Environmental effects on the growth and biochemical composition of four micoalgae, in relation to their use as food for Mytilus edulis larval rearing
Environmental conditions in form of light intensity, phosphorus and nitrogen limitation were used to manipulate the biochemical composition of continuous cultures of Skeletonema costatum, Chaetoceros muelleri, Rhinomonas reticulata and Pavlova lutheri. Crude protein, carbohydrate and chlorophyll content as well as the fatty acid profile was determined in the combinations of two light intensities (high and low light, HL and LL) and three nutrient conditions (no nutrient limitation, f/2, phosphorus limitation, P, and nitrogen limitation, N). They were fed to Mytilus edulis larvae over a two week period and the larval size and mortality were assessed; the larval fatty acid profile of various batches of eggs as well as after the end of the feeding trial was also determined. A novel computer aided image analysis technique was used for measuring the length ofthe larvae. All monospecific diets supported good growth, sometimes equal or better to a control diet which was a mixture of species (R. reticulata and P. lutheri). In general survival was not affected by the diets and was found to be related more with the specific batch of larvae used. On the contrary growth was correlated with the diet. Ranking of the S. costatum diets was: LL N =LL f/2 =LL P =HL N >Control >HL f/2 =HL P. The C. muelleri diets were ranked as: LL N =LL f/2 >Control >HL f/2 =HL P =HL N >LL P. The R. reticulata diets are ranked, again in decreasing quality order as: HL N =LL f/2 =Control >LL N >HL f/2 =HL P =LL P. The P. lutheri ranking order was: HL N =HL f/2 =HL P =LL P >LL N =LL f/2 >Control. The larvae were analyzed for their fatty acid profile and relative content and some fatty acids were significantly correlated with growth thus enabling the usage of certain fatty acids as an index of growth for M. edulis larvae. Larval 20:5(0:3 and Polyunsaturated fatty acids (PUFA) were a positive index of growth while 15:0 and Saturated Fatty acids (SaFA) were a negative index. A multidimensional model was used in an effort to correlate algal biochemical components with larval growth. Some fatty acids were found to be the main factors in determining the algal biochemical composition with protein and carbohydrate playing a secondary "modifying" role. In the case of P. lutheri the 16:0 and SaFA were positively correlated with larval growth in an almost linear fashion while omega: 3 fatty acids were negatively correlated with larval growth. A positive correlation concerning the 16:0 and a negative one for the PUFA was also established in S. costatum and R. rcticulata.