Optimum ranges of combined abiotic factor for Gracilaria gracilis aquaculture

Abstract

Laboratory culture of the rhodophyte Gracilaria gracilis (Stackhouse) M. Steentoft, L.M.Irvine & W.F.Farnham which often grows in many stressed Mediterranean coasts, mainly lagoons, was carried out to ascertain the optimum range and interaction of environmental factors on growth and composition with a view to potential mariculture. The methodology adopted was based on the synergy between mathematical modeling and laboratory experiments. An experimental design and a response surface methodology (RSM) were used to simulate field conditions. Experiments were done at 20 °C. The studied responses were daily growth rate (DGR), agar yields, and protein and R-phycoerythrin (RPE) contents. Second-order quadratic polynomial models for all studied responses were deduced and ANOVA showed them to be significant (p < 0.05). The RSM was successfully applied to assess the effects of abiotic factors (light, salinity, ammonium, and nitrate) and their interaction on the DGR, agar yields, and protein content. Gracilaria gracilis grew well (> 3% day−1) under all the combined abiotic factor conditions that were simulated. Hence, the optimum (5% day−1) was obtained around seawater salinity (36 psu), 198 μmol photons m−2 s−1 irradiance and nitrogen concentration of 80 μmol ammonium g−1 fw L−1, and 210 μmol nitrate g−1 fw L−1. Optimum agar yields (15%) were obtained at a lower irradiance (70 μmol photons m−2 s−1) and a salinity of 24 psu combined with similar ammonium (80 μmol g−1 fw L−1) and nitrate (100 μmol g−1 fw L−1) concentrations. Optimum protein contents (23%) and RPE contents (1.2 mg g−1 fw) were obtained at lower irradiance (70 μmol photons m−2 s−1), a salinity of 24 psu, an ammonium concentration of 80 μmol g−1 fw L−1, and lower nitrate concentration (10 μmol g−1 fw L−1). In addition, abiotic factor interaction effects occurred when one of them significantly contributes to response reduction. In conclusion, potential mariculture of G. gracilis in the lagoon is possible but related to their proteins and pigments (essentially RPE) contents in addition to agar yields.