Photoprotection mechanisms against ultraviolet radiation in Heterocapsa sp. (Dinophyceae) are influenced by nitrogen availability: Mycosporine-like amino acids vs. xanthophyll cycle
Limnol. Oceanogr., 55(2), 2010, 899-908 | DOI: 10.4319/lo.2010.55.2.0899
ABSTRACT: The photoprotective and acclimation capacity against ultraviolet radiation (UVR) was assessed for the dinoflagellate Heterocapsa sp. We examined the effect of UVR and N availability on photosynthetic activity and on the accumulation of photoprotective substances such as mycosporine-like amino acids (MAAs) and xanthophyll cycle pigments. Cells were cultivated under two different light treatments, photosynthetically active radiation (PAR) and PAR + UVR, and at two NaNO3 concentrations, mid nitrogen (MN, 0.1 mmol L-1) and high nitrogen (HN, 1 mmol L-1) for 6 d. MAA and photosynthetic pigment contents as well as maximum quantum yield of fluorescence (Fv : Fm) and electron transport rate were analyzed at the initial time and after 3 and 6 d of experimentation. Fv : Fm decreased because of UVR and N limitation. N enrichment reduced the deleterious effect of UVR on photosynthesis. The content of photosynthetic pigments and MAAs was higher at HN than at MN supply and a positive effect of UVR on MAA and pigment accumulation was observed, suggesting that under HN conditions the deleterious UVR effect is counteracted by MAAs. Under N limitation thermal energy dissipation takes place by the xanthophyll cycle, i.e., an increase of the de-epoxidation degree was observed under these culture conditions. However, UVR seems to favor diadinoxanthin accumulation; thus, no photoprotection through the xanthophyll cycle seems available in cells exposed to PAR + UVR and MN. We conclude that not only MAA accumulation but also N availability is very important to determine the photoprotective capacity against UVR of Heterocapsa sp.