A comparative study of microalgae isolated from flooded rice paddies: light-limited growth,C fixation,growth efficiency and relative N and P requirement

Seven unialgal isolates from rice paddies,Ankistrodesmus convolutus, Chlorella sp.,Scenedesmus quadricauda, Frustulia vulgaris, Anabaena sp.,Microcystis aeruginosa andPhormidium mucicola, were investigated for their light-limited growth and photosynthetic characteristics, growth efficiency, and relative requirement for N and P as well as their storage potentials. The maximum growth rate (μ_max) ranged from 1.27 to 2.72 d^?1 among species. The slope of light-limited growth (α_g) showed only small interspecies differences (0.126–0.204 d^?1 W^?1 m²) except inS. quadricauda (0.092 d^?1 W^?1 m²). Similarly, the ratio of μ_max to α_g, I_k(g), varied within a narrow range (8.7–13.9 W m^?2) with the exception ofS. quadricauda (19.5 W m^?2). The slope of the photosynthetic curve based on chlorophylla (chla), α_p(a), was significantly higher in cyanobacteria (0.118–0.189 mg C mg chla ^?1h^?1W^?1m²) than in other species (0.070–0.094 mg C mg chla ^?1h^?1W^?1m²). The maximum photosynthetic rate based on chla (P_max(a)) was similar (2.95–3.83 mg C mg chla ^?1 h^?1) with the exception of a high value (6.17 mg C mg chla ^?1 h^?1) inM. aeruginosa. A significant correlation (P<0.001) existed between α_p(a) and P_max(a). The C-specific maximum photosynthetic rate (P_max(c)) was inversely correlated to the C/chla ratio. Growth efficiency (μ_eff), the efficiency to retain photosynthetically-fixed C for growth, showed an interspecies variation ranging from 73–93% and was correlated to the μ_max:P_max(c) ratio. Cyanobacteria exhibited higher efficiency than others. The ratio of P_max:α_p, I_k(p), was higher than I_k(g) by 2.0 to 3.8 times. The optimum N:P ratio, determined as the ratio of minimum cell quotas of N to P (q_oN:q_oP), showed a wide interspecies variability. It was highest inP. mucicola (54) and lowest inF. vulgaris (10). The maximum storage capacity for excess P and N also varied among species; the maximum capacity for P ranged from 2 to 14 times of its immediate need and the capacity for N varied from 1.2 to 4 times.