48 June 2012 Effects of Metabolic Waste Products - CO2, NH3 and NO2 Carbon dioxide In intensive aquaculture, particularly those relying on partial or full recycling of seawater within the production system, the waste compounds that cause most negative effects on growth and development are normally CO2 and NH3 (in that order). During aerobic metabolism CO2 is produced at a rate of 0.7 to 1.0 gram per gram O2 consumed. The effect of CO2 on sea urchins and other marine organisms has recently attracted attention due to ocean acidification caused by anthropogenic CO2 release. It is predicted that the surface pH of the sea will drop by 0.4 units to 7.6 - 7.8 by 2100. Several studies have demonstrated that reducing pH to such values will have negative effects on fertilization and development of sea urchin larva. Downregulation of genes (reduced activity of enzymes) involved in calcification and metabolism has also been demonstrated (Moulin et al. 2011). In contrast, the effect of CO2 on sea urchin culture has not been adequately studied. Test growth of Paracentrotus lividus is completely inhibited when the partial pressure of CO2 is elevated five to nine times above the normal level in seawater (Grosjean et al. 1998). An increase in CO2 concentration from 1.1 to 18.1 mgL-1 leads to a 67 percent decrease in gonad growth of adult S. droebachiensis (Siikavuopio et al. 2007a), a decrease attributed to reduced feed intake and less efficient feed conversion. The explanation for the negative effect of increased CO2 on test growth is probably that CO2, through its pH effect, impairs the precipitation of bicarbonates into calcium carbonate, which is the main constituent of the sea urchin test. Fortunately carbon dioxide is easily stripped from water in recirculating aquaculture systems using trickle towers or other methods to degas the CO2. In flow-through or sea-based systems, CO2 is unlikely to be an issue in the immediate future. Ammonia Ammonia excretion by sea urchins has been the subject of several ecology-oriented articles, but there is a paucity of studies looking at the effects of elevated ammonia concentration on sea urchin growth. Sea urchins excrete several nitrogenous compounds, but the main excretory product is ammonia (NH3). The release of NH3 varies according to the substrate used for energy metabolism. Brockington and Peck (2001) found an O:N ratio of 7 in Sterechinus neumayeri during the onset of the austral summer when sea urchins use protein as their main source of energy. A ratio of the same magnitude may be expected for cultured sea urchins, which are fed diets with higher protein content than normal natural-food diets. As is the case with CO2, accumulation of NH3 is generally not a problem in flow-through culture systems but may be a problem in recirculating systems, depending on the degree of water reuse and the ammonia tolerance of the specific urchin species. Siikavuopio et al. (2004a) tested the effects of different ammonia concentrations on gonad growth of S. droebachiensis, and found that gonad growth was significantly reduced at an un-ionized ammonia (UIA) concentration of 16 µg/L during a 43-day experiment. At UIA levels of 32 and 68 µg/L, mortalities of 45 and 76 percent were recorded. A surprising result in this experiment was that feed intake was not significantly affected by increasing concentrations of UIA, meaning that reduced gonad growth was mainly the result of reduced feed conversion efficiency. S. droebachiensis has a low tolerance for UIA compared to many cultured marine fish species and invertebrates. Nitrite and Nitrate In recirculating systems, ammonia is oxidized to nitrite (NO2) and then nitrate (NO3) by autotrophic bacteria in biofilters. While nitrate is a relatively non-toxic compound, nitrite may cause problems if it accumulates. Gonad growth of S. droebachiensis is significantly impaired at a NO2-N concentration of 0.55 mg/L, which is relatively low compared to many species of fish and invertebrates (Siikavuopio et al. 2004b). As occurs with ammonia, increasing levels of nitrite does not affect feed intake, and there was no mortality during the 42-day experiment, where the highest concentration of NO2-N tested was 10 mg/L. Phosphate Phosphorus is normally found in minute quantities in aquatic environments. In natural seawater, dissolved phosphorus, which exists predominantly as inorganic phosphate, is usually found at very low (< 1 uM) concentrations (Barr et al. 2008). Concentrations of phosphate may be greatly elevated in intensive aquaculture systems, particularly those where there is very little water replacement and where phosphate-rich manufactured feeds are used. However, there is limited research on the specific effects of elevated phosphate on any aquaculture species. A phosphate concentration of 1.25 µM has an inhibitory effect on the rate of in vitro calcium carbonate crystallization in both the marine bivalve Rangia cuneata and the freshwater gastropod Helisoma duryi over a 24-hr period (Bernhardt et al. 1985). Exposure to elevated phosphate concentration for one month has an inhibitory effect on shell growth and causes increases in mortality in both species. Exposure of the sea urchin Lytechinus variegates to phosphate concentrations greater than 1.6 mg/L (17 µM) has a negative effect on urchin growth and righting behavior and inhibits feeding, fecal production and nutrient absorption (Böttger et al. 2001). There is a significant negative impact on growth (both shell length and wet weight) when small abalone Haliotis iris are exposed to phosphate concentrations greater than 10.7 µM and for larger abalone when exposed to phosphate concentrations greater than 60.9 µM. However, elevated phosphate concentrations do not significantly impact abalone survival (James and Barr 2010). Inorganic phosphate should be maintained below 10.7 µM to optimize growth of abalone in intensive aquaculture systems and this recommendation likely applies to other benthic invertebrates such as sea urchins.
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