Effects of irradiance on growth, pigment content and photosynthetic efficiency on three peridinin-containing dinoflagellates

• A. sanguinea, P. micans and S. trochoidea grow equally well under irradiances of 87-90 and 450-490 µmol photons m–2 s–1.
• The photosynthetic efficiency of A. sanguinea was low and possibly related to its mixotrophic behaviour.
• The relationship between high light and increased lipid droplets seems a common strategy to prevent physiological stress.

The effects of two irradiance levels on growth, cell volume, pigment content and photosynthetic efficiency were analysed in cultures of the peridinin-containing dinoflagellates Akashiwo sanguinea, Prorocentrum micans and Scrippsiella trochoidea. Strains were isolated from coastal waters in southern Brazil, and cells were grown at low (LL, 87–90 μmol photons m− 2 s− 1) and high (HL, 450–490 μmol photons m− 2 s− 1) light intensities. The composition of photosynthetic pigments was analysed by high-performance-liquid-chromatography (HPLC) in samples collected at days 2, 7 and 14 of growth, and the photosynthetic efficiency (Fv/Fm) was determined by pulse-amplitude-modulated fluorometry. On day 14, cells were stained with Nile Red and observed under the fluorescence microscope for lipid content determination. Similar growth rates were observed for A. sanguinea (μ = 0,26–0,33 d− 1) and P. micans (μ = 0,25–0,31 d− 1) under HL and LL, whereas S. trochoidea presented significantly higher values under HL (0,29 ± 0.02; p < 0,001) than LL (μ = 0,23 ± 0.01 d− 1). Increase in the cell volume was observed for P. micans along the growth curve in both irradiances while for A. sanguinea at LL only. The three species presented low concentration of photoprotective pigments under both HL and LL, whereas S. trochoidea and P. micans showed higher photosynthetic efficiency (average: Fv/Fm > 0.5) compared to A. sanguinea (Fv/Fm ~ 0.2). Microscope observation revealed that intracellular lipid concentration increased at HL in the three species, in all growth stages. In short, the three dinoflagellates are adapted to HL intensity and increased lipids might serve as a protective mechanism for the cells.