Diurnal regulation of photosynthetic light absorption, electron transport and carbon fixation in two contrasting oceanic environments

• Published in: Biogeosciences
• Main Authors: N. Schuback, P. D. Tortell
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2019
• Subjects: Ecology; QH540-549.5; Life; QH501-531; Geology; QE1-996.5; Pacific; Subarctic
• Online Access: https://doi.org/10.5194/bg-16-1381-2019; https://doaj.org/article/e641ec1dd4e740ef98f2239684569d90

Understanding the dynamics of marine phytoplankton productivity requires mechanistic insight into the non-linear coupling of light absorption, photosynthetic electron transport and carbon fixation in response to environmental variability. In the present study, we examined the variability of phytoplankton light absorption characteristics, light-dependent electron transport and 14 C -uptake rates over a 48 h period in the coastal subarctic north-east (NE) Pacific. We observed an intricately coordinated response of the different components of the photosynthetic process to diurnal irradiance cycles, which acted to maximize carbon fixation, while simultaneously preventing damage by excess absorbed light energy. In particular, we found diurnal adjustments in pigment ratios, excitation energy transfer to reaction centre II (RCII), the capacity for non-photochemical quenching (NPQ), and the light efficiency ( α ) and maximum rates ( P max ) of RCII electron transport (ETR RCII ) and 14 C uptake. Comparison of these results from coastal waters to previous observations in offshore waters of the subarctic NE Pacific provides insight into the effects of iron limitation on the optimization of photosynthesis. Under iron-limited, low-biomass conditions, there was a significant reduction of iron-rich photosynthetic units per chlorophyll a , which was partly offset by higher light absorption and electron transport per photosystem II (PSII). Iron deficiency limited the capacity of phytoplankton to utilize peak midday irradiance for carbon fixation and caused an upregulation of photoprotective mechanisms, including NPQ, and the decoupling of light absorption, electron transport and carbon fixation. Such decoupling resulted in an increased electron requirement ( Φ e,C ) and decreased quantum efficiency ( Φ C ) of carbon fixation at the iron-limited station. In both coastal and offshore waters, Φ e,C and Φ C correlated strongly to NPQ, albeit with a significantly different slope. We discuss the implications of our results for the ...