Introduction
The microalgae species Rhodomonas salina is known to be an excellent diet for many aquaculture species. I t has been shown that R. salina contributes to egg production, growth, survival, reproduction and lipid content of copepods, brine shrimps and scallops
and as specialty diet in the refinement of oysters
. The beneficial aspects of using R. salina as feed are attributed to their favourable content of polyunsaturated fatty acids (PUFAs), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are essential for the growth, survival and pigmentation of the aquaculture species mentioned above
. In addition, R. salina contains, besides the common pigments such as chlorophyll a/c and several carotenoids, the water-soluble pigment Cr-phycoerythrin 545 (PE) that harvests light in the green wavelength (λmax = 545 nm)
. PE is also used in food colouring and cosmetics
and linked to anti-parasitic and anti-tumour activities in studies using the marine mollusc Aplysia californica
. Optimization of R. salina. cultivation has been conducted in aspects of biomass production rate and biochemical composition, but no commercially stable cultivation plan has been applied in aquaculture yet
. The goal of our research was to optimize the biomass, fatty acid and PE producti on, by altering the nitrogen availability and the light quality.
Methodology
Two experiments were conducted in order to optimize R. salina production: 1. Effect of light wavelength on growth rate and biomass composition of R. salina. 2. Effect of nitrogen starvation on the lipid profile of R. salina. In both experiments R. salina was cultivated in 400 ml photobioreactors (Algaemist-S) , where temperature was stable at 22 oC and pH at 7.5, regulated by CO2 flow.
In the f irst experiment R. salina cultures were exposed to 50 μmolphotons m-2 s-1 of four different light wavelengths, blue (380-520 nm), green (520-600 nm), red (600-700 nm) , and warm light as reference. The photobioreactors were maintained in turbidostat mode with outgoing light of 15 μmolphotons m-2 s-1.
The second experiment was divided into two stages. In the first stage R. salina was cultivated under optimal conditions in turbidostat mode, which reached a dilution rate of 1.3 d-1. The turbidostat was maintained for more than a week to reach a steady culture. Samples were taken every 24 hours. In the next stage, the biomass was washed and transferred in a reactor filled with N- depleted medium. During nitrogen starvation, samples were taken over a period of 8 days. Samples were taken for biomass concentration, cell size, PE, and fatty acid composition.
Results
The results of the first experiment demonstrate that the highest productivity in volumetric biomass (0.20 gdry weight L-1 day-1) was observed under green light conditions. Blue and red light illumination resulted in lower productivities, 0.11 gdry weight L-1 day -1 and 0.02 gdry weight L-1 day-1 respectively. The differences in production could be ascribed to increased absorption of green and blue wavelength by phycoerythrin, chlorophyll and carotenoids, causing higher photosynthetically usable radiation (PUR) from equal photosynthetically absorbed irradiance (PAR). Moreover, phycoerythrin concentration (281.16 mg gdry weight-1 ) was stimulated under red light illumination. Because photosystemII (PSII) absorbs poorly red light, the algae had to induce more pigments in order to negate the lower absorption per unit pigment of the incident available photons. The results of this study indicate that green light can be used in the initial growth of R. salina to produce more biomass and, at a later stage, red light could be implemented to stimulate the synthesis of PE.
The results of the nitrogen starvation experiment demonstrate that the lipid content of biomass increased significantly from t=0h to t=120h from 6% to 28.2% of dry weight respectively. Furthermore, the highest increase within 120h was illustrated for C16:0, C:18:1, C18:2, C18:3. However, the maximum EPA and DHA concentration was observed after 48 hours of stress , while the maximum DHA to EPA ratio was detected at the end of the starvation.
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