A useful analogy might be the conception of “bristle worms.” For years, aquarists saw bristle worms as scary and assumed they were to be eliminated. Bristle worm traps and other devices were marketed and sold to cope with these “problem” creatures. Eventually, more people became familiar with the real nature of these worms and learned that they were not detrimental, but rather beneficial – with the exception of a very few types that could be problematic under certain conditions and to certain organisms – for example, Hermodice carunculata to gorgonians and soft corals, and the Bobbit Worm to fishes (and divers!). The same is true with bacteria as with polychaetes, but taken to the nth degree in terms of numbers and types. There are lots and lots of innocuous types, lots of good types, and a few troublesome types that may become apparent under certain conditions and to certain organisms.

As an example, consider a sterile culture, or the sterile conditions of an operating room. Anyone who has tried to culture phytoplankton under sterile conditions understands how difficult it is to create sterile conditions. Even surgeons working in operating rooms use multiple techniques to try and maintain the highest probability of sterility. Even so, infections occur very often in hospitals, despite the use of antibiotics, antiseptics, sterile conditions, and sterilizing sprays. These infections are not due to any single cause, but rather many such infections (called nosocomial infections) occur because the patients in a hospital have immune systems that are compromised to one degree or another. Bacteria are so omnipresent that creating the nurturing conditions in a hospital for human life allows the very quick appearance of bacteria in that environment.

We eat bacteria with every mouthful of food we eat. We are covered in bacteria. Bacteria line our mouths, our guts, cover our hair, our clothes. The bacterium, Escherichia coli, is a species that has allowed for tremendous advances in all fields of biological sciences as a model system and is partly responsible for allowing us to digest and absorb food as the primary commensal component of our intestinal flora. Yet, most people probably associate E. coli with the rarer pathogenic strains that cause disease and have been in countless newspapers headlines. Clearly, we live surrounded by and enveloped with bacteria all the time, and they do not cause us disease. In the majority of cases, diseases occur when conditions in the environment or with the host allow for disease to occur. The same will be seen for the marine environment.

Bacteria in the Marine Environment

Although there are certain areas and species that have attracted some degree of study, marine bacteria are, in the scheme of things, virtually unknown. Even their ecological roles are often generalized and assumptions often based on better-studied terrestrial systems. However, they are no less numerous than in terrestrial environments, and are in many cases, and probably most cases, likely more numerous and important.

It became obvious that the field of marine microbiology as it relates to understanding coral reefs had to advance, as best I can parse from the literature, when several key events and areas of study emerged.

First, the study of nutrient dynamics, by necessity, had to include processes mediated by bacteria in the benthic habitat.
Second, the observation and recognition of marine diseases causing mass mortalities suggested bacteria as potential candidates and required investigation.
Third, the existence of numerous microbial food webs that encompassed widely varying species groups, from larvae to large invertebrates, which included bacteria as a significant link in the chain.
Fourth, bacteria were mucking up otherwise good studies in biology. For example, in coral studies, the outer surface of mucus is so enriched with bacteria that investigators who did not flush the bacterial community from the surface discovered that measurements – such as respiration - were skewed by the presence of this active and productive surface flora.
Finally, it was becoming difficult to assess and even to separate the parts from the whole, since bacteria are practically inseparable from any surface, and there was beginning to be more and more evidence of highly dynamic relationships between organisms and bacteria.

Given the enormous bacterial biomass in all ecosystems, it should be of little surprise that they are food for something, if not many things. Bacteria, being composed of living material, contain a relatively large amount of nitrogen, an element in very short supply in coral reef waters. Nitrogen is closely recycled and horded in such environments, and many organisms may be limited by it. Therefore, attempts to acquire nitrogen might very well include methods of acquiring bacteria, an abundant resource.

For years, bacteria were lumped as being part of particulate organic material (including reef snow) because it was difficult to quantify the bacterial component, but possible to quantify the particulates, and it was known that bacteria coated these surfaces (and every other surface, for that matter). In other words, it was rather convenient to deal with them in this way without actually investigating a vast new unseen and unknown microbial world independently, especially since 80-99% of them will not grow in cultures and are quite difficult to study and characterize.

