Sterol dynamics of heterotrophic Tetraselmis suecica and its nutritional implication in the bivalve aquaculture

Employment of heterotrophic culture of microalgal foods in the hatchery-based seed production of oysters is still controversial because some algae produced using the method appear to loose sterols, a key nutritional factor for bivalve growth. We traced the changes in sterol content of Tetraselmis suecica growing under photoautotrophic and heterotrophic conditions with the aid of gas chromatography (GC) and GC-mass spectrometry. The photoautotrophic T. suecica at the mid-logarithmic growth phase contained six major (cholesta-5,22-dien-3(beta)-ol, ergost-5en-3(beta)-ol, cholest-5-en-3(beta)-ol, 24-methylcholesta5,22-dien-3(beta)-ol 24-methylcholesta-5,24-dien-3(beta)-ol, and 24-ethylchlolesta-5,24-dien-3(beta)-ol) and two minor sterols (24-methylcholesta-5-en-3(beta)-o1 and 24ethylchlolesta-5en-3(beta)-ol). In the comparison of algal growth and sterol level, photoautotrophic alga appeared to need higher amounts of major sterols for cell growth over heterotrophic alga. These findings, that heterotrophic alga needed less amount of sterols for growth, may have significant implications in the introduction of the method in bivalve hatcheries. We also reviewed the sterolic properties of the alga obtained from heterotrophic method with respect to bivalve aquaculture.