Haslea ostrearia, the peculiar diatom that develops in oyster-ponds and synthesizes a water-soluble blue pigment (marennine), was grown
in a semi-continuous mode over a wide range of irradiances (20–750 μmol; m−2 s−1). Growth, photosynthesis and pigment content were
determined for algae maintained in exponential growth phase by regular dilution with fresh medium. Increasing the growth irradiance
resulted in a decrease in the chloroplast length, but had no clear influence on the size of the cellular compartments pigmented by
marennine accumulation. Growth rates increased with irradiance from 20 to 100 μmol; m−2 s−1 and were constant from 100 to 750 μmol;
m−2 s−1. Increasing growth irradiance caused a decrease in the cellular content of chlorophylls a and c and fucoxanthin, in contrast to
diadinoxanthin. Algae acclimated to high irradiance had lower maximum photosynthetic rate (Pnm) and maximum light utilization
coefficient (α) when expressed on a per cell basis. On a chlorophyll a basis, the higher the growth irradiance, the lower the maximum
light utilization coefficient and the higher the maximum photosynthetic rate. No photoinhibition was observed at irradiances up to
1500 μmol; 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 irradiance) 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 characterized by low turbidity and shallow depth.