The Plenum Method

(This article written especially for my good friend Todd Schwartz)

To better understand the value of sandbeds, and possibly the Jaubert Plenum method, its necessary to look back in time when the reef portion of the hobby first began in the mid 80’s. That’s actually when more attention began to be paid to nitrate accumulation. A few years later the Berlin method came about and utilized live rock and efficient protein skimming to help lower nitrate concentrations, and it was a major steppingstone in the field of aquarium husbandry. A few years later an intriguing technique devised by Dr. Jean Jaubert and aptly called the Jaubert Plenum Method began to draw much attention. Both methods appeared to reduce nitrate accumulation, but ‘exactly’ how was not fully understood by most aquarists.

In 1992 I established my first Jaubert system and over the following six years with the help of marine scientist Sam Gamble it became evident there was much to understand why there was a major nitrate reduction with this method! Research went into the microbial processes in all types of sandbeds, whether in the wild or closed systems. Different depth beds consisting of different grain size material were also explored. One of the many things learned was that more focus needed to be applied to several classes of bacterium that inhabited the sandbed.

Among the more commonly known important microbes are the aerobic heterotrophs that live in oxygen rich areas and breakdown organic matter, such as some waste products and/or dead animals. The inorganic results, i.e., ammonia, is then utilized by aerobic autotrophs living in the upper level of substrate, which reduce it to less toxic substances such as nitrite, then nitrate. The resulting nitrate is then acted upon in a lower area of the substrate (sand or rock) that contains little or no oxygen, generally referred to as the anaerobic area. And it is there we found two different classes of bacteria going two different paths, one reducing nitrate to only that of ammonium (the primary alga nutrient), and the other reducing it to nitrogen gas. This caused us to question the long standing aquarist understanding of the denitrification process, as I and many other aquarists always thought that with nitrate being an alga encouraging nutrient and/or water quality issue, its reduction in ‘so-called’ anaerobic areas would ‘always’ result in it being reduced to nitrogen gas and harmlessly dissipating upwards through the substrate and water column.

But what we found was that a far more important nutrient to algae than nitrate, i.e., ammonium, was being produced in this ‘anaerobic’ area. And this ammonium could diffuse upward in the substrate/sediment and enter the bulk water or again simply be oxidized back into nitrite/nitrate, which may also diffuse upward into the bulk water! Therefore, it became necessary to look more closely at what was called the ‘anaerobic’ area and define the logistics associated with its bacterium.

Further research showed that facultative anaerobic heterotrophs live in an area that contain a small amount of oxygen, i.e., approximately 0.5 - 2.0 mg/l (per Sam Gamble and defined in our previous writings as the ‘anoxic’ area). They generate dissimilatory denitrification where nitrate is reduced to its basic elemental form — nitrogen gas. In an area of less oxygen content, more precisely called the anaerobic area, obligate anaerobic heterotrophs existed and the ‘end result’ of their process, technically called assimilatory denitrification, is ammonium. This is generally referred to as the ammonification process. And a continuing reprocessing of this ammonium in the lower ‘anaerobic’ level of the substrate back into nitrite/nitrate in the upper reaches of the substrate is quite feasible, with any of them, i.e., ammonium, nitrite, nitrate, possibly leaching back into the bulk water!

With that information in hand and the results from many different ‘plenum’ installations, it was found that the space under the grid, called the plenum, actually contained a small amount of dissolved oxygen, usually somewhere between .5 to .8 mg/l oxygen. And that was sufficient enough to keep the majority of the bed above in the ‘anoxic’ state/condition. This meant only facultative anaerobic heterotrophs were living in about 90% of the plenum bed, (from about one half inch on down the bottom of the bed) and actually converting nitrate that diffused downward from above to nitrogen gas! No ammonium was being produced! Of course, this doesn’t happen in a bed directly on the bottom of the aquarium!

Another factor to consider is there must not only be proper oxygen gradients to have an efficient bed, they must be accompanied by the proper sequence of electrical charges for efficient use of energy. As for the electrical charge, which is measured in millivolts (mV) with an ORP meter, which most aquarists are already familiar with it as its why protein skimmers work as they do, i.e., the positive charge on nutrients is attracted to the negative charge of its air bubbles, hence collecting/binding them together and removing them in the collected foam. Keep in mind that any type sandbed is a chemical sink where the diffusion of nutrients to and through it is also a function of an electrical charge.

