Ingrid Zondervan

Reduced calcification of marine plankton in response to increased atmospheric CO2.

The formation of calcareous skeletons by marine planktonic organisms and their subsequent sinking to depth generates a continuous rain of calcium carbonate to the deep ocean and underlying sediments. This is important in regulating marine carbon cycling and ocean–atmosphere CO2 exchange. The present rise in atmospheric CO2 levels causes significant changes in surface ocean pH and carbonate chemistry. Such changes have been shown to slow down calcification in corals and coralline macroalgae,, but the majority of marine calcification occurs in planktonic organisms. Here we report reduced calcite production at increased CO2 concentrations in monospecific cultures of two dominant marine calcifying phytoplankton species, the coccolithophorids Emiliania huxleyi and Gephyrocapsa oceanica . This was accompanied by an increased proportion of malformed coccoliths and incomplete coccospheres. Diminished calcification led to a reduction in the ratio of calcite precipitation to organic matter production. Similar results were obtained in incubations of natural plankton assemblages from the north Pacific ocean when exposed to experimentally elevated CO2 levels. We suggest that the progressive increase in atmospheric CO2 concentrations may therefore slow down the production of calcium carbonate in the surface ocean. As the process of calcification releases CO2 to the atmosphere, the response observed here could potentially act as a negative feedback on atmospheric CO2 levels.

Decreasing marine biogenic calcification: A negative feedback on rising atmospheric pCO2

In laboratory experiments with the coccolithophore species Emiliania huxleyi and Gephyrocapsa oceanica, the ratio of particulate inorganic carbon (PIC) to particulate organic carbon (POC) production decreased with increasing CO2 concentration ([CO2]). This was due to both reduced PIC and enhanced POC production at elevated [CO2]. Carbon dioxide concentrations covered a range from a preindustrial level to a value predicted for 2100 according to a “business as usual” anthropogenic CO2 emission scenario. The laboratory results were used to employ a model in which the immediate effect of a decrease in global marine calcification relative to POC production on the potential capacity for oceanic CO2 uptake was simulated. Assuming that overall marine biogenic calcification shows a similar response as obtained for E. huxleyi or G. oceanica in the present study, the model reveals a negative feedback on increasing atmospheric CO2 concentrations owing to a decrease in the PIC/POC ratio.

Effects of growth rate, CO2 concentration, and cell size on the stable carbon isotope fractionation in marine phytoplankton

Stable carbon isotope fractionation (ep) 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, ep increased with increasing [CO2,aq]. Among the diatoms, the degree of CO2-related variability in ep 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 ep 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 ep 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 ep 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 ep and μ/[CO2,aq], even when differences in cellular carbon content and cell geometry are accounted for.

Effect of CO2 concentration on the PIC/POC ratio in the coccolithophore Emiliania huxleyi grown under light-limiting conditions and different daylengths.

We compared the effect of CO2 concentration ([CO2], ranging from ∼5 to ∼34 μmol l−1) at four different photon flux densities (PFD=15, 30, 80 and 150 μmol m−2 s−1) and two light/dark (L/D) cycles (16/8 and 24/0 h) on the coccolithophore Emiliania huxleyi. With increasing [CO2], a decrease in the particulate inorganic carbon to particulate organic carbon (PIC/POC) ratio was observed at all light intensities and L/D cycles tested. The individual response in cellular PIC and POC to [CO2] depended strongly on the PFD. POC production increased with rising [CO2], irrespective of the light intensity, and PIC production decreased with increasing [CO2] at a PFD of 150 μmol m−2 s−1, whereas below this light level it was unaffected by [CO2]. Cell growth rate decreased with decreasing PFD, but was largely independent of ambient [CO2]. The diurnal variation in PIC and POC content, monitored over a 38-h period (16/8 h L/D, PFD=150 μmol m−2 s−1), exceeded the difference in carbon content between cells grown at high (∼29 μmol l−1) and low (∼4 μmol l−1) [CO2]. However, consistent with the results described above, cellular POC content was higher and PIC content lower at high [CO2], compared to the values at low [CO2], and the offset was observed throughout the day. It is suggested that the observed sensitivity of POC production for ambient [CO2] may be of importance in regulating species-specific primary production and species composition.

