Arctic Micromonas uses protein pools and non-photochemical quenching to cope with temperature restrictions on Photosystem II protein turnover

Abstract

Micromonas strains of small prasinophyte green algae are found throughout the world’s oceans, exploiting widely different niches. We grew arctic and temperate strains of Micromonas and compared their susceptibilities to photoinactivation of Photosystem II, their counteracting Photosystem II repair capacities, their Photosystem II content, and their induction and relaxation of non-photochemical quenching. In the arctic strain Micromonas NCMA 2099, the cellular content of active Photosystem II represents only about 50 % of total Photosystem II protein, as a slow rate constant for clearance of PsbA protein limits instantaneous repair. In contrast, the temperate strain NCMA 1646 shows a faster clearance of PsbA protein which allows it to maintain active Photosystem II content equivalent to total Photosystem II protein. Under growth at 2 °C, the arctic Micromonas maintains a constitutive induction of xanthophyll deepoxidation, shown by second-derivative whole-cell spectra, which supports strong induction of non-photochemical quenching under low to moderate light, even if xanthophyll cycling is blocked. This non-photochemical quenching, however, relaxes during subsequent darkness with kinetics nearly comparable to the temperate Micromonas NCMA 1646, thereby limiting the opportunity cost of sustained downregulation of PSII function after a decrease in light.

Phytoplankton σPSII and Excitation Dissipation; Implications for Estimates of Primary Productivity

“…In fact, across our studies to date (Ni et al, 2017; Figure S7) the maximum YNPQ we have consistently observed is ∼0.6 in an arctic strain of the prasinophyte Micromonas. As a conceptual exercise we used the chlorophyllspecific absorption coefficient, corrected for cell size (Fujiki and Taguchi, 2002), and the 35 chlorophyll content of the core of PSII (Umena et al, 2011) to estimate the approximate effective absorption cross section for PSII in the absence of any associated antenna complexes.…”

“…Indeed, for the second derivative of whole cell spectra, we found a difference at 487 nm before and after 8 min of actinic light treatment in Ostreococcus from growth saturating light (Figure S5B), likely reflecting an increase in the de-epoxidised xanthophyll cycle pigment content zeaxanthin (485 nm) (Méléder et al, 2013;Ni et al, 2017). This lightdependent difference at 487 nm was not found in Ostreococcus from growth limiting light (Figure S5A), reflecting a smaller content of xanthophyll cycle pigments.…”

“…Beyond these four taxa we include Supplementary Data from Chorella vulgaris (chlorophyte, chl a/b) (Bonnanfant and Campbell, unpublished); Haslea ostrearia and Haslea sp. (temperate and tropical pennate diatom strains, chl a/c) (Lavaud et al, unpublished) and Micromonas NCMA 2099 (arctic chl a/b Prasinophyte) (Ni et al, 2017). These strains were grown under comparable conditions, for different studies.…”

“…These strains were grown under comparable conditions, for different studies. When cultures reached mid-exponential phase we took samples for whole cell absorbance measures in an integrating cavity spectrophotometer (OLIS-Cary14 with DSPC) (Ni et al, 2017). From these spectra we took second derivative spectra to detect changes in carotenoid absorbance resulting from xanthophyll cycling (Jesus et al, 2008).…”

“…For each FRRf induction curve we exported the data from the FluorWin data capture software to fit a model with four parameters ( Table 2): minimal fluorescence, F 0 (or F S ); maximal fluorescence, F M (or (or F M '); effective absorption cross section for PSII photochemistry, σ PSII (or σ PSII ′ ); and the coefficient of excitonic connectivity ρ (Kolber et al, 1998) using the PSIWORX-R package (A. Barnett, sourceforge.net) (Murphy et al, 2016;Ni et al, 2017). For each measurement we did an FRRf induction before and then again after a 1 s period of darkness to allow PSII to re-open after illumination.…”