Understanding photoautotrophic, autotrophic, and heterotrophic bacterial based systems using basic water quality parameters

Many of the current greenhouse based intensive marine shrimp production systems have used photoautotrophic algae systems to control inorganic nitrogen accumulation. A second strategy of nitrogen control used in some indoor intensive recirculating production systems is using large fixed-film bioreactors. This option utilizes chemosynthetic autotrophic bacteria, Ammonia Oxidizing Bacteria (AOB) and Nitrite Oxidizing Bacteria (NOB) for the nitrification of ammonia-N to nitrite-N and finally to nitrate-N. Recently, zero-exchange management systems have been developed which encourage heterotrophic bacterial growth. These systems have been promoted for the intensive production of marine shrimp in both ponds and raceways. In this system, heterotrophic bacterial growth is stimulated through the addition of carbonaceous substrate. At high carbon to nitrogen (C/N) feed ratios, heterotrophic bacteria will assimilate ammonia-N directly from the water and produce cellular protein. A review of the literature on zero-exchange systems yielded little description of the pathways of ammonia removal employed and whether the removal was fundamentally photoautotrophic, autotrophic or heterotrophic bacterial based or in reality some mixture of the three. The fundamental stoichiometry of the three pathways is reviewed here and the impact on water quality for each summarized. By keeping in mind the fundamentals of the three predominant pathways for ammonia-N assimilation in aquaculture production systems, it is possible to track which pathway is dominating at critical times during startup and production phases. In addition, by manipulating key water quality parameters and ambient conditions, it becomes possible to guide a system towards a specific treatment process. For example, heterotrophic systems can be selected for by adjusting the carbon/nitrogen ratio of the feed or through supplemental addition of carbon in the form of carbohydrates, e.g., sugar. These pathway indicators were then examined in a series of replicate research trials conducted at the Conservation Fund’s Freshwater Institute examining water quality in juvenile marine shrimp, Penaeus vannamei, production tanks.