Influence of two different green algal diets on specific dynamic action and incorporation of carbon into biochemical fractions in the copepod Acartia tonsa

Previous studies have shown that the two green algae Tetraselmis sp. (Prasinophyceae) and Dunaliella tertiolecta (Chlorophyceae) induce high and low egg production rates in Acartia tonsa. The primary goal of the present study was to investigate if this is attributable to differences in the specific dynamic action (SDA) of the two diets. Secondly, we wanted to investigate if any qualitative differences in the incorporation of nutritional constituents from the two diets are influencing SDA. The functional response of ingestion was very different with the two diets. Ingestion of T impellucida was relatively high even at low food concentrations with a maximum of 19 mug C ind(-1) day(-1). The functional response was more clearly sigmoidal on D. tertiolecta with a maximum of 7.3 mug C ind (1) day(-1). The higher ingestion rate of T impellucida also induced higher respiration rates. Maximum respiration rates were 3.0 nl O-2 ind (1) min (1) on T. impellucida and 1.5 nl O-2 ind (1) min(-1) on D. tertiolecta. This created significantly different SDA coefficients: 0.19 on T impellucida and 0.06 on D. tertiolecta, which implies that the magnitude of SDA is strongly influenced by the composition of the diet. The incorporation of carbon into lipids was significantly higher on D. tertiolecta. However, because of lack of longer chain fatty acids in D. tertiolecta the copepods did not benefit from this. Thus, the proportion of carbon allocated to egg lipids was much lower than when feeding on T. impellucida. Acartia tonsa incorporated relatively more carbon into proteins when feeding on T. impellucida than on D. tertiolecta. Since protein synthesis is energetically very demanding this is probably the reason for the higher SDA coefficient in those feeding on T. impellucida.