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Coastal kelp forests produce substantial marine carbon due to high annual net primary production (NPP) rates, but upscaling of NPP estimates over time and space remains difficult. We investigated the impact of variable underwater photosynthetically active radiation (PAR) and photosynthetic parameters on photosynthetic oxygen production of Laminaria hyperborea, the dominant NE-Atlantic kelp species, throughout summer 2014. Collection depth of kelp had no effect on chlorophyll a content, pointing to a high photoacclimation potential of L. hyperborea towards incident light. However, chlorophyll a and photosynthesis versus irradiance parameters differed significantly along the blade gradient when normalized to fresh mass, potentially introducing large uncertainties in NPP upscaling to whole thalli. Therefore, we recommend a normalization to kelp tissue area, which is stable over the blade gradient. Continuous PAR measurements revealed a highly variable underwater light climate at our study site (Helgoland, North Sea) in summer 2014, reflected by PAR attenuation coefficients (K d ) between 0.28 and 0.87 m -1 . Our data highlight the importance of continuous underwater light measurements or representative average values using a weighted K d to account for large PAR variability in NPP calculations. Strong winds in August increased turbidity, resulting in a negative carbon balance at depths >3-4 m over several weeks, considerably impacting kelp productivity. Estimated daily summer NPP over all four depths was 1.48 ± 0.97 g C · m -2 seafloor · d -1 for the Helgolandic kelp forest, which is in the range of other kelp forests along European coastlines., (© 2023 The Authors. Journal of Phycology published by Wiley Periodicals LLC on behalf of Phycological Society of America.)

At high latitudes, the polar night poses a great challenge to photosynthetic organisms that must survive up to six months without light. Numerous studies have already shed light on the physiological changes involved in the acclimation of microalgae to prolonged darkness and subsequent re-illumination. However, these studies have never considered inter-individual variability because they have mainly been conducted with bulk measurements. On the other hand, such long periods are likely to impact within-population selection processes. In this study, we hypothesized that distinct subpopulations with specific traits may emerge during acclimation of a population of diatoms to darkness. We addressed this hypothesis using flow cytometry (FCM), which allow to individually characterize large numbers of cells. The ecologically dominant polar pennate diatom Fragilariopsis cylindrus was subjected to three dark acclimation (DA) experiments of one, three, and five months duration, during which all cultures showed signs of recovery once light became available again. Our results suggest that darkness survival of F. cylindrus relies on reduction of metabolic activity and consumption of carbon reserves. In addition, FCM allowed us to record three different causes of death, each shared by significant numbers of individuals. The first rendered cells were unable to survive the stress caused by the return to light, probably due to a lack of sufficient photoprotective defenses. The other two were observed in two subpopulations of cells whose physiological state deviated from the original population. The data suggest that starvation and failure to maintain dormancy were the cause of cell mortality in these two subpopulations., (© 2022 Phycological Society of America.)