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by Carrie Holl, Ph.D., Oceanic Institute


The Shrimp Department’s Microbial Ecology Group had an eventful year, and 2007 promises to be equally as productive. We finished 2006 by completing the ATP recirculating aquaculture project.

Our portion of the project was to understand how the microbial community can facilitate closed or recirculating aquaculture by its capacity to cycle nitrogen and carbon. The removal of ammonia and nitrite, which are toxic to shrimp, is crucial for animal health. Therefore, we spent a considerable amount of time quantifying the incorporation of tracers (15NH4Cl or Na15NO2) into algal and heterotrophic biomass, and measuring the rates of bacterial ammonium and nitrite oxidation. We found a very nice, linear increase in nitrite oxidation rates with time throughout the shrimp growout, which is likely in response to increases in ammonium oxidation rates with time.

We also continued to measure the natural abundance stable isotope signatures of water column particles and of shrimp and shrimp feed. From these measurements we have determined that shrimp are probably not using the natural microbial community as a significant source of nutrition, at least not at high-stocking densities in our concrete raceways, but these analyses are ongoing.
We began 2007 with a trial testing the effects of light intensity on the ammonium oxidizing capacity of the microbial community. We grew shrimp in replicated light and dark raceways for three months and measured ammonium uptake and oxidation rates in both the light and the dark. These measurements were made during the daytime a few times each week. We also tested the hypothesis that in the light raceways phytoplankton ammonium uptake would be dominant during the day and bacterial ammonium oxidation would dominate at night, sort of a temporal “niche partitioning” within the water column. In order to explore this hypothesis we also measured ammonium oxidation at night several times throughout the trial.

At first glance it looks like our hypothesis was probably right but we have a lot of data analysis to finish up before we can definitively say that our hypothesis was supported by the data. Also, it seemed that the microbes in the dark had lower rates of ammonium and nitrite removal but not as low as we might have predicted from past light/dark trials. The shrimp, surprisingly, did quite well in the dark. Growth rates were higher in the light treatments which meant that by the end of the trial the shrimp were a little bigger in the light (about 3 grams) but the dark treatments had slightly higher survival which means that shrimp production was not significantly different between the light and dark treatments and, interestingly, the shrimp were quite tasty in both treatments!

Later this year we have made plans for another growout trial again designed to explore how the microbial community, which is essential for closed, zero-effluent aquaculture, responds to an environmental change. As always we continue to encourage students to participate in our research as interns. The work is fun and the benefits are tasty.




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