Turbulence simultaneously stimulates small- and large-scale CO2 sequestration by chain-forming diatoms in the sea

Abstract

Chain-forming diatoms are key CO2-fixing organisms in the ocean. Under turbulent conditions they form fast-sinking aggregates that are exported from the upper sunlit ocean to the ocean interior. A decade-old paradigm states that primary production in chain-forming diatoms is stimulated by turbulence. Yet, direct measurements of cell-specific primary production in individual field populations of chain-forming diatoms are poorly documented. Here we measured cell-specific carbon, nitrate and ammonium assimilation in two field populations of chain-forming diatoms (Skeletonema and Chaetoceros) at low-nutrient concentrations under still conditions and turbulent shear using secondary ion mass spectrometry combined with stable isotopic tracers and compared our data with those predicted by mass transfer theory. Turbulent shear significantly increases cell-specific C assimilation compared to still conditions in the cells/chains that also form fast-sinking, aggregates rich in carbon and ammonium. Thus, turbulence simultaneously stimulates small-scale biological CO2 assimilation and large-scale biogeochemical C and N cycles in the ocean.

High single‐cell diversity in carbon and nitrogen assimilations by a chain‐forming diatom across a century

“…). Recently, a similar variation of carbon and nitrate assimilation rates at a single‐cell level was reported in two field populations of chain‐forming diatoms ( Skeletonema and Chaetoceros ) during a spring bloom (Bergkvist et al ., ). In those field populations, the individual cell‐specific carbon and nitrate assimilation rates varied even more than one order of magnitude while the average carbon‐to‐nitrate assimilation ratio was close to the Redfield ratio, and ammonium assimilation rates were only 14% of nitrate assimilation rates.…”

“…Ion transporters limit the nutrient uptake across the cell membrane (reaction limitation) under nutrient‐replete conditions and exponential growth, whereas diffusion and turbulent shear can limit transport of nutrients to the cell membrane under nutrient‐limiting conditions. Skeletonema forms long cell‐chains both in cultures and in the field (up to 22 cells chain −1 ) (Bergkvist et al ., ; ; Sildever et al ., ). Such long cell chains are beneficial for growth in a turbulent environment, and carbon assimilation rate in Skeletonema increased by about 30% at a turbulence level equal to that occurring in the wave breaking zone when nitrate concentrations were < 0.3 μM (Bergkvist et al ., ).…”

“…Skeletonema forms long cell‐chains both in cultures and in the field (up to 22 cells chain −1 ) (Bergkvist et al ., ; ; Sildever et al ., ). Such long cell chains are beneficial for growth in a turbulent environment, and carbon assimilation rate in Skeletonema increased by about 30% at a turbulence level equal to that occurring in the wave breaking zone when nitrate concentrations were < 0.3 μM (Bergkvist et al ., ). Turbulence was absent during the present study.…”

“…Chain-forming diatoms are key organisms in the biological fixation and sequestration of CO 2 in the ocean (Nelson et al, 1995;Bergkvist et al, 2018). In the euphotic zone, they form fast-sinking aggregates, which transport fixed inorganic carbon and nutrients to the deep sea or shallow sediments (Fowler and Knauer, 1986;Alldredge and Gotschalk, 1988).…”

Application of the isotope pairing technique in sediments: Use, challenges, and new directions

“…Long (> 3 h) pre‐incubation times may be useful to ensure the 15 NO 3 − has can penetrate through the oxic layer to the zone of NO 3 − reduction; however, this also increases the time available to MPB to continuously take up NO 3 − . Additional experiments are required to specifically assess intracellular NO 3 − uptake, storage, and respiration during light and dark exposure such as spatially resolving isotope measurements by Secondary Ion Mass Spectrometry (SIMS) in combination with labeling experiments (e.g., Klawonn et al ; Bergkvist et al ).…”