| 24 Apr 2026
Impact of light, turbulent mixing, and nutrients on phytoplankton photoinhibition in the euphotic zone from the subtropical to subpolar North Atlantic
Abstract. Photoinhibition, a condition caused when high photon fluxes lead to the rate of electron transport from light receptors in photosystem II exceeding a cell’s photochemical capacity, causes damage to the photosynthetic apparatus and reduces photosynthesis. The balance between damage and repair processes in cells determines the degree of photoinhibition (PI). PI is here shown to be ubiquitous in natural phytoplankton communities in the North Atlantic. PI effects were determined as the difference between the maximum photosynthetic quantum yield (Fv/Fm) recorded at the time of sampling and following a 4 h low-light incubation. Samples from the euphotic zone in the North Sea showed significant daily variation of PI extending to depths of 20 m. Principal component analysis identified a strong correlation between PI and light availability, whereas nutrient distributions and other environmental variables were less influential on PI. Furthermore, an almost immediate response of PI to changes in light intensity was demonstrated. A light-dependent linear regression model of PI for the northern North Sea explained a significant increase in PI (R2 = 0.4) with intensity of insolation. PI in samples from the euphotic zone in the subpolar Irminger Sea and the subtropical Sargasso Sea also showed significantly elevated PI at higher light levels. Our results show that PI in natural phytoplankton communities is closely related to, and occurs almost immediately, in response to changes in insolation. The sensitivity of phytoplankton communities’ photo-chemical capacity to insolation changes may help to explain the wide variation in photosynthetic parameters reported in nature and should be taken into account when estimating ocean primary production.
The authors have systematically determined the increment between dark-adapted Fv/Fm and Fv/Fm after a 4 hour incubation in low light in profiles of discrete samples in the mixed layer of several areas in the Atlantic Ocean. The increment is interpreted as the degree to which the function of PSII has been inhibited by in situ irradiance exposure. This relationship is shown in greatest detail in field measurements in the North Sea, from which a regression relationship is developed between in situ PAR and the incubation-based increase in Fv/Fm, either as an absolute change or relative to post-incubation Fv/Fm. The greatest changes are observed near the surface and around mid-day under cloud-free conditions. Smaller datasets show that similar phenomena can be observed in the sub-polar (Irminger Sea) and sub-tropical (Sargasso Sea) ocean, though the relationship with PAR is not the same as observed in the North Sea.
The exposure-sensitivity of Fv/Fm to high light and the recovery under low-light is a well known and widely observed characteristic of phytoplankton sampled from open waters, so the observations reported are not surprising. Nevertheless, their field observations constitute one of the most extensive datasets demonstrating its occurrence in a variety of ocean regimes and validates the suggestions of the earlier From et al (2014) study that these incubations provide not only a “corrected” Fv/Fm but also a widely applicable indicator of photoinhibition. An added feature of the dataset are the accompanying observations of microscale turbulence and standard oceanographic measurements.
A general comment is that the N. Sea field studies, like many, were made under what look like relatively calm conditions. Thus, the observations relating to light history probably do not consider the effect of large-scale vertical transport as could occur due, e.g. Langmuir circulation cells, on the profile of photoinhibition and how it would be affected by mixing.
The authors might want to consider changing the title, the current one doesn’t convey that the majority of the data is from the North Sea. Neither is enough said about nutrients to justify its inclusion in the title. An alternative might be: Phytoplankton photoinhibition and turbulent mixing in the euphotic zone of the North Sea and selected regions of the subtropical and subpolar Atlantic Ocean.
Minor Comments:
Section 2.2 turbulence: – Was this only measured in the North Sea?
Section 2.4 PAR: OK to assume that albedo is more-or-less the same over a given area, but albedo will vary through the day, this may have bearing on the regressions evaluating the effect of light history.
Section 2.5 Low-light incubation: In some species, including cyanobacteria and diatoms (e.g. Goss and Jakob 2010 ), dark adaptation does not completely reverse NPQ due to dark electron flow through the PQ pool. This can be reversed by giving the samples a short exposure to very low light (< 10 micromol m-2 s-1) just before the Fv/Fm determination. Did the authors check for this in their samples?
For the Sargasso Sea, From et al (2014) noted that there was a bottle effect which progressively reduced Fv/Fm with time. They made a bottle effect correction to their data, was this similarly applied in this study?
From et al (2014) also noted that incubation period needed for recovery varies between sites, a primary variable being temperature. What was the incubation temperature in the Irminger Sea samples and was it verified that recovery had reached an asymptote after 4 hours?
Line 146: Note typo 0^(-3) I assume it should be 10^(-3)
Supplemental Information – It would be good to include the full field data like that provided for Vermix also for the Sargasso Sea and Irminger Sea
Reference:
Goss, R., Jakob, T. Regulation and function of xanthophyll cycle-dependent photoprotection in algae. Photosynth Res 106, 103–122 (2010). https://doi.org/10.1007/s11120-010-9536-x