Physiological basis for high resistance to photoinhibition under nitrogen depletion in Emiliania huxleyi

We quantified and compared the photophysiological characteristics of Emiliania huxleyi with two marine diatoms Thalassiosira pseudonana and Coscinodiscus sp. under nitrogen- and phosphorus-replete and -depleted conditions. Under nitrogen-depleted conditions, E. huxleyi maintained significant photosystem II (PSII) function over at least 38 d, whereas for both diatoms, PSII function declined to marginal levels within 8–10 d of nitrogen depletion. In contrast, under phosphorus-depleted conditions, PSII function in E. huxleyi declined sharply within 7 d and in T. pseudonana within 3 d. Under nutrient-replete conditions, E. huxleyi had one of the highest PSII repair rate constants among phytoplankton examined to date. Under nitrogen depletion, E. huxleyi exhibited a decrease in susceptibility to photoinactivation of PSII, while the diatoms showed no significant change in susceptibility. The superior ability of E. huxleyi relative to the diatoms to limit PSII photoinactivation as well as to maintain PSII repair for many days of nitrogen depletion could help to explain why E. huxleyi is able to form blooms under the low-nitrate, high-light conditions that frequently occur in stratified surface waters.