Back
Sand and Natural Nitrate Reduction
 
by Sam Gamble
 
 
I would like to formally say hello to the members and readers of MARS. I am very fortunate to learn about the group and their interest in aquariums. It is particularly great to realize the interest in marine reef aquariums. My challange started many years ago when I became a scuba diver and saw the real thing offshore in the lower United States (Florida Keys) and the Bahamas. I now live in the Florida Keys with the reefs within a short boat trip distance. It will probably be an everlasting challenge to successfully reproduce what has already been presented to us in Nature.
 
This includes for us the unique coral reefs. The closeness and involvement with the environment, allows natural nitrate reduction to become an open book test with some very hard questions. Getting nature to divulge secrets is not easy and we have been lax because of the success by more unatural methods. Thanks to several pioneers and new work that can be seen, there is a movement spreading. From better understanding and getting involved, we will hopefully in the future maintain improved exhibits that explain our resource to our chidren. Just as importantly the communication will help others, like you.
 
Back about five years ago the feasibility of coral reefs in aquariums had been making very real progress. At first the concepts heavily favored technical assitance from devices like external filtration, ozone injection, CO2 injection, foam fractionators, and various other devices. After Dr. J. Jaubert publicised his method a new direction started. The word "new" really has to be explained, because it actually relied heavily on natural systems or the field of ecology. However, many of the details were missing.
 
In the United States the idea was first tested by Bob Goemans, a well known author and aquariast for over 20 years. His experiments caught my interest and we began to correspond. In the process we decided to formally investigate more of the workings of the sand bed system that has the space underneath the sand called a "plenum". My first discovery was a paradox. The concept of the system is simple, but works by comlex relationships. The more you look, the more intricate and beautiful it becomes. Part of that intricate and complex relationship can be realized from the "Vital Energy" story.
 
The concept can be used without a college degree to explain why if we admitt and accept the reality of the natural needs and characteristics of the phsical and biological laws governing it. Perhaps this is why the words "biogeochemical pathways" was invented. It covers most of the subjects at once; bio = biology, geo = geology, chemical = chemistry. Life has a balance in every event from microscopic to macroscopic. We observe balance as conducive to our way of life and the sustaining events of things or creatures we wish to preserve. if you are trying to maintain an aquarium, you must consider the main culture you wish to preserve and then understand that countless microscopic events must happen to maintain the macro cultures. The best way to understand the system is to understand the single cell and what it needs to promote its equilibrium. This includes biogeochemical pathways.
 
With the systems that preceeded the sand bed system, a big problem to solve was nitrate levels increasing to dangerous amounts. The need to reduce the amount of nitrate developed a natural method to eliminate it. Natural nitrate reduction was known to occur in natural systems like the sand of natural marine environments. Importantly how it works was also realized and that it could be used in marine aquariums. Thanks to Dr. J. jaubert.
 
To help understand the key features of natural nitrate reduction and the role of sand filters we must focus on the open systems in nature for our more finite marine application. The goal is a nutrient poor environment as per the guideline for coral reef ecosystems. In the case of our reef aquaria, we first build the environment trying to achieve ecosystem's goal. To do so successfully the crucial elements must be provided in the ratios necessary for balance and determinative survival and growth.
 
The driving forces are chemical and radiant energy. These elements are interrelated complexly and mediated biotically and/or abiotically (living and nonliving). With the topic of NNR and living sand filtration, the process of denitrification is of foremost importance. The process is mediated by bacteria (microbes). The deposition of organic detritus on the surface layer of marine sediments supports an elevated microbial metabolism and limits the penetration of O2 into substrata. An ideal environment for microbial NO3- reduction is thus created where NO3- can be in ample supply to substitute for O2 in the process of organic matter degradation (Koike & Sorensen, 1988).
 
