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Dependency of UVR-induced photoinhibition on atomic ratio of N to P in the dinoflagellate Karenia mikimotoi

Guan, Wanchun, Li, Ping
Marine biology 2017 v.164 no.2 pp. 31
Karenia, absorbance, carotenoids, chlorophyll, correlation, nitrogen, photoinhibition, photophosphorylation, photosynthetically active radiation, photosystem II, phytoplankton, seawater, superoxide dismutase, ultraviolet radiation
To investigate the effects of Nitrogen (N):phosphorus (P) ratio and solar ultraviolet radiation (UVR) on the growth and photophosphorylation of oceanic phytoplankton, the effects of UVR and N:P ratio on K. mikimotoi were evaluated. K. mikimotoi cells were cultured in artificial seawater (ASW) under five different N:P ratios (1:1, 16:1, 50:1, 100:1 and 200:1) for 15 days (phase 1: 1–5 day; phase 2: 6–10 day; phase 3: 11–15 days). Next, K. mikimotoi cultures were exposed to photosynthetically active radiation (PAR), PAB (PAR + UV-A + UV-B) and PA (PAR + UV-A), respectively. Finally, K. mikimotoi cells grown under the Redfield ratio (16:1) had the largest growth rate (0.257day⁻¹), highest pigment concentrations (chlorophyll a (Chla): 0.465 × 10⁻ ⁵ μg cell⁻¹, carotenoid (Caro): 0.249 × 10⁻⁵ μg cell⁻¹) and maximum superoxide dismutase (SOD) activity (0.9 U × 10⁻⁶ cell⁻¹) compared with those grown under other N:P ratios. The UV-absorbing compounds’ (UVₐbc) absorbance was largest at the N:P ratio of 50:1 during the range of 310–360 nm, with a relatively higher value at the N:P ratio of 16:1. UV intensity was positively correlated with photoinhibition. K. mikimotoi cells grown at the N:P ratio of 16:1 showed the smallest photoinhibition compared to those grown at other N:P ratios under the PAR treatment with the largest r:k ratio; similar results were found for both the PA and PAB treatments. The Redfield ratio of 16:1 is optimal for the growth and photophosphorylation of K. mikimotoi. The weakest UVR-induced photoinhibition at this ratio may be attributed to the highest photosystem II (PSII) repair rate and SOD activity.