Utilising microalgal mixotrophy to maximise larval nutrition

Mussel aquaculture is a low carbon source of protein and the mussel aquaculture industry in Scotland aims to double production. Reliance upon wild spat collection means that the industry is currently 'at the mercy' of spatially and temporally variable supply, with only 40% of spat collection sites producing sufficient larvae. One solution to this issue is to develop a larval hatchery; however, the requirement by larvae for large quantities of live microalgae results in high costs, relative to the value of spat. In this study, three microalgal species: Tetraselmis suecica, Phaeodactylum tricornutum and Cyclotella cryptica were screened for mixotrophic or heterotrophic growth with glucose, glycerol or acetate. These strains had a shift in the fatty acid profile in favour of shorter chain, saturated fatty acids and an increased carbohydrate content, depending upon strain, carbon source and harvesting time. Isotopically labelled glucose and inorganic carbon indicated that there was an increased partitioning of inorganic derived carbon into total fatty acids (4.19 % dry weight compared to 2.13 %) in autotrophic cultures, alongside lipid membrane remodelling in T. suecica. Other cost reduction strategies were trialled including media optimisation, screening for growth of P. tricornutum with crude glycerol and quantifying changes in the nutrient removal from the media. Utilising the identified strains and culture conditions, these microalgae were successfully used as feed for mussel larvae, both as single species and mixed diets. These diets performed as well as an 'industry standard reference' diet in terms of larval growth and survival. Furthermore, optimisation of the mixotrophic diet to account for increasing larval size resulted in both maximum larval growth (174.35 µm) and total fatty acid content (5.84 % dry weight). First order modelling indicated that mixotrophic culture of microalgae may reduce the costs of microalgal culture for a larval hatchery. The ability to maximise cellular density and tailor the biochemical profiles of these mixotrophic algae to develop 'designer' feed has potential to expand the uses of microalgae to other aquaculture species and the biotechnology industry.