Light and temperature acclimation of Rhodomonas salina (Cryptophyceae):: photosynthetic performance

Blooms of phototrophic cryptophytes have been observed in the highly eutrophic estuarine Darss-Zingst Bodden Chain (DZBC), Germany, during prolonged periods of light limitation due to ice and snow covering. The present study analyses possible mechanisms by which Rhodomonas salina, as a surrogate for bloom forming DZBC cryptophytes, maintains large densities during these low light/low temperature conditions. Growth, photosynthetic activity and pigment content were examined under 16 combinations of temperature (5 to 20degreesC) and irradiance (10 to 150 mumol photons m(-2) s(-1)) under nutrient-saturated conditions in a seawater-based medium. R. salina was tested for its capacity to photoacclimate to different light intensities in relation to temperature by calculating the photoadaptive index E-k (light saturation point of photosynthesis, P-max/alpha). P-max, the maximum photosynthesis rate and alpha, the efficiency of light utilisation at limited light intensities remained unchanged with respect to irradiance for every temperature tested. Consequently Ek, the irradiance at which photosynthesis rate ceased to be light-limited was constant (mean 49 mumol photons m(-2) s(-1)) within the chosen range of irradiances. This indicated that R. salina failed to adapt to down-shift changes in the light regime, at least in terms of photosynthetic parameters. Pigmentation analyses supported these results showing no acclimation of pigment ratios with regard to growth irradiance for a particular temperature. The calculated irradiance needed for 0 net photosynthesis (E-c was about 26 mumol photons m(-2) s(-1) and did not show any significant variation in light or temperature. The failure of R. salina to respond to down-shift changes in the light regime did not result, however, in a reduction in growth at low irradiances (10 mumol photons m(-2) s(-1)). Judging from these results, R. salina seems to pursue an alternative strategy to capture energy under low light conditions which we hypothesise to be uptake of dissolved organic carbon from the seawater-based medium. Follow-up research will concentrate on the relative contribution of heterotrophy to the overall nutrition of R. salina under white ice covering.