It’s a fact the water’s surface and the air above it is a negative mV. (That is the reason why supplying water from near the aquarium surface to a skimmer is far more effective than from deeper levels, as those nutrients are naturally closer to the water’s surface to satisfy their charges!) In the bulk water there is many charged molecules with much of it a positive mV. So is most of the living biomass, e.g., corals and fish. Substrate surfaces are largely a negative mV. The sandbed itself is negative with increasing magnitude with depth. The deeper the sand, the more negative it becomes and the more nutrients are attracted to lower depths. The bottoms of beds directly on the aquarium bottom are a dead end for nutrients because that is where the greatest negative charge occurs.

Even though plenum ‘bed’ charges become more negative with depth, the fluid in the plenum itself has a less negative charge than the sand above it because of its oxygen content! Therefore, those various nutrient accumulations sometimes seen in the plenum (some seen to correspond with overfeeding, etc.) are consistently being ‘attracted back’ to the bacteria living on the sand particles above (because there’s a higher negative charge there), or what could be called a natural supply and demand process! It seems like the space (plenum) acts like a temporary reservoir and helps prevent buildup of nutrients in the bulk water from possible overfeeding or other temporary mishaps in systems with many hiding places. How much accumulates in the plenum seems to depend upon system bioload, and moreover, we know of no leaching of these excesses back into the bulk water. An amazing process and only found in plenum beds!

Another interesting research fact, one with deep and shallow beds directly on the aquarium bottom, was they are apt to counter a certain process/chemical compound (enzyme) that is advantageous to the efficient use of available energy. In fact, nitrogen fixation — the utilization and production of energy, can be thought of as hinging upon the production of the enzyme ‘Nitrogenase,’ and that oxygen controls several important aspects of the nitrogenase activity.

In shallow bed systems without plenums, especially in fine grain beds, oxygen decreases rapidly with depth. In this type environment microbial metabolism uses most of the oxygen within a few millimeters of the sediment-bulk water interface. This condition can also be compounded by the result of microbial activity under raised nutrient loads, which significantly decrease oxygen further. In either situation decreased oxygen quickly falls below the anoxic level needed for the more energy efficient destructive denitrification process. This less efficient shift, now into an anaerobic condition, can cause storage of nutrients via assimilatory denitrification (reassembly of ammonium, i.e., ammonification). In view of the fact that nitrogenase is extremely oxygen sensitive, it does best in fully aerobic and anoxic zones. To limit these areas or their overall area volume would logically reduce nitrogenase production. In such a case there would be decreased dissimilatory denitrification and possibly increased nutrient storage (nitrogen compounds – which by the way can be considered green colored – algae!).

Ammonium also effects nitrogenase production. Experiments have shown ammonium additions cause a rapid reduction of its activity. Therefore it appears there is a need for a barrier or distance between the mineralization/nitrification zone and that of the destructive denitrification zone. To increase the depth of the oxic-anoxic layer or zone, as with the plenum method, is very beneficial to efficient denitrification by further separating the negative influence of ammonium on nitrogenase.

Since ammonium inhibits nitrogenase production it can also be surmised that nitrogenase inhibition by ammonium is a likely nucleus for unwanted plant growth. Think about that for a moment! In deep beds directly on the aquarium bottom most of its volume is anaerobic where ammonium is a nitrogen product continuing to be recycled over and over! It may fit a situation where persistent algal growth in some aquariums continues in spite of low nutrients (nitrate) in the bulk water. Keep in mind algal growth is a form of nitrogen storage (incorporation).

In my opinion, if a closed system, no matter what its physical size (home or public aquarium), contained more anoxic area than anaerobic area as defined above, its bulk water may possibly contain less inorganic nitrogen laden products. And one must keep mind that these substances are often seen as ‘green’ alga, as they are quickly incorporated into its structure. Now armed with this information I often look backwards and wonder if aquarists have miscalculated the value of deep sandbeds and/or the use of an excessive amount of live rock to reduce ‘nitrate’ concentrations, as both substances contain the same processes. In fact, a question often asked of me is why is there increasing amounts of unwanted green algae in some reef aquaria where nitrates appear low? Answer; a possible ammonium source from a deep bed or too much live rock, as its non-detectable and quickly incorporated into the color ‘green.’

The above leads me to believe it’s important for aquarists to give more thought to the ‘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, its wise in my opinion to concentrate on how to enlarge the volume of anoxic zones and reduce the volume of anaerobic zones. And some possibilities to consider would include shallow sandbeds and use of course grained sand, i.e., 2 - 5 mm. Also, minimizing the use of live rock should be considered. The Jaubert plenum method is another approach as it helps maintain the greater portion of its sandbed in an anoxic condition. In fact, it has been used in both public and private aquariums worldwide with great success, either as part of a new installation or in an interconnected system.