Long-term photoacclimation of Haslea ostrearia (Bacillariophyta): effect of irradiance on growth rates, pigment content and photosynthesis

Haslea ostrearia, the peculiar diatom that develops in oyster-ponds and syn thesizes a water-soluble blue pigment (marennine), was grown in a semi-cont inuous mode over a wide range of irradiances (20-750 mu mol m(-2) s(-1)), G rowth, photosynthesis and pigment content were determined for algae maintai ned in exponential growth phase by regular dilution with fresh medium. Incr easing the growth irradiance resulted in a decrease in the chloroplast leng th, but had no clear influence on the size of the cellular compartments pig mented by marennine accumulation. Growth rates increased with irradiance fr om 20 to 100 mu mol m(-1) s(-1) and were constant from 100 to 750 mu mol m( -2) s(-1). Increasing growth irradiance caused a decrease in the cellular c ontent of chlorophylls a and c and fucoxanthin, in contrast to diadinoxanth in. Algae acclimated to high irradiance had lower maximum photosynthetic ra te (P-m(n)) and maximum light utilization coefficient (alpha) when expresse d on a per cell basis. On a chlorophyll a basis, the higher the growth irra diance, the lower the maximum light utilization coefficient and the higher the maximum photosynthetic rate. No photoinhibition was observed at irradia nces up to 1500 mu mol m(-2) s(-1), whatever the growth irradiance. In vivo chlorophyll a fluorescence showed that cells grown at high irradiance had effective photosystem II quantum efficiency (measured at the growth irradia nce) considerably lower than that of cells grown at low irradiance. Thus H. ostrearia withstands high light exposure, consistent with the observation that this alga can outcompete other diatoms encountered in oyster-ponds cha racterized by low turbidity and shallow depth.