In sediments (top layer of benthic substratum), this would involve only a very thin layer that interfaces with the bulk water of an aquarium. In systems using porous rocks for filtration i.e. Berlin Method, this would include the rock to water surficial interface. In natural marine sediments the oxic/anoxic (high oxygen/minimal oxygen) interface varies from a few millimeters to several centimeters. So then, what is a segregated sand bed of about 5 cm, with a plenum space underneath going to do for the axiom of nitrate reduction (denitrification). Plenty ! !
The presence of the surficial interface, the area at the sand's surface where oxic conditions are changed to anoxic conditions by microbial metabolism of organic matter does not change in the living sand filter. In fact it seems extended or enhanced. What goes on below is where the difference becomes apparent after measuring variables like nitrate, oxygen, pH, H2S, and alkalinity.
Normally in marine sediments the area below the oxic/anoxic microzone is essentially anaerobic. The diffusion of elements to and from the obligate anaerobes is slow and at a reduced capacity compared to the surface microzones. Any increased activity is most often the result of plant and animal infauna increasing the net volume of surfaces.
In the sand bed system the elements traverse horizontally and vertically throughout the sediment substrate and plenum. Oxygen can be measured at anoxic levels at varying times throughout the sand bed and plenum. Oxygen can be present even if the sand layer above is temporarily anaerrobic.
Nitrate concentrations have been seen to have definite magnetism toward the plenum and often accumulate there in higher concentration than the aquariums bulk water. Nitrate production from an organic load can be seen to diffuse sequentially through the sand layers to the plenum. Associated with this transition is a gradual reduction of nitrate, oxygen levels, and pH. Interestingly, total alkalinity is relatively associated and observed to increase slightly with proportional pH decrease.
To surmise the observations it would be possible to conclude one of the biggest contributions of this sand bed (NNR) system is to expedite facultative microbes as opposed to obligate microbes. Obligate anaerobes are found mostly in the natural marine sediment environments below the anoxic microzone. They only metabolize and grow in anaerobic conditions. However, the facultative microbial populations can do either - metabolize and grow with oxygen or reduced oxygen. This ability really enhances nitrate reduction capability. To implement a facultative design that is at least three inches deep by the length and width of the aquarium provides tremendous potential to filtration in this regard.
To make quantitative and qualitative identification of the microbial populations directly takes specialized equipment. Indirectly the chemistry techniques measuring general water quality parameters helps to illustrate. Taking water samples in the aquarium, middle of the top sand layer, middle of the bottom sand layer, and the middle of the plenum help provide observations we can draw conclusions that stem from recent research done with sediments in the marine environment. A good example is the presence of sulfur reducing bacteria.
Sulfide which is produced by sulfate-reducing bacteria is oxidized by different microorganisms. Under anoxic conditions anoxygenic phototrophic bacteria utilize sulfide as electron donors, while colorless sulfur bacteria oxidize sulfide under oxic conditions. The different groups of microorganisms show strong interrelationships (van Gemerden, 1994).
Dissimilatory sulfate-reducing bacteria (SRB), using excretion-, lysis-, and decomposition produce sulfide. The sulfide can reoxidize to sulfate by colorless sulfur bacteria (CRB) and purple sulfur bacteria (PSB). Aerobic heterotrophic organisms are functionally important as their activity leads to oxygen depletion, and fermentative organisms provide growth substances for SRB, (van den Ende F.P., van Gemerden H. 1994).
Apart from their collective effort in sulfide removal, CSB and PSB have little in common and will compete for this reduced sulfur compound. The extent to which H2S is oxidized by either group very much depends on the availability of oxygen. With no oxygen available, sulfide will be exclusively oxidized by PSB, provided light is available. With excess oxygen, virtually all sulfide will be oxidized by CSB, despite the fact that PSB are capable of chemotrophic growth. This is explained by the fact that CSB have much higher affinities for sulfide than PSB. In microbial mats most of the sulfide is oxidized at the oxygen/sulfide interface at low oxygen concentrations, (van den Ende F.P., van Gemerden H. 1994).
The microorganisms in the uppermost sediment layers thus influence the nitrogen cycle by high rates of both incorporation and mineralization of nitrogen compounds, and also by changing the chemical microenvironment. Both total photosynthetic activities and oxygen penetration increase when the light intensity is increased. The oxygen consumption is very high at the lower boundary of the oxic zone where intense biologically mediated oxidation of reduced sulfur compounds, and probably also ammonium, takes place, (Niels Peter Revsbech, Janne Nielson & Pia Kupka Hansen 1988).
We know that sulfide oxidation in sediments where sulfide diffuses up to the oxic zone can be restricted to a 50-100 micrometer thick layer, in which a dense population of sulfide-oxidizing bacteria mediates the process (Jorgensen & Revsbech, 1983; Revsbech et Al., 1983). The oxic-anoxic interface moves up and down in diurnal cycles in photosynthetically active sediments. It is therefore advantageous for microorganisms utilizing chemical species found near the interface, to be motile, so that they can follow the interface when it moves. Many sulfide-oxidizing microorganisms are also mostly motile, (Niels Peter Revsbech, Janne Nielson & Pia Kupka Hansen 1988 ).
The factors controlling bacterial abundance in marine sediments are complex. It is a widely held view that bacterial abundance is directly controlled by sediment surface area. This relationship suggests that bacterial abundance is the result of density-dependent processes which are in turn regulated by particle surface area. Even though particle surface area may not be the primary factor involved in determining bacterial numbers, it appears that most of the controlling factors have some areal dimension. The areal dimension of greatest consequence appears to be only the simple planar surface area of a given particle. Other sedimentological parameters, such as the three-dimensional arrangement of particles, the topography of individual grains, and the distance between particles as a result of packing seem of secondary importance.
Protein enriched samples had consistently higher bacterial abundances than those untreated. Adsorbed protein may be important for the growth of bacteria as a nutrient and energy source. Protein as a renewable nitrogen source encounters loss in recycling, while as an energy source it is not renewable, (Yomomato N. & Lopez G. (1985).
There is a large collection of information on the table now from this article, and the one about energy. The heavy weight concepts and definitions are now on file for later review. Next we need to look at what this all means for using it in our aquarium. The type of sand for the sand bed is perhaps an important area to look at next. There's more than meets the eye. References Cited
 
