Nature’s Pathways, Part II
by Bob Goemans
As mentioned in Part I, there’s an increased desire to maintain what is loosely called “natural marine aquariums,” where methods used to maintain them are more inline with processes that occur in the wild, than what high tech equipment can provide. In my opinion, if aquarists are going to maintain non-high tech aquariums, it’s a must they understand how the usage of energy, whether that’s light or chemical energy, become the most important players in whether or not they are going to be successful.
The importance of quality light and water pathways were discussed last month. It should have been clear from that discussion that high quality lamps rendering the correct spectrum and intensity are needed in reef aquaria containing organisms that require such energy to accomplish photosynthesis. Of course, if the goal is keeping the fish-only aquarium, then light quality and quantity is not as important.
But where reef aquariums are concerned, seawater is a complex maze of different elements and compounds that can somewhat diminish light intensity and spectrum as it passes through. And in closed systems, where some aquarists over-indulge in the use of some additives, and/or allow their systems to become nutrient rich, quality light transmissions become even more affected. The point here is to make this water pathway as easy as possible for light energy to traverse by maintaining its parameters as close as possible to that which surrounds the coral reefs in the wild.
And if aquarists have done what’s necessary to insure the correct quantity and quality of light will be emitted from those lamps and can pass through the seawater without being radically affected by an excess of minerals and nutrients, there’s now the need to better understand the inhabitants in sandbeds, and that includes who they are, why they are important, how they work, how to cultivate them, and how to maintain them. And as for those inhabitants, in my opinion, they would be those where we can predict their pathways and results, and they would be its microbial inhabitants. Now keep in mind, this is not to say bioturbation and/or infauna are not helpful, but my focus here is on what is predicable and controllable!
Prior to the sandbed itself, living on all wet oxygen-rich surfaces, even on the sides of fish, the aerobic heterotrophs breakdown organic matter such as waste products and/or dead animals. The then inorganic result, ammonia, is utilized by aerobic autotrophs (most upper areas of substrate), which reduce it to less toxic substances such as nitrite, then nitrate. The resulting nitrate is then acted upon in an area below that containing little or no oxygen, generally called the anaerobic area. However, that so-called anaerobic area is subdivided into two zones, each having a class of bacteria that cannot live in the others area. In the upper portion of this so-called anaerobic area, properly called the anoxic zone, where there still is a marginal amount of oxygen, i.e., 0.5 – 2.0 ppm, the facultative anaerobic heterotrophs exist and reduce the incoming nitrate to nitrogen gas (dissimilatory denitrification). Below this zone where less or no oxygen exists, more properly called the anaerobic zone, obligate anaerobic heterotrophs exist and reduce nitrate to ammonium (assimilatory denitrification), no further! Keep in mind ‘ammonium’ is a far greater alga nutrient than nitrate!
Furthermore, both aerobic autotrophs and facultative anaerobic heterotrophs produce adenosine triphosphate (ATP) during their respiration process. However, obligate anaerobic heterotrophs are inefficient, as they produce none. Bear in mind ATP production equates to efficient energy production and improved energy cycling. This would mean that those producing ATP are by far more efficient in reducing large amounts of nutrients, and that fluxes in system nutrients can be quickly utilized/better controlled.
If the efficiency of aerobic autotrophs and facultative anaerobic heterotrophs were not important enough to ‘cultivate’ these bacterium/their zones, then nitrogen fixation — the utilization and production of energy, should be thought of as hinging upon an enzyme called Nitrogenase. That’s important because nitrogenase is extremely oxygen sensitive, as it does best in fully aerobic and anoxic zones. In fact, experiments have shown ammonium additions cause a rapid reduction of its activity. Therefore, it appears that limited anaerobic areas, where ammonium is produced, are in the best interest of the more efficient aerobic and anoxic bacterium, which then can more efficiently carry on with their superior processes.
How to accomplish that brings several methods to mind. Probably the most efficient is the plenum method, as the far greater percentage of its bed depth, about 4 inches, is the anoxic zone. Or, utilize a shallow bed consisting of 2 – 5 mm grains, and keep the bed clean, thereby eliminating clogging detritus from encouraging anaerobic/ammonium-producing zones. Keep in mind, when using this grain size sand; aerobic autotrophs approximately occupy the upper half-inch. The next half-inch is inhabited by facultative anaerobic heterotrophs. The remaining depth contains the obligate anaerobic heterotrophs!
Additionally, since ammonium inhibits nitrogenase production, it can be surmised that it’s a likely nucleus for unwanted plant growth. That’s because inhibited nitrogen fixation becomes a revolving door so to speak where instead of efficiently processing ‘new’ incoming nutrients in the upper reaches of the bed, its own ‘products’ may continue to be recycled! Think about that for a moment! In deep sandbeds directly on the aquarium bottom most of its volume is anaerobic where ammonium is produced. Besides the overall reduced efficiency, unprocessed excess nitrate and ammonium may leach upwards through the bed to encourage unwanted alga growths in the bulk water. In fact, such a happening may fit a situation where persistent algal growths in some aquariums continue in spite of low, ‘readable’ nutrients in the bulk water. Keep in mind algal growth is a form of nitrogen storage (incorporation), therefore a sign that nitrite, nitrate, and/or ammonium are available for its use.
