Preprints
https://doi.org/10.5194/egusphere-2024-3812
https://doi.org/10.5194/egusphere-2024-3812
06 Jan 2025
 | 06 Jan 2025

Photosynthetic electron, carbon and oxygen fluxes within a mosaic of Fe limitation in the California Current Upwelling System

Yayla Sezginer, Kate Schuler, Emily Speciale, Adrian Marchetti, Claire Till, Ralph Till, and Philippe Tortell

Abstract. We compare primary productivity estimates based on different photosynthetic ‘currencies’ (electrons, O2 and carbon) collected from the dynamic coastal upwelling waters of the California Current. Fast Repetition Rate Fluorometry and O2/N2 measurements were used to collect high-resolution underway estimates of photosynthetic electron transport rates and net community productivity, respectively, alongside on-station 14C uptake experiments to measure gross carbon fixation rates. Our survey captured two upwelling filaments at Cape Blanco and Cape Mendocino with distinct biogeochemical signatures and iron availabilities, enabling us to examine photosynthetic processes along a natural iron gradient. Significant differences in photo-physiology, cell sizes, Si:NO3- draw-down ratios, and molecular markers of Fe-stress indicated that phytoplankton assemblages near Cape Mendocino were Fe-stressed, while those near Cape Blanco were Fe-replete. Upwelling of O2-poor deep water to the surface complicated O2-based net community productivity estimates, but we were able to correct for these vertical mixing effects using continuous [N2O] surface measurements and depth-profiles of ∂[O2]∂[N2O]. Vertical mixing corrections were strongly correlated to sea surface temperature, which serves as an N2O-independent proxy for upwelling. Following vertical mixing corrections, all three productivity estimates reflected trends in Fe-stress physiology, indicating greater productivity near Cape Blanco compared to Cape Mendocino. For all assemblages, carbon fixation varied as a hyperbolic function of electron transport rates, but the derived parameters of this relationship were highly variable and significantly correlated with physiological indicators of Fe-stress (σPSII, FV/FM, Si:NO3- and diatom-specific PSI gene expression), suggesting that iron availability influenced the coupling between photosynthetic electron transport and subsequent carbon fixation. Net community productivity showed strong coherence with daily-integrated photosynthetic electron transport rates across the entire cruise track, with no apparent relationship with Fe-stress. This result suggests that fluorescence-based estimates of gross photochemistry are still a good indicator for bulk primary productivity, even if Fe-limitation influences the stoichiometric relationship between productivity currencies.

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Journal article(s) based on this preprint

02 Oct 2025
Photosynthetic electron, carbon, and oxygen fluxes within a mosaic of Fe limitation in the California Current upwelling system
Yayla Sezginer, Kate Schuler, Emily Speciale, Adrian Marchetti, Claire Till, Ralph Till, and Philippe Tortell
Biogeosciences, 22, 5257–5281, https://doi.org/10.5194/bg-22-5257-2025,https://doi.org/10.5194/bg-22-5257-2025, 2025
Short summary
Yayla Sezginer, Kate Schuler, Emily Speciale, Adrian Marchetti, Claire Till, Ralph Till, and Philippe Tortell

Interactive discussion

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2024-3812', Anonymous Referee #1, 04 Feb 2025
    • AC1: 'Reply on RC1', Yayla Sezginer, 26 Mar 2025
  • RC2: 'Comment on egusphere-2024-3812', Kazuhiro Yoshida, 28 Feb 2025
    • AC2: 'Reply on RC2', Yayla Sezginer, 26 Mar 2025

Interactive discussion

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2024-3812', Anonymous Referee #1, 04 Feb 2025
    • AC1: 'Reply on RC1', Yayla Sezginer, 26 Mar 2025
  • RC2: 'Comment on egusphere-2024-3812', Kazuhiro Yoshida, 28 Feb 2025
    • AC2: 'Reply on RC2', Yayla Sezginer, 26 Mar 2025

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload
ED: Reconsider after major revisions (11 Apr 2025) by Koji Suzuki
AR by Yayla Sezginer on behalf of the Authors (24 Apr 2025)  Author's response   Author's tracked changes   Manuscript 
ED: Referee Nomination & Report Request started (29 Apr 2025) by Koji Suzuki
RR by Kazuhiro Yoshida (09 Jun 2025)
ED: Reconsider after major revisions (10 Jun 2025) by Koji Suzuki
AR by Yayla Sezginer on behalf of the Authors (07 Jul 2025)  Author's response   Author's tracked changes   Manuscript 
ED: Referee Nomination & Report Request started (08 Jul 2025) by Koji Suzuki
RR by Kazuhiro Yoshida (09 Jul 2025)
ED: Publish subject to minor revisions (review by editor) (12 Jul 2025) by Koji Suzuki
AR by Yayla Sezginer on behalf of the Authors (18 Jul 2025)  Author's response   Author's tracked changes   Manuscript 
ED: Publish as is (28 Jul 2025) by Koji Suzuki
AR by Yayla Sezginer on behalf of the Authors (29 Jul 2025)  Manuscript 

Journal article(s) based on this preprint

02 Oct 2025
Photosynthetic electron, carbon, and oxygen fluxes within a mosaic of Fe limitation in the California Current upwelling system
Yayla Sezginer, Kate Schuler, Emily Speciale, Adrian Marchetti, Claire Till, Ralph Till, and Philippe Tortell
Biogeosciences, 22, 5257–5281, https://doi.org/10.5194/bg-22-5257-2025,https://doi.org/10.5194/bg-22-5257-2025, 2025
Short summary
Yayla Sezginer, Kate Schuler, Emily Speciale, Adrian Marchetti, Claire Till, Ralph Till, and Philippe Tortell
Yayla Sezginer, Kate Schuler, Emily Speciale, Adrian Marchetti, Claire Till, Ralph Till, and Philippe Tortell

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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

Short summary
We recorded three metrics of photosynthesis in the California Current. Real-time observations of microalgae physiology and productivity revealed signs of iron limitation where the continental shelf rapidly dropped off. Iron limitation influenced how efficiently light was absorbed and used for carbon fixation but did not appear to affect net photosynthetic oxygen production. Our results offer useful insights towards efforts to model carbon fixation rates from microalgae optical properties.
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