Bacteria as Coral Reef Food

The biomass and productivity of bacteria on coral reefs are as great as those in nutrient-enriched (or “eutrophic”) lakes and up to a hundred times greater than in the open ocean. Planktonic bacteria in coral reefs areas are dominated by coccoid, rod, and horseshoe shaped forms from 0.3-0.8?m in size which have filamentous processes to allow them to absorb and consume dissolved organic molecules. Many of the bacteria ingested as bacterioplankton are associated with detrital material and phytoplankton, to which they attach with adhesin-molecules and cellular attachment processes, such as fimbriae. Filamentous bacteria tend to predominate in periphytonic (on the outside of plants) communities, whereas motile and attached rods tend to form the primary components of bottom sediment surface layers. Deeper in the sediments, coccoid and spore-forms predominate. It has also been found that the diversity and composition of pelagic forms tends to change as the water column of an area “matures,” with the highest diversity found in mature nutrient poor (or “oligotrophic”) waters, such as coral reefs. This is because many of the diverse forms found there are oligocarbophylic (thriving in low carbohydrate levels) and do not thrive in the nutrient concentrations found in eutrophic waters, even though the total biomass there may be significantly higher. However, some studies have found that bacterial biomass in polluted eutrophic waters and sediments may not be any higher than in pristine areas. Levels can vary over several orders of magnitude from 5 x 103 to 2 x 106 cells/ml of water. Levels in sediments and detrital pools can reach 3-10 x 109 cells/gram with a biomass of 2-5mg/g. The highest levels are reached in detritally-enriched fine silts (1 x 1010 cells/gram). Bacteria in enriched sediments are estimated to be found in extraordinarily high numbers, comprising somewhere between two to five percent of the total weight or organic material present. This may be notable below as to why corals “open up and feed” when the sediments of a tank are stirred. Even more notable is the efficiency (60-80% efficiency, compared with about 20% for deposit-feeding animals) with which bacteria decompose organic material and put it into their own biomass. It is estimated that decomposed organic material can result in 30% of the newly formed protein in waters from the production of microbial biomass.

Detrital food chains are found to predominate in most marine ecosystems, and it has been found that the bulk of the diet of herbivores, as well as their nutritional requirements, comes not from direct consumption of phytoplankton but by the consumption of the adherent periphyton and detrital material that is enriched by attached microbial communities. The importance of these components of the diet appears nearly universal in marine ecosystems. In pelagic plankton, estuarine, coastal, and coral reef communities, the consumption of these components encompasses an estimated 60% –90% of the total energy flow through pelagic and coastal tropical systems, respectively. Bacterial primary production or photosynthesis is highest in sediments and can even equal that of the phytoplankton in pelagic systems. Photosynthetic rates within the sediments can be equal to or even higher than on those on the reef itself. In summary for this section, in virtually all studied marine environments, bacteria are water purifiers, decomposers of organic material, and a primary source of protein for both those animals that directly graze on them and those that acquire them indirectly through secondary consumption.

Bacteria as Coral Food

Given the importance of bacteria as a food source in marine ecosystems, it might not be surprising to learn that they are also a primary food source for corals. It has been found that bacteria alone can supply up to 100% of both the daily carbon and nitrogen requirements of corals. All corals studied consume dissolved organic material, bacteria, and detrital material. This is more than can be said for any other food source, including zooplankton and light.

Coral consume bacteria in a number of ways. First, they can use their mucus and well-developed epithelial cilia to entrap and consume both attached and pelagic bacteria. Some corals, such as Turbinaria species, can beat their mucus into webs with their cilia, and these webs are cast out like a net into the water column to ensnare particulate material, primarily bacteria. The cilia then pull the net backwards towards the colony where the polyps consume the bounty. The amount of nutrition gained by bacteriovory under normal conditions is unusually high in terms of efficiency of capture and ingestion, and studies show a range of average gains from such resources that depend on both the species and the environment (in terms of the availability of bacteria in the water column). Table 1 presents some data from Sorokin (1979, 1991).

Coral species
feeding rate, % of expenditure on metabolism
Capnella sp. 1
Cladiella humesi 1
Sinularia densa 1
Sarcophyton trocheliophorum 1
Tunicate, Ascidia nigra 1.6
Rumphella aggregata 3
Sponge, Toxidocea violacea 3.4
Xenia elongata 4
Lemnalia rhabdota 5
Favites abdita 5
Hicksonella princeps 5
Plexauroides lenzii 5
Goniastrea pectinata 5
Montipora verrucosa 5.8
Acabaria nicksoni 7
Litophyton arboreum 7
Dendronephthya gigantean 7
Zoanthus sociatus 8
Isis hippurus 9
Bebryce indica 9
Merulina ampliata 10/11
Lobophytum gazellae 10
Holothurian, Ophiodesoma spectabilis 10.4
Paralemnalia clavata 11
Stylophora pistillata 11/25
Montipora eryhtrea 12
Tubastraea sp. 13
Goniopora sp. 13
Sinularia sp. 15
Symphyllia sp. 15
Mopsella aurantia 17
Palythoa caesia 19
Fungia scutaria 20
Pocillopora damicornis 20
Seriatopora hystrix 22
Fungia actinformis 22
Acropora hyacinthus 22
Acropora squamosa 22
Porites annae 31/27
Pavona cactus 41
Hydroid, Pennaria tiarella 43.5
Tubipora musica 75
Hydnophora exaesa 75
Leptastrea transversa 85/84

Bacteria not only provide carbon and nitrogen for the polyp, but also provide an important source of phosphorous for the zooxanthellae, in addition to other elements such as vitamins and iron. In fact, shallow water corals at Heron Island gained a larger proportion of their metabolic needs from feeding on bacteria and particulate organic material than from zooplankton.

