Carbon Dioxide

Nitrogen is the most abundant gas in aquatic systems, but oxygen and CO₂ concentrations are the most dynamic because of manipulation by biologic activity. In an ecologically balanced system, oxygen and CO₂ tend to be inversely related in terms of their activity because one is constantly being exchanged for the other between plants or photosynthetic protoctists (e.g., algae, cyanobacteria) and animals, which has resulted in many ocean symbioses; thus, this effectively using light energy to drive chemical equilibria and stability in the system.¹ Probably the most important part that CO₂ plays in the aquarium setting is its role in the carbonate alkalinity system, which controls the pH level and its stability; this system is crucial to maintaining organisms’ health. Other factors affecting this system include carbonic acid, carbonate, bicarbonate, and hydrogen ion levels. This is especially true in marine environments, in which animals have adapted to a very stable pH and narrow pH range.

CO₂ reacts with water to form carbonic acid (H₂CO₃), which dissociates to bicarbonate (HCO₃⁻) and then carbonate (CO₃²⁻) and hydrogen ions (H⁺), chemically proportioned by temperature, pressure, and salinity equilibria²⁴:

Nitrifying and some denitrifying biologic water treatments use autotrophic microorganisms that metabolize the carbon from the carbonate system into biomass and hydrogen ions, driving the alkalinity and pH of the system downward. The mineralizing (biologic processes that convert organic to inorganic substances) and some denitrifying heterotrophic bacteria involved in organic decay in the system metabolize their carbon from biomass and respire CO₂ along with the fish and invertebrates, which easily becomes excessive in a typical aquarium with a high bioloading. Another source of excess CO₂ in systems is from rain water, which usually has at least a moderately acidic pH because of interactions with the atmosphere, even if unpolluted.

Too rapid an input of CO₂ relative to photosynthetic activity and gas exchange in a system results in an accumulation of CO₂ and H₂CO_3, increasing the H⁺ concentration and decreasing pH, which may be alleviated with vigorous gas exchange. A simple jar test for this in the laboratory is to stir a water sample from the system vigorously with a vortex while monitoring pH and maintaining system temperature. An increase in pH indicates inadequate gas exchange in the system relative to total CO₂ input. If inadequate gas exchange cannot be easily addressed, the addition of sodium hydroxide (NaOH) to a system may help drive the conversion of excess H₂CO₃ from dissolved CO₂ to bicarbonate:

It also converts bicarbonate into carbonate¹³:

Extreme caution must be taken not to overdose with this artificial method, which would result in too great or too rapid a pH change for the animals. We have found that adding it too rapidly to a seawater system by pipe injection may result in the precipitation of calcium carbonate, rapidly giving the exhibit water the appearance of skim milk. A more natural balancing method is to add photosynthesis to the treatment system and/or exhibit with aquatic plants and/or photosynthetic protoctists, although this approach should be in conjunction with adequate gas exchange for good pH balance.

Denitrification

In cases in which water changes are not feasible or at lower levels than desired, denitrification is the process of turning nitrate into nitrogen gas through anaerobic microbial action. An additional carbon source is needed through the use of methanol or sulfur, which helps facilitate the autotrophic denitrification process in which the carbon issued is from the CO₂. These are complex processes and denitrification should not be attempted without experienced staff because toxic elements may be produced and released into the main water system if the procedure is not performed correctly. It is important to note that our understanding of bacteria in the nitrogen cycle is in its infancy; once more information is gained, we could significantly alter our water-conditioning capabilities. For example, as recently as 2002, it was discovered that bacteria could perform anaerobic ammonium oxidation (anammox) to N₂ gas. These four genera of anammox bacteria were identified as Brocadia, Kuenenia, Scalindula, and Anammoxoblobus and are responsible for 24% to 67% of nitrogen loss in marine systems.⁹ They may be grown autotrophically with CO₂ as the only carbon source, thus eliminating the complex dependency on several bacterial pathways, including anoxic denitrification using methanol or sulfur. It is currently being applied in industrial use and is highly experimental but could be used in zoo and aquarium settings. We would then not have to depend on complex biologic filtration systems, which are prone to failures.