Seawater carbonate chemistry and biological processed during experiments with corals Porites sp. & Stylophora pistillata, 2010

Uptake of anthropogenic CO2 by the oceans is altering seawater chemistry with potentially serious consequences for coral reef ecosystems due to the reduction of seawater pH and aragonite saturation state (omega arag). The objectives of this long-term study were to investigate the viability of two ecologically important reef-building coral species, massive Porites sp. and Stylophora pistilata, exposed to high pCO2(or low pH) conditions and to observe possible changes in physiologically related parameters as well as skeletal isotopic composition. Fragments of Porites sp. and S. pistilata were kept for 6-14 months under controlled aquarium conditions characterized by normal and elevated pCO2 conditions, corresponding to pHTvalues of 8.09, 7.49, and 7.19, respectively. In contrast with shorter, and therefore more transient experiments, the long experimental timescale achieved in this study ensures complete equilibration and steady state with the experimental environment and guarantees that the data provide insights into viable and stably growing corals. During the experiments, all coral fragments survived and added new skeleton, even at seawater omega arag <1, implying that the coral skeleton is formed by mechanisms under strong biological control. Measurements of boron (B), carbon (C) and oxygen (O) isotopic composition of skeleton, C isotopic composition of coral tissue and symbiont zooxanthellae, along with physiological data (such as skeletal growth, tissue biomass, zooxanthellae cell density and chlorophyll concentration) allow for a direct comparison with corals living under normal conditions and sampled simultaneously. Skeletal growth and zooxanthellae density were found to decrease, whereas coral tissue biomass (measured as protein concentration) and zooxanthellae chlorophyll concentrations increased under high pCO2 (low pH) conditions. Both species showed similar trends of delta11B depletion and delta18O enrichment under reduced pH, whereas the delta13C results imply species-specific metabolic response to high pCO2 conditions. The skeletal delta11B values plot above seawater delta11B vs. pH borate fractionation curves calculated using either the theoretically derived deltaB value of 1.0194 (Kakihana et al., Bull. Chem. Soc. Jpn. 50(1977), 158) or the empirical deltaB value of 1.0272 (Klochko et al., EPSL 248 (2006), 261). However, the effective deltaB must be greater than 1.0200 in order to yield calculated coral skeletal delta11B values for pH conditions where omega arag >1. The delta11B vs. pH offset from the literature seawater delta11B vs. pH fractionation curves suggests a change in the ratio of skeletal material laid down during dark and light calcification and/or an internal pH regulation, presumably controlled by ion-transport enzymes. Finally, seawater pH significantly influences skeletal delta13C and delta18O. This must be taken into consideration when reconstructing paleo-environmental conditions from coral skeleton

Effects of CO2 and iron availability on rbcL gene expression in Bering Sea diatoms

Iron (Fe) can limit phytoplankton productivity in approximately 40% of the global ocean, including in high-nutrient, low-chlorophyll (HNLC) waters. However, there is little information available on the impact of CO2-induced seawater acidification on natural phytoplankton assemblages in HNLC regions. We therefore conducted an on-deck experiment manipulating CO2 and Fe using Fe-deficient Bering Sea water during the summer of 2009. The concentrations of CO2 in the incubation bottles were set at 380 and 600 ppm in the non-Fe-added (control) bottles and 180, 380, 600, and 1000 ppm in the Fe-added bottles. The phytoplankton assemblages were primarily composed of diatoms followed by haptophytes in all incubation bottles as estimated by pigment signatures throughout the 5-day (control) or 6-day (Fe-added treatment) incubation period. At the end of incubation, the relative contribution of diatoms to chlorophyll a biomass was significantly higher in the 380 ppm CO2 treatment than in the 600 ppm treatment in the controls, whereas minimal changes were found in the Fe-added treatments. These results indicate that, under Fe-deficient conditions, the growth of diatoms could be negatively affected by the increase in CO2 availability. To further support this finding, we estimated the expression and phylogeny of rbcL (which encodes the large subunit of RuBisCO) mRNA in diatoms by quantitative reverse transcription polymerase chain reaction (PCR) and clone library techniques, respectively. Interestingly, regardless of Fe availability, the transcript abundance of rbcL decreased in the high CO2 treatments (600 and 1000 ppm). The present study suggests that the projected future increase in seawater pCO2 could reduce the RuBisCO transcription of diatoms, resulting in a decrease in primary productivity and a shift in the food web structure of the Bering Sea.