Effect of trace metal availability on coccolithophorid calcification

The deposition of atmospheric dust into the ocean has varied considerably over geological time. Because some of the trace metals contained in dust are essential plant nutrients which can limit phytoplankton growth in parts of the ocean, it has been suggested that variations in dust supply to the surface ocean might influence primary production. Whereas the role of trace metal availability in photosynthetic carbon fixation has received considerable attention, its effect on biogenic calcification is virtually unknown. The production of both particulate organic carbon and calcium carbonate (CaCO3) drives the oceans biological carbon pump. The ratio of particulate organic carbon to CaCO3 export, the so-called rain ratio, is one of the factors determining CO2 sequestration in the deep ocean. Here we investigate the influence of the essential trace metals iron and zinc on the prominent CaCO3-producing microalga Emiliania huxleyi. We show that whereas at low iron concentrations growth and calcification are equally reduced, low zinc concentrations result in a de-coupling of the two processes. Despite the reduced growth rate of zinc-limited cells, CaCO3 production rates per cell remain unaffected, thus leading to highly calcified cells. These results suggest that changes in dust deposition can affect biogenic calcification in oceanic regions characterized by trace metal limitation, with possible consequences for CO2 partitioning between the atmosphere and the ocean.

Growth rate dependence of Sr incorporation during calcification of Emiliania huxleyi

carbon relies on accurate determination of the growth rate of phytoplankton contributing to sedimentary organic matter. We demonstrate that the Sr/Ca ratio in the coccoliths of Emiliania huxleyi is correlated with the rates of both organic carbon fixation and calcification. An investigation of biomineralization models suggests that these three factors may be linked by the differential pumping rates of calcium relative to strontium ions across the cellular membranes which supply ions for photosynthetic carbon fixation and calcification and derives energy from photosynthetic products. A quantitative relationship is derived between calcification, organic carbon fixation, and Sr/Ca such that a combined geochemical proxy approach using ep and Sr/Ca can be applied to the paleoceanographic record. INDEX TERMS: 1045 Geochemistry: Lowtemperature geochemistry; 4808 Oceanography: Biological and Chemical: Chemical tracers; 4855 Oceanography: Biological and Chemical: Plankton; 4875 Oceanography: Biological and Chemical: Trace elements; KEYWORDS: Sr/Ca, growth rate, E. huxleyi, coccolithophore, proxy

Biological versus physical processes as drivers of large oscillations of the air-sea CO2 flux in the Antarctic marginal ice zone during summer

The fugacity of CO2 andabund ance of chlorophyll a (Chla) were determined in two long transects from the Polar Front to the Antarctic Continent in austral summer, December 1995–January 1996. Large undersaturations of CO2 in the surface water were observedcoinciding with high Chl a content. In the major hydrographic regions the mean air–sea fluxes were foundto range from � 3 to +7 mmol m � 2 d � 1 making these regions act as a sink as well as a source for CO2. In the total 40-d period, the summation of the several strong source and sink regions revealed an overall modest net source of 0.3 mmol m � 2 d � 1 , this basedon the Wanninkhof (J. Geophys. Res. 97 (1992) 7373) quadratic relationship at in situ windspeed. A simple budget approach was used to quantify the role of phytoplankton blooms in the inorganic carbonate system of the Antarctic seas in a time frame spanning several weeks. The major controlling physical factors such as air–sea flux, Ekman pumping and upwelling are included. Net community production varies between � 9 and +7 mmol m � 2 d � 1 , because of the large oscillations in the dominance of autotrophic (CO2 fixation) versus heterotrophic (CO2 respiration) activity. Here the mixedlayer d epth is the major controlling factor. When integratedover time the gross influx andefflux of CO 2 from air to sea is large, but the net residual air/sea exchange is a modest efflux from sea to atmosphere.