Effect of intermittent agitation on the productivity of the marine diatom Chaetoceros gracilis

The demand for global fisheries protein source continues to increase with the increase of the worldʼs population, causing decrease of natural fishery resources due to the overfishing and the degradation of habitats. Under these circumstances, the fisheries industry for marine products has rapidly grown in recent decades to promote the stable production and utilization of fishery resources while managing and conserving them. Mass cultivation of microalgae is essential for efficient production of artificial seedlings of bivalves whose resources are rapidly decreasing. Diatoms can provide high utility as a primary feed because they contain a high content of fucoxanthin, a carotenoid pigment. The marine diatom, Chaetoceros gracilis which can accumulate fucoxanthin, is a promising feed for fishery products. However, marine diatoms are known as a taxonomic group in which stable cultivation is difficult, and it is necessary to establish a cultivation method that enables high-density cultivation. In previous studies, the factors involved in the growth of marine diatoms have been investigated by controlling the external environment such as light intensity, water temperature and nutrient concentration. However, the establishment of high-density cultivation for marine diatoms under each optimal environmental condition has still not been achieved. A problematic point in the high-density cultivation involves the heterogeneity of the light intensity, water temperature, and nutrient concentration in the culture tank, which limits the growth of algae. Previous studies have reported that “agitation” of the culture solution has the effect of promoting the supply of CO2, as a carbon source, and the removal of dissolved oxygen by equalizing the internal environment of the culture tank, and is essential to improve the productivity of microalgae. However, the performance of using the agitation method remain to be solved in mass culture operation. Intermittent agitation frequency is one of the solutions for low-energy cost operation. There is no previous research comparing biomass productivity per unit of agitation energy input between the same species using both continuous and intermittent agitation methods. The objective of this study was to evaluate the productivity per aeration energy under different agitation frequencies toward high-density cultivation of marine diatom C. gracilis. C. gracilis (UPMC-A0010-2) isolated from Malaysian coastal waters was semi-continuously cultured using modified Conway medium in 1.2-L bubble column reactors (n=2). The culture was conducted under stable light intensity 300 μmol m-2 s-1 (12 hours cycle of light and dark) and temperature (25oC). The dilution rates were adjusted at 0.3 d-1. The aeration rate in each reactor was set to 0.2 L min-1 (2% CO2), and aeration frequency was set to the following three conditions: (1) continuous agitation condition, (2) intermittent agitation condition once 9 every minutes, and (3) intermittent agitation condition once 18 every minutes. The cultivation period was a stationary phase of the biomass for up to 5 days, and the absorbance (750 nm) and pH of the culture, and the dry weight were measured. The biomass productivity per algal volume was equivalent to that under continuous agitation conditions even with relatively high frequency of intermittent agitation once 9 every minutes. This may be caused by the high light utilization efficiency of the cells because the cells in the culture are distributed on an average without sedimentation even after the aeration pause period without agitation. The biomass productivity per unit of agitation energy input in the intermittent agitation conditions showed a higher value than the continuous agitation condition by several times. Intermittent agitation performance can be evaluated as an environmental control technology that can maintain biomass productivity equivalent to continuous agitation conditions. The reduction of energy consumption by intermittent agitation is expected to significantly contribute to the reduction of operating costs for outdoor closed-system reactors.