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Bio‐optical properties of marine phytoplankton retrieved through satellite remote sensing are used to estimate ocean productivity and carbon cycling. Daily activity of phytoplankton is attuned to the predictable light fluctuations of the diel cycle. Field and laboratory studies have documented diel changes in phytoplankton growth, division, and respiration, carbon and pigment content, cell size, photosynthetic efficiency and rate, and DNA replication and transcription. Many of these physiological changes can alter cellular optical properties and contribute to diel variations in bulk absorption and scattering properties. Consequently, understanding phytoplankton contributions to diel optical cycles is essential for improving algorithms that convert remote sensing data to biological rates and stocks. Here, we describe time‐resolved cellular, photophysiological, and bio‐optical properties for three cultured phytoplankton ranging in cell size from ~ 1 to 6 μm: Ostreococcus lucimarinus, Synechococcus (WH8102), and Thalassiosira pseudonana, all of which can significantly contribute to phytoplankton abundance and/or biomass in the open and coastal ocean. This work is the first to characterize complete diel cycles in absorption and attenuation for O. lucimarinus and backscattering for all these species. Results show that the percent increase from the minimum to maximum values over the diel cycle ranged between ~ 24–121%, ~ 31–235%, and ~ 25–198% for particulate absorption, attenuation, and backscattering, respectively. Diel changes in bio‐optical properties also differed in both timing and magnitude across phytoplankton species, demonstrating the importance of contextualizing remote sensing observations with phytoplankton community composition. [ABSTRACT FROM AUTHOR]

Episodic deep mixing events are one component of the biological carbon pump that physically transports organic carbon into the mesopelagic. Episodic deep mixing also disrupts summertime thermal stratification thereby changing the light field and nutrient concentrations available for phytoplankton growth. Phytoplankton survival and growth below the mixed layer following restratification depends on how rapidly cells can employ a variety of photoacclimation processes in response to the environmental changes. To compare the relative timescales of summertime episodic deep mixing events with the timescales of phytoplankton photoacclimation processes, we first analyzed autonomous float data to survey the frequency and magnitude of deep mixing events in the western North Atlantic Ocean. Next, we simulated a sustained deep mixing event in the laboratory and measured rates of acclimation processes ranging from light harvesting to growth in a model diatom and green alga. In both algae increases in chlorophyll (Chl) were coupled to growth, but growth of the green alga lagged the diatom by about a day. In float profiles, significant increases in Chl and phytoplankton carbon (Cphyto) were detected below the mixed layer following episodic deep mixing events. These events pose a previously unrecognized source of new production below the mixed layer that can significantly boost the amount of carbon available for export to the deep ocean. [ABSTRACT FROM AUTHOR]

Acetaldehyde, methanol, acetone, and isoprene are important reactive volatile organic compounds (VOCs) in the oceans that partition to the atmosphere in significant amounts. Reports of potentially high rates of VOC turnover in the North Atlantic suggested that both biotic and abiotic processes are involved. The biological basis for VOC cycling by ocean plankton is unknown, but is potentially important because of VOC contributions to carbon cycle budgets and atmospheric chemistry. We designed dynamic stripping chambers that coupled to a proton transfer reaction mass spectrometer to measure VOC production and consumption by cultured phytoplankton and bacterioplankton. The diatom, Thalassiosira pseudonana, produced acetaldehyde in a light-dependent manner. Acetaldehyde was oxidized by the chemoheterotrophic bacterium, Pelagibacter, at rates that suggest that most acetaldehyde is recycled in the ocean before escaping to the atmosphere. These results show that field observations of acetaldehyde turnover reported previously could be explained by biological activity. Rates of production by phytoplankton cultures of methanol, acetone, and isoprene were also measured. These findings support the conclusion that VOCs are a conduit for carbon transfer directly from phytoplankton to bacterioplankton, with the remainder available for escape to the atmosphere. [ABSTRACT FROM AUTHOR]