Hydrothermal Vent Clam and Tube Worm 13C/12C: Further Evidence of Nonphotosynthetic Food Sources.

The stable carbon isotope ratios in clam mantle tissues taken from both Galápagos and 21 degrees N hydrothermal vent sites were similar to the unusually low ratios of carbon-13 to carbon-12 previously reported for a Galápagos hydrothermal vent mussel. In marked contrast to these bivalves, vestimentiferan worm tissues from a Galápagos vent had isotope ratios that were higher than those of open ocean biota. These observations suggest that more than one nonpelagic and nonphotosynthetic carbon fixation pathway is of nutritional importance to vent animals, and that at least one of these pathways is common to two geographically separated vent sites.

Carbon-13 depletion in a hydrothermal vent mussel: suggestion of a chemosynthetic food source.

Tissues of a mytilid mussel from the Clambake I hydrothermal vent in the Galápagos Rift zone are strikingly depleted in carbon-13 relative to the tissues of other marine organisms. The stable carbon isotope composition of this mussel suggests that chemoautotrophic bacteria present in the hydrothermal waters are a major food source for filter-feeding organisms in this abyssal environment.

Isotopic evidence for reduced productivity in the glacial southern ocean.

Records of carbon and nitrogen isotopes in biogenic silica and carbon isotopes in planktonic foraminifera from deep-sea sediment cores from the Southern Ocean reveal that the primary production during the last glacial maximum was lower than Holocene productivity. These observations conflict with the hypothesis that the low atmospheric carbon dioxide concentrations were introduced by an increase in the efficiency of the high-latitude biological pump. Instead, different oceanic sectors may have had high glacial productivity, or alternative mechanisms that do not involve the biological pump must be considered as the primary cause of the low glacial atmospheric carbon dioxide concentrations.

Does sedimentary organic delta 13C record variations in quaternary ocean [CO2(aq)]?

Ocean surface water [CO2(aq)] variations based on glacial/interglacial changes in sediment delta 13Corg are shown to compare favorably with reconstructions based on ice core [CO2]. In particular, an approximate 80 microatmospheres increase in atmospheric pCO2 during the last glacial-interglacial transition is calculated to correspond to a 3-4 micromolar increase in ocean surface water [CO2(aq)] at atmospheric equilibrium. A widespread marine delta 13Corg decrease of 1-2% accompanied this event and was not preceded by an equivalent isotopic change in surface water total dissolved inorganic carbon. These observations support the hypothesis that [CO2(aq)] influences photosynthetic isotope fractionation between marine inorganic and organic carbon pools, and therefore that plankton/sediment delta 13Corg may serve as a proxy for surface water [CO2(aq)].

Particulate organic matter delta 13C variations across the Drake Passage.

A 7 % gradient in the delta 13C of suspended particulate organic matter (POM) was observed in samples taken during two transects across the Drake Passage during March 1986. This POM delta 13C transition from -23.2 % at 53.3 degrees S to values as low as -30.3 % at > 62 degrees S does not track previously reported abrupt changes in water chemistry and plankton species composition associated with the Polar Front Zone that resides at approximately 58 degrees S in this region. Also, the north-south isotopic trend is not accompanied by significant changes in POM carbon or nitrogen concentrations, or in POM C/N. Differences in plankton standing crop or biochemistry (e.g., lipid content) therefore do not appear responsible for the isotopic trends observed. The latitudinal change in POM delta 13C is, however, highly correlated with water temperature and with the calculated concentration of CO2 (aq) at equilibrium with atmospheric CO2. These observations are consistent with the hypothesis that [CO2 (aq)] significantly influences POM delta 13C in ocean surface waters.

Latitudinal variations in plankton delta 13C: implications for CO2 and productivity in past oceans.

The stable-carbon isotopic composition of marine organic material has varied significantly over geological time, and reflects significant excursions in the isotopic fractionation associated with the uptake of carbon by marine biota. For example, low 13C/12C in Cretaceous sediments has been attributed to elevated atmospheric (and hence oceanic) CO2 partial pressures. A similar depletion in 13C present-day Antarctic plankton has also been ascribed to high CO2 availability. We report, however, that this high-latitude isotope depletion develops at CO2 partial pressures (pCO2 levels) that are often below that of the present atmosphere (340 microatmospheres) , and usually below that of equatorial upwelling systems (> 340 microatmospheres). Nevertheless, because of the much lower water temperatures and, hence, greater CO2 solubility at high latitude, the preceding pCO2 measurements translate into Antarctic surface-water CO2 (aq) concentrations that are as much as 2.5 times higher than in equatorial waters. We calculate that an oceanic pCO2 level of > 800 microatmospheres (over twice the present atmospheric pCO2) in a warmer low-latitude Cretaceous ocean would have been required to produce the plankton 13C depletion preserved in Cretaceous sediments.

The relationship between delta 13C of organic matter and [CO2(aq)] in ocean surface water: data from a JGOFS site in the northeast Atlantic Ocean and a model.

The delta 13C of suspended particulate organic matter (SPOM) in surface waters increased from -22.9 to -18.1% during April 25-May 31, 1989 at the JGOFS North Atlantic Bloom Experiment Site (NABE Site; 47 degrees N, 20 degrees W). During the same period, nearly parallel increases in sinking POM delta 13C were also found, although these values were usually lower than those of the corresponding SPOM. Consistent with the hypothesis that plankton delta 13C and [CO2 (aq)] are inversely related, the increases in both sinking and suspended POM delta 13C were highly negatively correlated with mixed-layer [CO2(aq)] that generally decreased from 13.2-10.1 micromoles/kg during the five weeks. The change in SPOM delta 13C per change in [CO2(aq)], however, appears to be somewhat greater than that expected from previous, though less direct, ocean and laboratory evidence. By adapting a model of plant delta 13C by FARQUHAR et al. (1982), it is shown that under a constant phytoplankton demand for CO2 an inverse, nonlinear SPOM delta 13C response to ambient [CO2(aq)] is expected. Such trends are unlike the negative linear relationships indicated by data from the NABE Site and or from Southern Hemisphere waters. Such differences between predicted and observed SPOM delta 13C vs. [CO2(aq)] trends and among observed relationships can be reconciled, however, if biological CO2 demand is allowed to vary. This has significant implications for the use of the delta 13C of plankton (or their organic subfractions or sedimentary remains) as a proxy for past or present ocean CO2 concentrations and biological productivity.

On the origin of terpenes in symbiotic associations between marine invertebrates and algae (zooxanthellae). Culture studies and an application of 13C/12C isotope ratio mass spectrometry.

13C/12C ratios of sets of compounds, algal sterols and terpenes, isolated from dinoflagellate symbiont (zooxanthellae)-bearing soft corals and gorgonians were determined. In most cases, a significant difference was found between the delta 13C values of the terpenes and of the algal sterols from the same set, the algal sterols containing less 13C than the terpenes. These results can only be explained if terpenes are synthesized by the host. Cultured zooxanthellae, isolated from symbiotic associations, do not make terpenes. Algal sterols of the various sets do not all have the same delta 13C value: average values range from -18.2 to -24.3%. A consistent difference of about 7% in the delta 13C values of sterols of cultured symbionts isolated from two of the gorgonians was found. This has potential applications for the taxonomy of zooxanthellae, most of which are believed by some specialists to be one discrete species.