Nutrient Limitation Induce Photoinhibition in a Planktonic Diatom Skeletonema costatum

Diatoms are main primary producers of ocean ecosystems. Usually in the summer season of Japan, decrease of riverine water and increase of water stratification could cause nutrients depletion in diatoms, and in the same time, diatoms are exposed to strong sunlight. Such situation may adversely cause combined effect in diatoms’ photosynthesis. To know this, a strain of the representative diatom species, Skeletonema costatum, was maintained in a continuous culture at two dilution rates (0.14/day and 0.53/day) in both N-limited (1/10 NaNO₃) and P-limited (1/10 NaH₂PO₄) f/2 media, and photosynthetic responses under various light intensities were measured in a PAM fluorometer. Decreases of maximum quantum yield (Fv/Fm) and effective quantum yield (Φ_PSII) of PSII as well as photochemical quenching (qP) were notable in the both N- and P- limited media when the dilution rates were 0.14/day. Under such severe conditions, Stern-Volmer quenching (NPQ), an index of heat dissipation of excess light, increased in the N-limited medium, meanwhile it was not observed in the P-limited medium under the rapid light measurement. Even under prolonged exposure (1 h) under strong light (800 µmol-photons m^-2 s^-1), while higher NPQ inductions were observed in less-starved cultures (i.e. dilution rate = 0.53/day), only half level of NPQ was observed in the P-limited 0.14/day culture even if their photochemical efficiency deterred as indicated with low Φ_PSII. This result indicated P-limitation could cause failure of heat dissipation and may induce photoinhibition. When 100 µM of NaH₂PO₄ was re-introduced to the culture, decrease of Φ_PSII was relieved, further indicating phosphate is essential to maintain photosynthesis under strong light. These results imply photosynthesis in S. costatum can be significantly suppressed with insufficient photochemical reaction in PSII and with failure of heat dissipation induction under phosphate limitation.