Description: Stable carbon isotope fractionation (εp) was measured in four marine diatom and one dinoflagellate species of different cell sizes. Monospecific cultures were incubated under high-light and nutrient-replete conditions at 16 h : 8 h and 24 h : 0 h light/dark cycles in dilute batch cultures at six CO2 concentrations, [CO2,aq], ranging from ca. 1 to 38 μmol kg−1. In all species, εp increased with increasing [CO2,aq]. Among the diatoms, the degree of CO2-related variability in εp was inversely correlated with cell size. Isotopic fractionation in the dinoflagellate differed in several aspects from that of the diatoms, which may reflect both morphological and physiological differences between taxa. Daylength-related changes in instantaneous growth rate, defined as the rate of C assimilation during the photoperiod, affected εp to a similar or greater extent than differences in experimental [CO2,aq] in three of the species tested. In contrast, the irradiance cycle had no effect on εp in 2 other species. With the exception of Phaeodactylum tricornutum, growth rate of all species declined below a critical [CO2,aq]. At these concentrations, we observed a reversal in the CO2-related εp trend, which we attribute to a decline in carbon assimilation efficiency. Although uncatalyzed passive diffusion of CO2 into the cell was sufficient to account for gross carbon uptake in most treatments, our results indicate that other processes contribute to inorganic carbon acquisition in all species even at [CO2,aq] 〉 10 μmol kg−1. These processes may include active C transport and/or catalyzed conversion of HCO3− to CO2 by carbonic anhydrase. A comparison of our results with data from the literature indicates significant deviations from previously reported correlations between εp and μ/[CO2,aq], even when differences in cellular carbon content and cell geometry are accounted for.

Description: Inorganic carbon uptake by phytoplankton depletes the immediate cell environment and disturbs the carbonate system equilibrium. Uptake is balanced by both diffusional transport across and chemical reactions within the depleted boundary layer. In this study, we have derived a model that simulates inorganic carbon diffusion and reactions in the vicinity of phytoplankton cells. To allow a general application of the model, the reaction kinetics of the carbonate system are reviewed and temperature- and salinity-dependence of the various rate constants are discussed. A consistency condition for some of the kinetic rates is derived. The effective thickness of the diffusive boundary layer in spherical and planar geometry is discussed. In addition, the effect of cell shape on diffusive transport to phytoplankton is examined and a simple means to account for this effect in model calculations is presented. In a second step, the complete description of the diffusion-reaction system is simplified to consider two special cases in which (1) algal production relies on CO2(aq) as the single source of inorganic carbon, and (2) CO2, HCO3−, or CO32− are utilized independently for organic matter production combined with calcite precipitation. In the size range typical for phytoplankton cells model predictions of these simplified versions are nearly identical to those of the complete model, indicating that the simplified models represent good approximations of the complete diffusion-reaction system.

Description: While the aggregation and mass settlement of diatoms at the termination of blooms results in significant export of carbon from the surface ocean, the mechanisms of bloom aggregation have been poorly understood. The aggregation of a multispecies diatom bloom was investigated under controlled conditions in a 1200 liter, nutrient-enriched, laboratory mesocosm in order to elucidate the parameters sufficient to accurately predict bloom aggregation. A diverse bloom of diatoms dominated by several species of Chaetoceros and Thalassiosira progressed through a classic pattern of exponential, stationary, and senescent phases in the mesocosm. Aggregates larger than 0.5 mm became detectable on the eighth day after inoculation, and aggregates 〉1 mm increased exponentially from Day 10 onward producing the appearance of a mass aggregation event late on Day 10. The bloom aggregated sequentially with Thalassiosira dominating early aggregates and Chaetoceros dominating later ones. Chaetoceros resting spores formed only in aggregates. Aggregation was not linked to nutrient depletion or to the physiological state of the cells since the onset of aggregation and the mass aggregation event occurred 1 to 3 days prior to nutrient depletion and while carbon:nitrogen ratios of cells were still very low and growth rates high. Moreover, visible aggregates did not form in the mesocosm until cell abundances were considerably higher than abundances observed to aggregate in nature, suggesting that aggregation was not strongly linked to phytoplankton cell concentration. Complementary studies in this volume clarify the role of non-phytoplankton particles in aggregation of the mesocosm bloom. The mesocosm approach proved highly effective in producing an aggregating diatom bloom under controlled conditions.