ALBERT

Description: During phytoplankton growth a fraction of dissolved inorganic carbon (DIC) assimilated by phytoplankton is exuded in the form of dissolved organic carbon (DOC), which can be transformed into extracellular particulate organic carbon (POC). A major fraction of extracellular POC is associated with carbon of transparent exopolymer particles (TEP; carbon content = TEPC) that form from dissolved polysaccharides (PCHO). The exudation of PCHO is linked to an excessive uptake of DIC that is not directly quantifiable from utilisation of dissolved inorganic nitrogen (DIN), called carbon overconsumption. Given these conditions, the concept of assuming a constant stoichiometric carbon-to-nitrogen (C:N) ratio for estimating new production of POC from DIN uptake becomes inappropriate. Here, a model of carbon overconsumption is analysed, combining phytoplankton growth with TEPC formation. The model describes two modes of carbon overconsumption. The first mode is associated with DOC exudation during phytoplankton biomass accumulation. The second mode is decoupled from algal growth, but leads to a continuous rise in POC while particulate organic nitrogen (PON) remains constant. While including PCHO coagulation, the model goes beyond a purely physiological explanation of building up carbon rich particulate organic matter (POM). The model is validated against observations from a mesocosm study. Maximum likelihood estimates of model parameters, such as nitrogen- and carbon loss rates of phytoplankton, are determined. The optimisation yields results with higher rates for carbon exudation than for the loss of organic nitrogen. It also suggests that the PCHO fraction of exuded DOC was 63±20% during the mesocosm experiment. Optimal estimates are obtained for coagulation kernels for PCHO transformation into TEPC. Model state estimates are consistent with observations, where 30% of the POC increase was attributed to TEPC formation. The proposed model is of low complexity and is applicable for large-scale biogeochemical simulations.

Description: During phytoplankton growth a fraction of dissolved inorganic carbon (DIC) assimilated by phytoplankton is exuded in the form of dissolved organic carbon (DOC), which can be transformed into extracellular particulate organic carbon (POC). A major fraction of extracellular POC is associated with carbon of transparent exopolymer particles (TEP; carbon content = TEPC) that form from dissolved polysaccharides (PCHO). The exudation of PCHO is linked to an excessive uptake of DIC that is not directly quantifiable from utilisation of dissolved inorganic nitrogen (DIN), called carbon overconsumption. Given these conditions, the concept of assuming a constant stoichiometric carbon-to-nitrogen (C:N) ratio for estimating new production of POC from DIN uptake becomes inappropriate. Here, a model of carbon overconsumption is analysed, combining phytoplankton growth with TEPC formation. The model describes two modes of carbon overconsumption. The first mode is associated with DOC exudation during phytoplankton biomass accumulation. The second mode is decoupled from algal growth, but leads to a continuous rise in POC while particulate organic nitrogen (PON) remains constant. While including PCHO coagulation, the model goes beyond a purely physiological explanation of building up carbon rich particulate organic matter (POM). The model is validated against observations from a mesocosm study. Maximum likelihood estimates of model parameters, such as nitrogen- and carbon loss rates of phytoplankton, are determined. The optimisation yields results with higher rates for carbon exudation than for the loss of organic nitrogen. It also suggests that the PCHO fraction of exuded DOC was 63±20% during the mesocosm experiment. Optimal estimates are obtained for coagulation kernels for PCHO transformation into TEPC. Model state estimates are consistent with observations, where 30% of the POC increase was attributed to TEPC formation. The proposed model is of low complexity and is applicable for large-scale biogeochemical simulations.

Description: A methane surplus relative to the atmospheric equilibrium is a frequent feature of ocean surface water. Despite the common fact that biological processes are responsible for its origin, the formation of methane in aerobic surface water is still poorly understood. We report on methane production in the central Arctic Ocean, which was exclusively detected in Pacific derived water but not nearby in Atlantic derived water. Both water masses are distinguished by their different nitrate to phosphate ratios. We show that methane production occurs if nitrate is depleted but phosphate as P source is available. Apparently the low N:P ratio enhances the ability of bacteria to compete for phosphate while the phytoplankton metabolite dimethylsulfoniopropionate (DMSP) is utilized as a C source. This was verified by experimentally induced methane production in DMSP spiked Arctic sea water. Accordingly we propose that methylated compounds may serve as precursors for methane and thermodynamic calculations show that methylothrophic methanogenesis can provide energy in aerobic environments.