Ironically, algal mats (Slime algae/cyanobacteria/hair algae) can develop and build the same or similar conditions. When an algal mat develops, its structure facilitates a similar mechanism for nitrate as occurs in some coastal environments. And when an aquarium sandbed becomes structured to favor a lesser efficient form of denitrification such as using nitrate to build ammonium (not breaking it down into lesser ions), it’s logical that algal mats will follow by ecological succession. Both seem to propagate their own environment by nitrogenase inhibition. In short, nutrient pathways shift toward propagation of plant cells or storage for that goal. This distracts greatly from efficient energy cycling and the balance of the system as a whole suffers.
One last subject that is always on aquarists’ minds is that of phosphate. Actually, most phosphate in our aquariums is due to the food fed and/or the quality of tap water used for evaporation makeup or water changes. It’s the reason why phosphate-removing media is so important in closed systems, especially the ‘iron’ based products, as they are the most efficient. Nevertheless, it’s been said anaerobic areas, where obligate anaerobic heterotrophs reside, accumulate phosphate. Actually, the word ‘accumulate’ is somewhat incorrect. Yes these bacteria are inefficient and produce copious amounts of phosphate. However, the anaerobic area with its lower pH and redox is a fairly efficient user of the oxygen electrons tied to the phosphorous element. This results in the phosphate being reduced back to phosphorus. That’s a good point for deep beds, (but the only one that I can think of) as most of its substrate is in a truly ‘anaerobic’ condition. It could also then be said phosphate accumulates anywhere its not attacked for its oxygen elements. That would tend to say that in the more aerobic/anoxic zones there would be greater accumulation, however, that’s also not 100% accurate! In those areas it’s mostly bound to calcium where it’s quite stable because it’s very easy there to maintain its ‘charge’ balance because of the surrounding diffusion of oxygen. Therefore, phosphate is usually not available for uptake in substrates unless directly associated with reducing conditions.
And if we take this one-step further, where there is infauna, they depend on getting dissolved oxygen to live and have to link with the substrate surface, whereas the bacterium described above do not. This opens the door to the irrigation/tunneling processes of infauna, which can bring phosphate to the bed surface; however, even though feasible it’s probably a rare happening. Nevertheless, it’s a possibility and when and if it occurs, it’s in the form of orthophosphate, something not registered on aquarium phosphate test kits and which can easily cause algae blooms. Keep in mind infauna also ingest sources of phosphate and produce phosphate-laden wastes. Nevertheless, they are more movers of the compound than users.
It should now be evident that if a closed system, no matter what its physical size (home or public aquarium), contained more anoxic than anaerobic area as defined above, its bulk water may possibly contain far less inorganic nitrogen laden products. Now armed with this information I often look back and wonder if aquarists have miscalculated the value of deep sandbeds and/or the use of an excessive amount of live rock. Those who say utilize deep beds and lots of live rocks because it will help lower nitrate accumulations are only somewhat correct. As you can see from the above, there’s much more to it, as it has a greater impact on system health than a narrow viewpoint on nitrate reduction! And as to the value of infauna, I can calculate the zones where efficient or inefficient bacteria reside, but I can’t give marching orders to infauna to carefully and evenly traverse the depths of my sandbeds (Bioturbation)! However, that is not to say they provide no value at all.
For the above reasons I believe it’s important for aquarists to give more thought to the “space/volume of area” that house facultative and obligate anaerobic heterotrophs in closed systems. It should be evident the denitrification path in an anoxic area is of far greater value than the denitrification path in anaerobic areas. Since the volume of area accomplishing ‘nitrification’ is usually fixed in closed systems, it’s wise in my opinion to concentrate on how to enlarge anoxic zones and reduce anaerobic zones.
In closing, our aquariums contain an intricate network of biological and chemical processes. You can present Mother Nature a roadmap that is designed for energy efficiency, or blindfold her. The choice is yours!
References and Further Reading
Brown, C.M. 1988. Nitrate Metabolism by Aquatic Bacteria. In Austin, B. (ed) Methods in Aquatic Bacteriology, John Wiley & Sons, Ltd. London.
Cammen, L.M. 1982. Effect of particle size on organic content and microbial abundance within four marine sediments. Mar. Ecol. Prog. Ser., Vol. 9, pp. 273-280.
Boudreau B.P., Jørgensen, B.B., 2000. The Benthic Boundary Layer: Transport Processes and Biogeochemistry. Oxford University Press (c) 2000
Holiday, L. 2002. In the Lagoon of Sharks. Practical Fishkeeping. May/2002
Howarth, R. 1993. Microbial Processes in Salt-Marsh Sediments. In: Ford T.E. (ed) Aquatic Microbiology, an ecological approach. Blackwell Scientific Publications, Inc., Cambridge, MA.
Jaubert, J. 1988. An integrated nitrifying-denitrifying biological system capable of purifying seawater in a closed circuit system. In Deuxieme Congress International d’Aquariologia Monaco. Culltein de l’Institut Oceanographique Monaco 5. Pp 101-106.
Jorgensen, B.B. & Des Marais, D.J. 1990. The diffusion boundary layer of sediments: Oxygen microgradients over a microbial mat. Limnol. Oceanogr., 35: 1343-1355.
Koike, I. & Sorensen, J., 1988. Nitrate Reduction and Denitrification in Marine Sediments, Nitrogen Cycling in Coastal Marine Environments. Edited by T.H. Blackburn and J. Sorenson. SCOPE. John Wiley & Sons.
Sprung, Julian. 2002. Jaubert's Method, the "Monaco System," Defined and Redefined. Advanced Aquarist Online Magazine. 2002