Fig 1.
DMSO reduction in response to high irradiance exposure from simulated vertical mixing.
(A) D6,13C2-DMSO reduction rate constants (kDMSOred, in d-1) in the low light (LL) and high light (HL) treatments across all stations. Data points represent individual replicate measurements, while mean rates (denoted by bars) are derived from n = 3 replicates, except for LBP8—LL and CS04—HL (n = 2). Significance is determined by two-tailed Student’s t-tests (*: p < 0.05, **: p < 0.01, n.s.: non-significant). (B, C) Changes in maximum photosynthetic efficiency (Fv/Fm, n = 3) under LL and HL exposure for stations LBP8 and SS2, respectively. Significant simple main effects between treatments were found for SS2 (two-way mixed effects ANOVA model; ***: p < 0.001). Dashed lines represent linear regression fits over time. (D) Sampling depth at the chlorophyll-a maximum (Chl-a max), euphotic zone depth (Zeu), and mixed layer depth (MLD) for each station. (E) Chl-a normalized concentrations of xanthophylls diadinoxanthin (Dd) and diatoxanthin (Dt) (μg μg-1; bars, n = 2) and their de-epoxidation ratios (blue markers, n = 2) for stations LBP8 and SS2. Higher concentrations and de-epoxidation ratios of Dd and Dt are indicative of increased photoprotection through thermal dissipation [29]. All error bars denote range (n = 2) or ± 1 s.d. (n = 3). (F) Conceptual schematic illustrating the hypothesized link between vertical mixing (dashed arrows), the light exposure history of the natural assemblages, and DMSO reduction rates (symbolized by the dashed arrow thickness). Processes shoaling the MLD (denoted by red dashed lines) will entrain sub-surface, LL-acclimated assemblages to higher irradiance at the surface, promoting increased DMSO reduction rates (thicker dashed arrows).
Fig 2.
Correlations between DMSO reduction (D6,13C2-DMS production) and Stern-Volmer non-photochemical quenching (NPQsv) at four sampling stations.
Data are plotted for the experimental treatment (either high light or DCMU; grey markers) and their respective control (black markers) for stations (A) LGO6, (B) LBP8, (C) JI22, and (D) SS2. (E) Pooled data for all three treatments at the four sampling stations illustrate the overall inverse correlation between DMSO reduction and NPQ. Error bars denote range (n = 2) or ± 1 s.d. (n = 3). Except for JI22 (D6,13C2-DMS: n = 2), all means are n = 3. Pearson correlations (r) for pooled data is -0.53, and individual treatments range from -0.82 to -0.97, except under DCMU exposure at LG06, where r = -0.41.
Fig 3.
Proposed mechanistic links between methylated sulfur cycling and photosynthetic electron transport in marine phytoplankton.
(A-B) The effect of exposure to DCMU (which specifically blocks electron flow past the secondary quinone (Qb) in PSII) [61] on D6,13C2-DMSO reduction rate constants (kDMSOred, in d-1; bars) at stations (A) SS5 (2022), (B) LG06 (2023) and (C) JI22 (2023). Data points represent individual replicate measurements, while bars represent the mean value for each treatment. Error bars indicate range (n = 2) or ± 1 s.d. (n = 3). Statistical significance is determined by two-tailed Student’s t-tests (*: p < 0.05, **: p < 0.01). (D) A schematic representation of the proposed energy dissipation pathway in which DMSO acts as a sink for excess energy downstream of photosystem II (PSII) that would otherwise produce ROS e.g. at photosystem I (PSI; see red box). The acidic lumen, produced from the water splitting reactions at PSII and the trans-thylakoid proton gradient at Cytochrome b6f (Cyt b6f), can satisfy the proton requirements for DMSO reduction. The oxidation of intracellular DMSP [19] and cleavage of the intermediate DMSOP [16] could act to replenish intracellular DMSO concentrations and further mitigate ROS build-up, providing an efficient antioxidant redox system [18,28]. Smaller arrows signify that the DMSP cleavage and DMS oxidation pathways likely provide an indirect, minor contribution towards this hypothetical antioxidant system (see main text) [21]. Electron flow across redox carriers is indicated by the red arrow, with dashes denoting inhibition by DCMU past Qb (purple arrow). Additional abbreviations in (D) are as follows: reduction (Red.), oxidation (Ox.), primary quinones (Qa/Qb), plastoquinone (PQ) plastocyanin (PC), ferredoxin (Fd), superoxide dismutase (SOD), ascorbate peroxidase (APX). Schematic created in BioRender. McNabb, B. (2024) https://BioRender.com/w07b221.