There is, however, significantly more to this story…it just gets better and better!

Corals not only use their mucus to trap bacteria, but the coral mucus also serves two other purposes. First, it is an incredibly good growth medium for marine microbes resulting in and the surface of corals containings bacterial communities with densities far in excess of the already significantly high levels in the surrounding waters and benthos. Second, and; coral mucus is a primary contributor to some kinds of detritus, where – the particulate aggregates are held together by coral mucus with levels of bacteria two to five2-5 times higher than in particulates without the presence of coral mucus. These aggregatesd not only provide food for corals, but for all manner of particulate filter feeders, a category – that encompasses directly or indirectly just about every living animal on the reef. Furthermore, these microaggregates contain a tiny recycling community where nutrients themselves are concentrated 2-3 times higher within the aggregates than in the surrounding water. These energy “packets” are extraordinarily important, both to individual organisms and the entire ecology of the reef.

Corals are also able to selectively culture specific strains and increase the density of bacteria in several ways. They “farm” bacteria within recesses and interstitial spaces of their branches and colonies. Reduced water flow and microenvironmental conditions allow the proliferation of microbes used as food. They can also change the composition of their mucus by altering the production of the mucosecretory cells of the epithelium. The change in mucosal composition allows for variations in the microbial community on the surface in that different species and strains are more adept at exploiting various components of mucus.

Additionally, particulate aggregates are concentrated in the coral gut or coelenteron and moderated by the composition and amounts of coelenteric fluid released into the gastric cavity. This is yet another way to alter, increase, or vary the types and amounts of bacteria available as food, and another method of “bacteria farming.”

Lastly, and only sort-of finally, specific strains of bacteria may be host specific in that species of corals may associate with one or several bacterial species in a symbiotic relationships. Increasing evidence points to the idea that the microbes on the surface are not only acting directly as food, but are involved in the production of specific compounds either in limited availability or unavailable by other means. The bacteria may provide these as “leaky” fluids resulting from their own metabolism. Additionally, nitrate reduction or nitrogen fixation is occurring, and the action of such microbes may provide an important source of inorganic nitrogen to both coral polyps and zooxanthellae. The specific associations of coral/bacteria may be related to the specific requirements of the individual coral species. There are even some corals, such as some Porites sp., that harbor bacteria intracellularly, and although it appears to be a commensal or symbiotic relationship rather than one of parasitism or pathogenicity, much more work is required to elucidate the true nature of these internal aggregates.

Summary and Aquarium Considerations:

Bacteria exist in very high diversity and biomass in the marine environment, and especially on coral reefs and on coral surfaces. They play critical roles in virtually all ecological processes that control reefs and are a major component of food webs. Corals feed on bacteria at levels and efficiencies that rival all other bacterial consumers (Figures 1 and 2, adapted from Sorokin 1973). Special adaptations exist with corals that allow them to be able to supplement their nutritional requirements with bacteria, directly, and the products of bacteria, indirectly, and even enhance an otherwise already abundant resource. Levels of bacteria in reef aquariums are largely unmeasured; however, they are likely to be comparable to those found in wild communities. As such, corals in aquariums are likely deriving a significant amount of energy from the consumption of bacteria on detrital aggregates, in the water column, and “farmed” on their surfaces. Attempts to “sterilize” coral surfaces or tanks by using prophylactic dips or antiseptics and antibiotics has the distinct potential to do more harm than good in the majority of cases. Most aquarists probably have a distinct impression that “good” and “bad” bacteria exist, but may not be aware of the relative levels of each, the importance of the diversity and roles of the microbial community, or the extent of the biomass involved. A more complete understanding of the roles and importance of bacterial communities in aquariums is essential in understanding how reef aquariums function and to provide for their success.

click here for full size picture
Figure 1. A comparison and quantification of bacteria as a food source by various coral reef organisms. Click on the table for a larger image.


Figure 2. Dependence and rate of feeding on bacterioplankton by various filter feeding animals. A – sponge, Toxadocea violacea, Bgastropod veligers, C – serpulid (feather) worms, D – coral, Pocillopora damicornis, E – oyster, Crassotrea sp., F – crustacean, Eucalanus antennatus.

Note: I put quite a bit of work into utilizing and providing references in my articles. I hope that interested readers will take the time to find these literature sources, for they provide a much more complete picture of my article content. In this particular case, some of the works are absolutely fascinating and simply should not be missed.

Links to Part 1, Part 2 , Part 3, Part 4, Part 6, Part 7

If you have any questions about this article, please visit my author forum on Reef Central.


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The Food of Reefs, Part 5: Bacteria by Eric Borneman -