Bioassays were carried out with the ‘blue diatom' Haslea ostrearia Simonsen, which is responsible for oyster greening during the fattening period of Crassostrea gigas Thunberg in oyster ponds. Samples of seawater were taken from two oyster ponds: one without oysters and the other with 20 oysters per m[sup2 ], maximal density allowed by the French AFNOR norm for ‘refinement'. The aims were to clarify the nutrient requirements of this diatom, also to elucidate the eventual influence of C. gigas at this density on the seawater fertility and to envisage the mass production of this diatom by pond fertilization. Examination of cell numeric densities at the end of bioassays allows us to conclude that silicate was the first limiting nutrient, closely followed by phosphate. Chlorophyll a concentrations led to different conclusions: phosphate was the first limiting factor, but after the seawater storage period in ponds, seawater quality evolved to a deficiency of nitrogen. Silicate addition increased cell division rate, and silicate depletion increased chl a synthesis for this species. Examination of nutrient assimilation ratios confirms that H. ostrearia requires a large amount of silicon. From these results, it was possible to prepare a N + P + Si simplified medium which has been tested in laboratory and field mesocosm conditions. In both conditions, similar results were observed: a significant increase in H. ostrearia cell concentrations and consequently an evolution up to the greening stage. Applications of this work are numerous; the principal permits us to envisage the production of this species in 25-m3 ponds, with the aim of allowing constant production of the greening phenomenon.

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.