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The cryptophyte Rhodomonas sp. is a potential feed source for aquaculture live feed and resource for phycoerythrin (PE) production. This research investigates the influence of light, both quality and quantity, on the biomass productivity, composition and growth rate of Rhodomonas sp. The incident light intensity used in the experiments was 50 μmolphotons m−2 s−1, irrespective of the colour of the light, and cultivation took place in lab-scale flat-panel photobioreactors in turbidostat mode. The highest productivity in volumetric biomass (0.20 gdry weight L−1 day−1), measured under continuous illumination, 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 g L−1 day−1 respectively. The differences in production are 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 gDW−1) was stimulated under red light illumination. Because photosystem II (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 Rhodomonas sp. to produce more biomass and, at a later stage, red light could be implemented to stimulate the synthesis of PE. Fourier-transform infrared spectroscopy (FTIR) analysis demonstrated a significant difference between the cells under different light quality, with higher contents of proteins for samples of Rhodomonas sp. cultivated under green light conditions. In comparison, higher carbohydrate contents were observed for cells that were grown under red and blue light.

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The microalgae Rhodomonas sp. is known as an excellent feed source for live feed organisms such as copepods. The main benefits of feeding Rhodomonas to live feed animals are attributed to the relative high polyunsaturated fatty acid (PUFA) level, the combination of containing both docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), and the ratio between these fatty acids (FA). It has been shown that microalgae are able to accumulate valuable metabolites, such as lipids, under adverse conditions. The easiest and most inexpensive method to induce stress to microalgae is through nitrogen (N) starvation. In this study, the effect of N-starvation on biomass concentration, cell volume, and cellular composition, such as fatty acid concentration and composition, and phycoerythrin (PE) concentration of Rhodomonas sp. during a period of 8 days, was investigated. The research was divided into two stages. In the first (growth) stage, Rhodomonas sp. was cultivated in small 400 ml photobioreactors (Algaemist-S) under optimal conditions in turbidostat mode, which reached a biomass concentration of 1.5 gDW L −1 and dilution rate of 1.3 d −1. Samples were taken every 24 h for cell density and volume and productivity measurements in order to ensure a healthy and stable culture. In the next stage (N-starvation), the biomass was washed and transferred in a reactor filled with N-depleted medium. During N-starvation, samples were taken for biomass concentration, cell volume, PE and FA composition. The results of this study demonstrate that the lipid content increased significantly from 9% (t = 0 h) to 30% (t = 120 h) of the dry weight. After 120 h of N-starvation, the total FA content of Rhodomonas sp. remained stable for the remainder of the experiment (next 72 h). The highest increase of the FA concentration was represented by C16:0, C:18:1, C18:2, and C18:3, with highest concentrations after 120 h of starvation. The maximum EPA and DHA concentrations were observed after 48 h of starvation, while the maximum DHA to EPA ratio was detected at the end of the starvation.

The cryptophyte Rhodomonas salina is widely used in aquaculture due to its high nutritional profile. This study aims to investigate the effect of salinity and pH on the growth, phycoerythrin concentrations, and concentrations of non-volatile umami taste active compounds of R. salina, using a design of experiment approach. Rhodomonas salina was cultivated in a flat-panel photobioreactor in turbidostat mode in a range of salinity (20–40 ‰) and pH (6.5–8.5). The strain was able to grow steadily under all conditions, but the optimal productivity of 1.17 g dry weight L−1 day−1 was observed in salinity 30 ‰ and pH 7.5. The phycoerythrin concentration was inversely related to productivity, presenting higher values in conditions that were not optimal for the growth of R. salina, 7% of dry weight at salinity 40 ‰, and pH 8.5. The identification of the umami taste of R. salina was based on the synergistic effect of umami compounds 5′-nucleotides (adenosine 5′-monophosphate, guanosine 5′-monophosphate, inosine 5′-monophosphate) and free amino acids (glutamic and aspartic acids), using the equivalent umami concentration (EUC). The results indicated that an increase in pH induces the accumulation of 5′-nucleotides, resulting in an EUC of 234 mg MSG g−1 at a salinity of 40 and pH 8.5. The EUC values that were observed in R. salina were higher compared to other aquatic animals, a fact that makes R. salina promising for further research and application in the food and feed sectors.