(Appendix) Carbon and nitrogen isotope ratios of ODP Site 160-964

Nitrogen isotopes of chlorins, degradation products of chlorophyll, reflect the isotopic composition of nutrient N utilized by marine phytoplankton communities. Here we show that in sediments of the eastern Mediterranean Pleistocene and Holocene, values of d15N for chlorins and total nitrogen vary in concert, with a consistent offset of ~5 per mil reflecting the fractionation imparted during chlorophyll biosynthesis. Samples from the Integrated Ocean Drilling Program Sites 964 and 969 were analyzed at a sampling resolution of ~4-10 cm, clustered around sapropel events 2, 3, 4 and 5 (~100-170 ka). In low organic content sediments, chlorin values of ~0 per mil coincident with total nitrogen values of ~+ 5 per mil indicate that the latter reflects the original biomass and is not a consequence of diagenetic isotope enrichment. In sapropel horizons, the chlorin and total nitrogen values are 5 per mil more negative (~-5 per mil and ~ 0 per mil, respectively), resembling previously-reported, modern-day water-column particulates (~0 per mil). We suggest that nutrient conditions in the Eastern Mediterranean correspond to three scenarios and that the similarity between sapropel and modern-day bulk d15N is coincidental. Organic-poor marl sediments formed under oligotrophic conditions where surface productivity resulted from upwelling of Atlantic-sourced nitrate. Sapropels were characterized by enhanced diazotrophy that was likely fueled by increased riverine P fluxes to surface waters. Present-day conditions are dominated by anthropogenic N sources. These scenarios agree with a model of sapropel formation in which stratification caused by increased fresh-water inputs led to N fixation due to P:N nutrient imbalance. Enhanced production combined with stratification promoted and maintained anoxic deep waters, consequently increasing organic matter preservation. Such a model may be relevant to interpreting other episodes of intense organic matter deposition in past oceans.

Chemical composition and phytoplankton characteristics of water samples obtained during POLARSTERN cruise ANT-XXIV/3

The fixation of dissolved inorganic carbon (DIC) by marine phytoplankton provides an important feedback mechanism on concentrations of CO2 in the atmosphere. As a consequence it is important to determine whether oceanic primary productivity is susceptible to changing atmospheric CO2 levels Among numerous other factors, the acquisition of DIC by microalgae particularly in the polar seas is projected to have a significant effect on future phytoplanktonic production and hence atmospheric CO2 concentrations. Using the isotopic disequilibrium technique the contribution of different carbon species (CO2 and bicarbonate) to the overall DIC uptake and the extent to which external Carbonic Anhydrase (eCA) plays a role in facilitating DIC uptake was estimated. Simultaneous uptake of CO2 and HCO3- was observed in all cases, but the proportions in which different DIC species contributed to carbon assimilation varied considerably between stations. Bicarbonate as well as CO2 could be the major DIC source for local phytoplankton assemblages. There was a positive correlation between the contribution of CO2 to total DIC uptake and ambient concentration of CO2 in seawater suggesting that Southern Ocean microalgae could increase the proportion of CO2 uptake under future high atmospheric CO2 levels. Results will be discussed in view of metabolic costs related to DIC acquisition of Southern Ocean phytoplankton.

Reduced calcification decreases photoprotective capability in the Coccolithophorid Emiliania huxleyi

Intracellular calcification of coccolithophores generates CO2 and consumes additional energy for acquisition of calcium and bicarbonate ions; therefore, it may correlate with photoprotective processes by influencing the energetics. To address this hypothesis, a calcifying Emiliania huxleyi strain (CS-369) was grown semi-continuously at reduced (0.1 mM, LCa) and ambient Ca2+ concentrations (10 mM, HCa) for 150 d (>200 generations). The HCa-grown cells had higher photosynthetic and calcification rates and higher contents of Chl a and carotenoids compared with the naked (bearing no coccoliths) LCa-grown cells. When exposed to stressfull levels of photosynthetically active radiation (PAR), LCa-grown cells displayed lower photochemical yield and less efficient non-photochemical quenching (NPQ). When the LCa- or HCa-grown cells were inversely shifted to their counterpart medium, LCa to HCa transfer increased photosynthetic carbon fixation (P), calcification rate (C), the C/P ratio, NPQ and pigment contents, whereas those shifted from HCa to LCa exhibited the opposite effects. Increased NPQ, carotenoids and quantum yield were clearly linked with increased or sustained calcification in E. huxleyi. The calcification must have played a role in dissipating excessive energy or as an additional drainage of electrons absorbed by the photosynthetic antennae. This phenomenon was further supported by testing two non-calcifying strains, which showed insignificant changes in photosynthetic carbon fixation and NPQ when transferred to LCa conditions

Biogeochemical and physico-chemical characteristics in the gulf of Trieste, Italy

In this paper we investigated, for two years and with a bi-monthly frequency, how physical, chemical, and biological processes affect the marine carbonate system in a coastal area characterized by high alkalinity riverine discharge (Gulf of Trieste, northern Adriatic Sea, Mediterranean Sea).