Influence of irradiance, dissolved nutrients and salinity on the colour and nutritional characteristics of Gracilariopsis longissima (Rhodophyta)

Colour is immediately perceived by the consumer and crucial for food product attractiveness. In this study, the possibility of obtaining different colour morphotypes in the rhodophyte Gracilariopsis longissima using short cultivation protocols (i.e. 9 days) was explored. In a first factorial experiment, the effects of salinity (25‰, 35‰ and 45‰), irradiance (60 and 270 μmol photons m−1 s−1) and nutrient conditions (N and P enrichment, no enrichment) on the growth, pigment concentration and colour of G. longissima were assessed. The results indicated that growth rate and phycocyanin were influenced by the three variables, whereas the concentration of other pigments (chlorophyll a, carotenoids and phycoerythrin) was significantly influenced by irradiance and nutrient conditions. The colour, defined as the Red, Green, Blue (RGB) signal from the photographed thalli, was mainly affected by irradiance and nutrient conditions. In a second experiment, four culture conditions yielding contrasting colour morphotypes and growth rates were repeatedly scaled up in the experiment to get enough biomass and to analyze nutritional properties (lipids, proteins, tissue N and P). The colour morphotypes and the corresponding culture conditions selected were: i) dark red; high salinity, low irradiance, nutrient enrichment; ii) light red; medium salinity, high irradiance, nutrient enrichment; iii) brownish; low salinity, low irradiance; no enrichment; and iv) green-yellowish; low salinity, high irradiance, no enrichment. These conditions caused marked differences in the nutritional properties of the thalli. The light red colour showed the best nutritional characteristics. Overall, G. longissima was not a suitable source of omega 3, but can be a source of Fe (0.34–0.53 mg g−1 DW) and K (103–161 mg g−1 DW), and its K:Na ratio can be enhanced under increasing salinity. The fast ecophysiological response, and its wide tolerance to environmental conditions, makes this species an excellent candidate for the development of short cultivation protocols.