Isolation of Australian microalgae and preparation of microalgal concentrates for use as aquaculture feeds

In this study, two types of microalgal feed were examined; 1) algal concentrates and 2) new Australian microalgae. Algal concentrates were examined because for aquaculturists, they could provide an off-the-shelf alternative to maintaining live algal cultures. Australian microalgae were examined to identify new feed species and to determine specific dietary requirements of local juvenile Pacific oysters (Crassostrea gigas). Ten diatoms, microalgae widely cultivated as feed for aquaculture species, were isolated and purified from local waters, Tasmania, Australia. Proximate analysis of logarithmic and stationary phase cultures showed major differences between species and in the effect of stationary phase on composition. Four species (Attheya septentrionalis, Entomoneis cf. punctulata, Extubocellulus spinifera and Thalassiosira oceanica) were evaluated as feed for juvenile Pacific oysters. Fed as the major component in a ternary algal diet, two algae (Attheya septentrionalis and Entomoneis cf. punctulata) supported over 80% of the growth of the T. pseudonana control. Centrifugation of eleven microalgae from a range of classes showed it to be an efficient (>80 % recovery) way to concentrate microalgae. Small chlorophytes survived the high shear forces best but, are documented poor algal diets for oysters. Of the diatoms Chaetoceros calcitrans and Thalassiosira pseudonana were least damaged. Juvenile oyster feeding trials showed that T. pseudonana pastes were capable of sustaining limited growth. However, nutritional deficiencies in the pastes were reflected in falling weekly growth rates, by the third week, oysters grew only marginally or lost organic weight. An alternative, low shear concentration process was developed based on chemical induced flocculation. Initial coagulation of algae was induced using Fe +3 or by increasing pH. Flocculated concentrates of T pseudonana were fed to juvenile Pacific oysters and compared to live T. pseudonana and centrifuged algal pastes. The pH flocculated algal diet was superior to all other test diets and lost nutritional value more slowly than centrifuged algal diets. In the discussion the nutritional composition of algal diets is related to the composition and nutritional requirements of the juvenile oysters. It is argued that the dietary protein content of live algal diets, not limited in essential nutrients, is the most significant factor in determining a nutritionally superior diet. The nutritional value of algal concentrates is discussed and flocculation is concluded to be a superior method to produce concentrates. It is shown that flocculation is applicable for concentrating many algal species and argued that it is a cost-effective process with commercial applications.