A Supercomplex, of Approximately 720 kDa and Composed of Both Photosystem Reaction Centers, Dissipates Excess Energy by PSI in Green Macroalgae Under Salt Stress

Photostasis and photosynthetic adaptation to polar life

Photostasis is the light-dependent maintenance of energy balance associated with cellular homeostasis in photoautotrophs. We review evidence that illustrates how photosynthetic adaptation in polar photoautrophs such as aquatic green algae, cyanobacteria, boreal conifers as well as terrestrial angiosperms exhibit an astonishing plasticity in structure and function of the photosynthetic apparatus. This plasticity contributes to the maintenance of photostasis, which is essential for the long-term survival in the seemingly inhospitable Antarctic and Arctic habitats. However, evidence indicates that polar photoautrophic species exhibit different functional solutions for the maintenance of photostasis. We suggest that this reflects, in part, the genetic diversity symbolized by inherent genetic redundancy characteristic of polar photoautotrophs which enhances their survival in a thermodynamically challenging environment.

Quantifying the Energy Spillover between Photosystems II and I in Cyanobacterial Thylakoid Membranes and Cells

The spatial separation of photosystems I and II (PSI and PSII) is thought to be essential for efficient photosynthesis by maintaining a balanced flow of excitation energy between them. Unlike the thylakoid membranes of plant chloroplasts, cyanobacterial thylakoids do not form tightly appressed grana stacks that enforce strict lateral separation. The coexistence of the two photosystems provides a ground for spillover-excitation energy transfer from PSII to PSI. Spillover has been considered as a pathway of energy transfer from the phycobilisomes to PSI and may also play a role in state transitions as means to avoid overexcitation of PSII. Here, we demonstrate a significant degree of energy spillover from PSII to PSI in reconstituted membranes and isolated thylakoid membranes of Thermosynechococcus (Thermostichus) vulcanus and Synechocystis sp. PCC 6803 by steady-state and time-resolved fluorescence spectroscopy. The quantum yield of spillover in these systems was determined to be up to 40%. Spillover was also found in intact cells but to a considerably lower degree (20%) than in isolated thylakoid membranes. The findings support a model of coexistence of laterally separated microdomains of PSI and PSII in the cyanobacterial cells as well as domains where the two photosystems are energetically connected. The methodology presented here can be applied to probe spillover in other photosynthetic organisms.

Cryptochrome PtCPF1 regulates high temperature acclimation of marine diatoms through coordination of iron and phosphorus uptake

Increasing ocean temperatures threaten the productivity and species composition of marine diatoms. High temperature response and regulation are important for the acclimation of marine diatoms to such environments. However, the molecular mechanisms behind their acclimation to high temperature are still largely unknown. In this study, the abundance of PtCPF1 homologs (a member of the cryptochrome-photolyase family in the model diatom Phaeodactylum tricornutum) transcripts in marine phytoplankton is shown to increase with rising temperature based on Tara Oceans datasets. Moreover, the expression of PtCPF1 in P. tricornutum at high temperature (26 °C) was much higher than that at optimum temperature (20 °C). Deletion of PtCPF1 in P. tricornutum disrupted the expression of genes encoding two phytotransferrins (ISIP2A and ISIP1) and two Na+/P co-transporters (PHATRDRAFT_47667 and PHATRDRAFT_40433) at 26 °C. This further impacted the uptake of Fe and P, and eventually caused the arrest of cell division. Gene expression, Fe and P uptake, and cell division were restored by rescue with the native PtCPF1 gene. Furthermore, PtCPF1 interacts with two putative transcription factors (BolA and TF IIA) that potentially regulate the expression of genes encoding phytotransferrins and Na+/P co-transporters. To the best of our knowledge, this is the first study to reveal PtCPF1 as an essential regulator in the acclimation of marine diatoms to high temperature through the coordination of Fe and P uptake. Therefore, these findings help elucidate how marine diatoms acclimate to high temperature.

Spillover in the direct-type PSI-PSII megacomplex isolated from Arabidopsis thaliana is regulated by pH

Various megacomplexes in which Photosystem I and Photosystem II are physically bound (PSI-PSII m.c.) have been found in many organisms. In terms of function, these can be divided into two groups: those in which PSII and PSI are closely coupled (direct-type, photoprotection), and those in which a large light-harvesting antenna is placed between PSII and PSI (bridged-type, energy sharing). Arabidopsis thaliana has been reported to use the direct-type, where fast energy transfer occurs from PSII to PSI (~20 ps, fast spillover). In this paper, we show that the fast spillover is reversibly regulated depending on pH.