Concentrations and carbon isotopic compositions of organic and inorganic compounds in sediment porewaters at IODP Site U1329

Ocean drilling has revealed the existence of vast microbial populations in the deep subseafloor, but to date little is known about their metabolic activities. To better understand the biogeochemical processes in the deep biosphere, we investigate the stable carbon isotope chemistry of acetate and other carbon-bearing metabolites in sediment pore-waters. Acetate is a key metabolite in the cycling of carbon in anoxic sediments. Its stable carbon isotopic composition provides information on the metabolic processes dominating acetate turnover in situ. This study reports our findings for a methane-rich site at the northern Cascadia Margin (NE Pacific) where Expedition 311 of the Integrated Ocean Drilling Program (IODP) sampled the upper 190 m of sediment. At Site U1329, d13C values of acetate span a wide range from -46.0 per mill to -11.0 per mill vs. VPDB and change systematically with sediment depth. In contrast, d13C values of both the bulk dissolved organic carbon (DOC) (-21.6 ± 1.3 per mill vs. VPDB) and the low-molecular-weight compound lactate (-20.9 ± 1.8 per mill vs. VPDB) show little variability. These species are interpreted to represent the carbon isotopic composition of fermentation products. Relative to DOC, acetate is up to 23.1 per mill depleted and up to 9.1 per mill enriched in 13C. Broadly, 13C-depletions of acetate relative to DOC indicate flux of carbon from acetogenesis into the acetate pool while 13C-enrichments of pore-water acetate relative to DOC suggest consumption of acetate by acetoclastic methanogenesis. Isotopic relationships between acetate and lactate or DOC provide new information on the carbon flow and the presence and activity of specific functional microbial communities in distinct biogeochemical horizons of the sediment. In particular, they suggest that acetogenic CO2-reduction can coexist with methanogenic CO2-reduction, a notion contrary to the hypothesis that hydrogen levels are controlled by the thermodynamically most favorable electron-accepting process. Further, the isotopic relationship suggests a relative increase in acetate flow to acetoclastic methanogenesis with depth although its contribution to total methanogenesis is probably small. Our study demonstrates how the stable carbon isotope biogeochemistry of acetate can be used to identify pathways of microbial carbon turnover in subsurface environments. Our observations also raise new questions regarding the factors controlling acetate turnover in marine sediments.

Abundance and biomass of dinoflagellates and ciliates at time series station Helgoland Roads, North Sea, 2007-2009

A monitoring programme for microzooplankton was started at the long-term sampling station ''Kabeltonne'' at Helgoland Roads (54°11.30' N; 7°54.00' E) in January 2007 in order to provide more detailed knowledge on microzooplankton occurrence, composition and seasonality patterns at this site and to complement the existing plankton data series. Ciliate and dinoflagellate cell concentration and carbon biomass were recorded on a weekly basis. Heterotrophic dinoflagellates were considerably more important in terms of biomass than ciliates, especially during the summer months. However, in early spring, ciliates were the major group of microzooplankton grazers as they responded more quickly to phytoplankton food availability. Mixotrophic dinoflagellates played a secondary role in terms of biomass when compared to heterotrophic species; nevertheless, they made up an intense late summer bloom in 2007. The photosynthetic ciliate Myrionecta rubra bloomed at the end of the sampling period. Due to its high biomass when compared to crustacean plankton especially during the spring bloom, microzooplankton should be regarded as the more important phytoplankton grazer group at Helgoland Roads. Based on these results, analyses of biotic and abiotic factors driving microzooplankton composition and abundance are necessary for a full understanding of this important component of the plankton.

Feeding parameters of copepods from upwelling areas

Feeding patterns of mass herbivorous copepods in upwelling areas are investigated. Daily rations and aspects of their formation are examined in Calanoides carinatus (Benguela upwelling), Calanus pacificus (off the California coast), and Calanus australis (Peru upwelling). Rations were calculated based on gut plant pigment contents obtained at daily stations using laser spectrofluorometry, experimental data on the rate of gut evacuation and data on the carbon/chlorophyll ratio in phytoplankton and particulate matter at the respective stations. When phytoplankton was abundant, diel feeding rhythms were not pronounced and gut pigment level was high during the entire 24-h period. When phytoplankton biomass was low, distinct feeding rhythms were pronounced with a nocturnal maximum. During active upwelling intensive feeding on phytoplankton supports energy (respiration) and plastic (growth, development, reproduction, accumulation of reserves) metabolism of copepods. When upwelling was inactive, the surface part of the population feeds less actively and is able only partially to cover its energy expenditures. The actively growing and reproducing populations of C. pacificus and C. carinatus may consume close to 20% of primary production, whereas the inactive population of C. australis consumed only 0.2% of primary production when upwelling weakened.

Seawater carbonate chemistry, gross photosynthesis and metabolically induced rate of pH change during experiments with macroalgae, 2012

Ocean acidification (OA) is a reduction in oceanic pH due to increased absorption of anthropogenically produced CO2. This change alters the seawater concentrations of inorganic carbon species that are utilized by macroalgae for photosynthesis and calcification: CO2 and HCO3 increase; CO32 decreases. Two common methods of experimentally reducing seawater pH differentially alter other aspects of carbonate chemistry: the addition of CO2 gas mimics changes predicted due to OA, while the addition of HCl results in a comparatively lower [HCO3]. We measured the short-term photosynthetic responses of five macroalgal species with various carbon-use strategies in one of three seawater pH treatments: pH 7.5 lowered by bubbling CO2 gas, pH 7.5 lowered by HCl, and ambient pH 7.9. There was no difference in photosynthetic rates between the CO2, HCl, or pH 7.9 treatments for any of the species examined. However, the ability of macroalgae to raise the pH of the surrounding seawater through carbon uptake was greatest in the pH 7.5 treatments. Modeling of pH change due to carbon assimilation indicated that macroalgal species that could utilize HCO3 increased their use of CO2 in the pH 7.5 treatments compared to pH 7.9 treatments. Species only capable of using CO2 did so exclusively in all treatments. Although CO2 is not likely to be limiting for photosynthesis for the macroalgal species examined, the diffusive uptake of CO2 is less energetically expensive than active HCO3 uptake, and so HCO3-using macroalgae may benefit in future seawater with elevated CO2.

Experiment: Ultraviolet radiation modulates the physiological responses of the calcified rhodophyte Corallina officinalis to elevated CO2

Ocean acidification reduces the concentration of carbonate ions and increases those of bicarbonate ions in seawater compared with the present oceanic conditions. This altered composition of inorganic carbon species may, by interacting with ultraviolet radiation (UVR), affect the physiology of macroalgal species. However, very little is known about how calcareous algae respond to UVR and ocean acidification. Therefore, we conducted an experiment to determine the effects of UVR and ocean acidification on the calcified rhodophyte Corallina officinalis using CO2-enriched cultures with and without UVR exposure. Low pH increased the relative electron transport rates (rETR) but decreased the CaCO3 content and had a miniscule effect on growth. However, UVA (4.25 W m-2) and a moderate level of UVB (0.5 W m-2) increased the rETR and growth rates in C. officinalis, and there was a significant interactive effect of pH and UVR on UVR-absorbing compound concentrations. Thus, at low irradiance, pH and UVR interact in a way that affects the multiple physiological responses of C. officinalis differently. In particular, changes in the skeletal content induced by low pH may affect how C. officinalis absorbs and uses light. Therefore, the light quality used in ocean acidification experiments will affect the predictions of how calcified macroalgae will respond to elevated CO2.