Program

Keywords: Cell size, Diatom, Growth, Photoinactivation, Photosystem II, Photoperiod, Light, Thalassiosira

Abstract

Light drives phytoplankton productivity, so phytoplankton must exploit variable intensities and durations of light exposure, depending upon season, latitude and depth. We analyzed the growth, photophysiology and composition of small, Thalassiosira pseudonana, and large, Thalassiosira punctigera, centric diatoms from temperate, coastal marine habitats, responding to a matrix of photoperiods and growth light intensities. T. pseudonana showed fastest growth rates under long photoperiods and low to moderate light intensities, while the larger T. punctigera showed fastest growth rates under short photoperiods and higher light intensities. Photosystem II function and content responded primarily to instantaneous growth light intensities during the photoperiod, while diel carbon fixation and RUBISCO content responded more to photoperiod duration than to instantaneous light intensity. Changing photoperiods caused species-specific changes in the responses of photochemical yield (e-) to growth light intensity. These photophysiological variables showed complex responses to photoperiod and to growth light intensity. Growth rate also showed complex responses to photoperiod and growth light intensity. But these complex responses resolved into a close relation between growth rate and the cumulative daily generation of reductant, across the matrix of photoperiods and light intensities.

We also found diatoms maintain an active cycling of Photosystem II (PSII) proteins even in the dark. PSII protein cycling saturates at low light, and continued cycling in dark periods allows the diatoms to catch up to excess photoinactivation accumulated over the preceding illuminated period. The large diatom suffers only limited photoinactivation of PSII, but cycling of PSII protein exceeds PSII cycling, so the diatom recycles functional PSII units before they are inactivated. Through the diel cycle the contents of active PSII centers and PSII proteins change predictably, but are not correlated, generating large changes in the fraction of total PSII that is active at a given time or growth condition. We propose that dark and steady cycling of PSII proteins is driven by the tight integration of chloroplastic and mitochondrial metabolism in diatoms, supporting their success in mixed water environments with variable light.