Variation in photosynthetic electron requirements for carbon fixation by Dunaliella tertiolecta during transient phases with different phosphorus availability.
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Marine phytoplankton is an essential contributor to global primary production (PP). Conventional methods to measure PP are time-consuming and the results are usually influenced by bottle effect. Radio isotope tracking methods using radioactive labels are very expensive and limited by health/safety regulations. Measuring PP through variable chlorophyll a (Chla) fluorescence based techniques is a potential cheaper, handier and safer choice. Especially its much higher temporal resolution is quite attractive. However, the regulation of the conversion factor -- the electron requirement for carbon fixation (Фe,C) -- from photosynthetically produced electrons to carbon based PP, is still not fully understood, especially under fluctuating conditions which are typical in natural environments. We investigated the robustness of Фe,C and the contributions of the xanthophyll cycle and alternative sinks of electron to Фe,C during transient phases, through Dunaliella tertiolecta culture experiments in which the phosphorus supply was varied. Besides, we also investigated whether the theory developed to obtain absolute rates of ETRPSII by Oxborough et al. 2012 and Silsbe et al. 2015 holds under such transient phase. Our results revealed that: 1) Фe,C doesn’t remain stable and could increase 3-fold from a P-replete condition to a P-limited environment. 2) The roles of the xanthophyll cycle in NPQ is limited, but cyclic electron flow around PSII (or PSI) by PTOX can contribute about 50% to electrons sink and it decreased a lot when transformed into P-limited environment. 3) The assumption used for derivations of [RCII] from sigma algorithm and optical absorption coefficient of PSII (aLHII) for the new absorption algorithm, that KR is constant, proposed by Oxborough et al. 2012 doesn’t hold during fluctuating environment with different P concentrations, and the KR values we got were quite higher than the default value. Our finding points to the limitations of ETRPSII derivations, and provides more information on the conversion of photosynthetically produced electrons to carbon-based PP. More studies are required before the wide spread of Chla fluorescence in measuring primary production.