Description: Uptake of half of the fossil fuel CO2 into the ocean causes gradual seawater acidification. This has been shown to slow down calcification of major calcifying groups, such as corals, foraminifera, and coccolithophores. Here we show that two of the most productive marine calcifying species, the coccolithophores Coccolithus pelagicus and Calcidiscus leptoporus, do not follow the CO2-related calcification response previously found. In batch culture experiments, particulate inorganic carbon (PIC) of C. leptoporus changes with increasing CO2 concentration in a nonlinear relationship. A PIC optimum curve is obtained, with a maximum value at present-day surface ocean pCO2 levels (∼360 ppm CO2). With particulate organic carbon (POC) remaining constant over the range of CO2 concentrations, the PIC/POC ratio also shows an optimum curve. In the C. pelagicus cultures, neither PIC nor POC changes significantly over the CO2 range tested, yielding a stable PIC/POC ratio. Since growth rate in both species did not change with pCO2, POC and PIC production show the same pattern as POC and PIC. The two investigated species respond differently to changes in the seawater carbonate chemistry, highlighting the need to consider species-specific effects when evaluating whole ecosystem responses. Changes of calcification rate (PIC production) were highly correlated to changes in coccolith morphology. Since our experimental results suggest altered coccolith morphology (at least in the case of C. leptoporus) in the geological past, coccoliths originating from sedimentary records of periods with different CO2 levels were analyzed. Analysis of sediment samples was performed on six cores obtained from locations well above the lysocline and covering a range of latitudes throughout the Atlantic Ocean. Scanning electron micrograph analysis of coccolith morphologies did not reveal any evidence for significant numbers of incomplete or malformed coccoliths of C. pelagicus and C. leptoporus in last glacial maximum and Holocene sediments. The discrepancy between experimental and geological results might be explained by adaptation to changing carbonate chemistry.

Description: In recent studies the Sr/Ca ratio of coccolithophore calcite was used as a proxy for past coccolithophore growth and calcification rates. Since Sr and Ca concentrations in seawater have not remained constant through time, interpretation of Sr/Ca data from the coccolith-dominated sedimentary record requires knowledge about the incorporation of seawater Sr into coccolith calcite during coccolithogenesis. Here we show that Sr/Ca of Emiliania huxleyi coccoliths is linearly related to seawater Sr/Ca, meaning that the Sr exchange coefficient does not change with changing seawater Sr/Ca. The exchange coefficient for Sr in this study, 0.39, is close to values presented in the literature and is high compared with values obtained by inorganic precipitation experiments. This suggests a strong effect of cell physiology on biogenic calcite precipitation in coccolithophores. We present a conceptual model, based on the transmembrane transport of Sr and Ca, which explains the offset.

Description: During phytoplankton growth a fraction of dissolved inorganic carbon (DIC) assimilated by phytoplankton is exuded in the form of dissolved organic carbon (DOC), which can be transformed into extracellular particulate organic carbon (POC). A major fraction of extracellular POC is associated with carbon of transparent exopolymer particles (TEP; carbon content = TEPC) that form from dissolved polysaccharides (PCHO). The exudation of PCHO is linked to an excessive uptake of DIC that is not directly quantifiable from utilisation of dissolved inorganic nitrogen (DIN), called carbon overconsumption. Given these conditions, the concept of assuming a constant stoichiometric carbon-to-nitrogen (C:N) ratio for estimating new production of POC from DIN uptake becomes inappropriate. Here, a model of carbon overconsumption is analysed, combining phytoplankton growth with TEPC formation. The model describes two modes of carbon overconsumption. The first mode is associated with DOC exudation during phytoplankton biomass accumulation. The second mode is decoupled from algal growth, but leads to a continuous rise in POC while particulate organic nitrogen (PON) remains constant. While including PCHO coagulation, the model goes beyond a purely physiological explanation of building up carbon rich particulate organic matter (POM). The model is validated against observations from a mesocosm study. Maximum likelihood estimates of model parameters, such as nitrogen- and carbon loss rates of phytoplankton, are determined. The optimisation yields results with higher rates for carbon exudation than for the loss of organic nitrogen. It also suggests that the PCHO fraction of exuded DOC was 63±20% during the mesocosm experiment. Optimal estimates are obtained for coagulation kernels for PCHO transformation into TEPC. Model state estimates are consistent with observations, where 30% of the POC increase was attributed to TEPC formation. The proposed model is of low complexity and is applicable for large-scale biogeochemical simulations.

Description: The marine calcifying algae Emiliania huxleyi (coccolithophores) was grown in laboratory cultures under varying conditions with respect to the environmental parameters of temperature and carbonate ion concentration [CO32-] concentration. The Ca isotope composition of E. huxleyi's coccoliths reveals new insights into fractionation processes during biomineralization. The temperature-dependent Ca isotope fractionation resembles previous calibrations of inorganic and biogenic calcite and aragonite. Unlike inorganically precipitated calcite, the [CO32-] concentration of the medium has no significant effect on the Ca isotope composition of the coccoliths. These results indicate a decoupling of the chemical properties of the bulk medium and the calcifying vesicle. Cellular Ca pathways of E. huxleyi indicate that fractionation cannot occur at the crystal surface, as occurs during inorganic precipitation. The dominant processes leading to the observed Ca isotope fractionation pattern in E. huxleyi are most likely the dehydration of the Ca aquocomplex at the plasma membrane and the attachment of dissolved Ca to proteins of Ca channels. The independence of Ca isotope fractionation from [CO32-] and the small temperature dependence of E. huxleyi are also important for defining the isotopic signature of the oceanic Ca sink. Since coccolithophores contribute to about half the global CaCO3 production, a relatively uniform isotopic composition of the oceanic Ca sink is further supported.