(Table 1) Biological and chemical characteristics of surface water stations during Nathaniel B. Palmer cruise NBP06-08 to the Ross Sea

We present results from a field study of inorganic carbon (C) acquisition by Ross Sea phytoplankton during Phaeocystis-dominated early season blooms. Isotope disequilibrium experiments revealed that HCO3? was the primary inorganic C source for photosynthesis in all phytoplankton assemblages. From these experiments, we also derived relative enhancement factors for HCO3?/CO2 interconversion as a measure of extracellular carbonic anhydrase activity (eCA). The enhancement factors ranged from 1.0 (no apparent eCA activity) to 6.4, with an overall mean of 2.9. Additional eCA measurements, made using membrane inlet mass spectrometry (MIMS), yielded activities ranging from 2.4 to 6.9 U/[?g chl a] (mean 4.1). Measurements of short-term C-fixation parameters revealed saturation kinetics with respect to external inorganic carbon, with a mean half-saturation constant for inorganic carbon uptake (K1/2) of ~380 ?M. Comparison of our early springtime results with published data from late-season Ross Sea assemblages showed that neither HCO3? utilization nor eCA activity was significantly correlated to ambient CO2 levels or phytoplankton taxonomic composition. We did, however, observe a strong negative relationship between surface water pCO2 and short-term 14C-fixation rates for the early season survey. Direct incubation experiments showed no statistically significant effects of pCO2 (10 to 80 Pa) on relative HCO3? utilization or eCA activity. Our results provide insight into the seasonal regulation of C uptake by Ross Sea phytoplankton across a range of pCO2 and phytoplankton taxonomic composition.

Alkenone and boron based Oligocene pCO2 records

The Pliocene period is the most recent time when the Earth was globally significantly (~3°C) warmer than today. However, the existing pCO2 data for the Pliocene are sparse and there is little agreement between the various techniques used to reconstruct palaeo-pCO2. Moreover, the temporal resolution of the published records does not allow a robust assessment of the role of declining pCO2 in the intensification of the Northern Hemisphere Glaciation (INHG) and a direct comparison to other proxy records are lacking. For the first time, we use a combination of foraminiferal (delta11B) and organic biomarker (alkenone-derived carbon isotopes) proxies to determine the concentration of atmospheric CO2 over the past 5 Ma. Both proxy records show that during the warm Pliocene pCO2 was between 330 and 400 ppm, i.e. similar to today. The decrease to values similar to pre-industrial times (275-285 ppm) occurred between 3.2 Ma and 2.8 Ma - coincident with the INHG and affirming the link between global climate, the cryosphere and pCO2.

Negative effects of ocean acidification on calcification vary within the coccolithophore genus Calcidiscus

A large percentage of CO2 emitted into the atmosphere is absorbed by the oceans, causing chemical changes in surface waters known as ocean acidification (OA). Despite the high interest and increased pace of OA research to understand the effects of OA on marine organisms, many ecologically important organisms remain unstudied. Calcidiscus is a heavily calcified coccolithophore genus that is widespread and genetically and morphologically diverse. It contributes substantially to global calcium carbonate production, organic carbon production, oceanic carbon burial, and ocean-atmosphere CO2 exchange. Despite the importance of this genus, relatively little work has examined its responses to OA. We examined changes in growth, morphology, and carbon allocation in multiple strains of Calcidiscus leptoporus in response to ocean acidification. We also, for the first time, examined the OA response of Calcidiscus quadriperforatus, a larger and more heavily calcified Calcidiscus congener. All Calcidiscus coccolithophores responded negatively to OA with impaired coccolith morphology and a decreased ratio of particulate inorganic to organic carbon (PIC:POC). However, strains responded variably; C. quadriperforatus showed the most sensitivity, while the most lightly calcified strain of C. leptoporus showed little response to OA. Our findings suggest that calcium carbonate production relative to organic carbon production by Calcidiscus coccolithophores may decrease in future oceans and that Calcidiscus distributions may shift if more resilient strains and species become dominant in assemblages. This study demonstrates that variable responses to OA may be strain or species specific in a way that is closely linked to physiological traits, such as cellular calcite quota.

Ocean acidification alters the calcareous microstructure of the green macro-alga Halimeda opuntia

Decreases in seawater pH and carbonate saturation state (Omega) following the continuous increase in atmospheric CO2 represent a process termed ocean acidification, which is predicted to become a main threat to marine calcifiers in the near future. Segmented, tropical, marine green macro-algae of the genus Halimeda form a calcareous skeleton that involves biotically initiated and induced calcification processes influenced by cell physiology. As Halimeda is an important habitat provider and major carbonate sediment producer in tropical shallow areas, alterations of these processes due to ocean acidification may cause changes in the skeletal microstructure that have major consequences for the alga and its environment, but related knowledge is scarce. This study used scanning electron microscopy to examine changes of the CaCO3 segment microstructure of Halimedaopuntia specimens that had been exposed to artificially elevated seawater pCO2 of 650 µatm for 45 d. In spite of elevated seawater pCO2, the calcification of needles, located at the former utricle walls, was not reduced as frequent initiation of new needle-shaped crystals was observed. Abundance of the needles was 22 %/µm**2 higher and needle crystal dimensions 14 % longer. However, those needles were 42 % thinner compared with the control treatment. Moreover, lifetime cementation of the segments decreased under elevated seawater pCO2 due to a loss in micro-anhedral carbonate as indicated by significantly thinner calcified rims of central utricles (35-173 % compared with the control treatment). Decreased micro-anhedral carbonate suggests that seawater within the inter-utricular space becomes CaCO3 undersaturated (Omega < 1) during nighttime under conditions of elevated seawater pCO2, thereby favoring CaCO3 dissolution over micro-anhedral carbonate accretion. Less-cemented segments of H. opuntia may impair the environmental success of the alga, its carbonate sediment contribution, and the temporal storage of atmospheric CO2 within Halimeda-derived sediments.