Samenvatting

The aquaculture sector is expanding worldwide, which gives this sector an important economic potential. A problem for the on land aquaculture sector is that there is insufficient quality feed for cultivating fish larvae and shellfish. Based on this problem the RAAQUA project was launched (Robust Algae cultivation for the Aquaculture) to realize a commercial achievable, qualitative high standard and stable algae cultivation which will improve the on land aquaculture production. The marine microalgae Rhodomonas sp. provides many benefits to the aquaculture, as feed but also in other applications. Therefore, it is necessary to design an applicable cultivation plan for this algae.
Rhodomonas sp. is a cryptophyte, a unicellular algae, that has specific characteristics. This algae consists of two plastids which contains the phycobilin protein phycoerythrin besides the common chlorophyll pigment. This additional protein acts as an antenna for harvesting light energy. Phycoerythrin is active at specific wavelengths in the visible light spectrum ranging from 460 to 610nm. Within this range phycoerythrin has three absorption peaks that can be found in the blue, green and yellow region of the spectrum. The position of this pigment in the chloroplast makes this algae special. Where pigments are normally positioned in the stromal side of the thylakoid, phycoerythrin is located at the lumen side. Only cryptophyte species have organized their pigmentation this way. The possession of the phycoerythrin protein makes it possible for Rhodomonas sp. to live in the deeper regions of the ocean, which is unreachable by red light that is captured by the chlorophyll in green algae (which consequently live in the upper layers of the ocean). Blue light is made up of longer wavelengths that consist of more energy (40% more) than red wavelengths and is therefore able to reach these depths.
Several studies have been performed with respect to irradiance for Rhodomonas sp. However, as of yet no study to the optimal cultivation wavelength has been performed for cryptophytic species, although wavelength studies have been done on green algae blue green algae and cyanobacteria. Therefore, the aim of this present study was to investigate what light intensity in combination with wavelengths are the most suitable for cultivating Rhodomonas sp. with respect to its growth and biochemical composition. Importantly, the findings of this study will help designing a quantitative and qualitative high and stable cultivation plan for Rhodomonas sp. Rhodomonas sp. was cultivated under three light conditions, low light (8 ± 10 μmol m-2 s-1), medium light (60 ± 10 μmol m-2 s-1) and high light (80 ± 20 μmol m-2 s-1) while being exposed to wavelengths representing the colours red (λpeak 630 nm), green (λpeak 517 nm), blue (λpeak 461 nm) and white as reference (λ range of 461 – 630 nm). Rhodomonas sp. was batch cultured (150ml) under continuous irradiance at 20 °C (salinity of 30 g L-1) in an incubator. Several parameters were monitored including biomass, productivity, cell density and the biochemical composition. The biochemical composition monitoring consisted of chlorophyll a and b, carotenoids, phycoerythrin, carbohydrate and protein content analysis.
This study has demonstrated that cultivating Rhodomonas sp. with an irradiance of 60 μmol m2 s-1 under blue wavelengths of light results in the highest biomass (1.64 g L-1), maximum productivity (0.58 g L-1 day-1) and chlorophyll a content (1.1 pg cell-1). In addition, the highest chlorophyll b (10.2 pg cell -1), carbohydrate (88 pg cell-1) and protein (496 pg cell-1) levels are found under the same light intensity for the colours red, green and white respectively. Phycoerythrin and carotenoids content will be the highest when Rhodomonas sp. is cultivated with a PFD of 8 μmol m2 s-1 with blue (19.6 pg cell-1) and red (8.6 p cell-1) light respectively.
In elaboration of this present study there is recommended to do more research to protein and carbohydrate behaviour in respect to growth phases under different light conditions for Rhodomonas sp. In addition, the effect of combining colours, changing colours per growth phase and colours in respect to the absorption range of the photosynthetic pigments on the cultivation of Rhodomonas sp. is recommended to investigate further. Moreover, implementing a light cycle might induce pigmentation and growth. At last, up scaling of the cultivation with the conditions found in the present study in continuous systems or photo bioreactors is recommended.