van den Ende F.P. & van Germerden H. (1994) Relationships between functional groups of organisms in microbial mats, In Stal L.J. & Caumette P. (eds) Microbial Mats: Structure, Development, and Environmental Significance, NATO ASI Series G, Ecological Sciences, Vol. 35
 
Gamble S. (1993), Hurricane Andrew and the John Pennekamp Coral Reef State Park, FAMA, November.
 
Koike, I. & Sorensen J. (1988), Nitrate Reduction and Denitrification in Marine Sediments, In, Blackburn T. H., Sorensen J. (eds) Nitrogen Cycling In Coastal Marine Environments, SCOPE 33, John Wiley & Sons.
Revsbech N.P., Nielson J. & Hansen P. K. (1988), Bentic Primary Production and Oxygen Profiles, In, Blackburn T. H., Sorensen J. (eds) Nitrogen Cycling In Coastal Marine Environments, SCOPE 33, John Wiley & Sons.
 
Spotte S. (1992) Captive Seawater Fishes, Science and Technology. John Wiley and Sons, New York
 
Yomomato N. & Lopez G. (1985), Bacterial Abundance In Relation To Surface Area and Organic Content of Marine Sediments, L. Exp. Mar. Ecol., Vol. 90, pp.209-220.
 
 
Discussion & questions:
102170.3150@compuserve.com
 

 

Biography :
 
Sam GAMBLE has a B.S. degree in Marine Science (Biology). He has spent over five years in marine aquaculture growing penneid shrimp, Artemia, and many kinds of algae. For ten years his experience has included maintaining over 30,000 gallons of marine aquarium exhibits at John Pennekamp Coral Reef State Park. Recently his time has been devoted to research and developpement of products for aquariums made for natural systems like sand beds. Since 1991 he has published over 20 articles about aquarium subjects for magazines and news letters, which includes a monthly column on the Internet.
 
 
MARS © Copyright 1997 - All Rights Reserved