SIMPLIFIED METHOD TO MEASURE Isochrysis galbana CELL DENSITY IN THE HATCHERY  

The flagellate, Isochrysis galbana Parke 3011, is a free living marine unicellular phytoflagellate of the order Chrysomonadales, which is rich in polyunsaturated fatty acids, such as eicosapentaenoic (EPA) and docosahexaenoic (DHA), that are of nutritional value in the rearing of various bivalve larvae. Therefore I. galbana is widely used in the aquaculture industry. In some hatcheries in China, workers usually rely on the color intensity of the microalgae culture to determine the amount of food supplied to any given larvae husbandry. Surprisingly, information regarding the relationship between the microalgae density and color is scarce. Thus the aim of this research was to develop a simple and rapid method to indirectly calculate the microalgal concentration by describing the relationship between color parameters and cell density of I. galbana.

An 10-days culturing trial using 0.1μm filtered seawater(S=30) in 5L flasks was conducted, at 26℃ and constant illumination (5000 lux) measured at the surface of the flasks, as well as 30 L/min constant ventilation. Density and color parameters were measured everyday. Microalgae density in the samples was calculated by counting the cells using a blood count plate under the microscope. Sample color parameters were then measured in a Spectrophotometer CM-5 (KONICA MINOLTA, INC. Japan). Color parameters consists of L*a*b units, and △E* that was calculated automatically. The L* parameter corresponds to the degree of luminance or lightness, ranging from 0 to 100, where L*=0 yields black and L*=100 indicates white. The parameters of a* and b* are the two chromatic components, in which +a* is red, -a* is green, +b* is yellow, -b* is blue, both a* and b* values ranging from -120 to 120. The color differences △E* of the various concentrations were calculated using FSW as reference to zeroed the spectrophotometer. All correlations were performed using the software DPS (Ver. 16.05)

The relationships between culturing time and cell density are shown in Table.1, as well as color parameter. Results shown a strong relationship between the culturing time and the cell density, L*, a*, b*, as well as △E*. This simplified method can be used to other species of microalage to indirectly calculate the microalgal concentration.