Evaluating Freshwater Mussel Shell δ13C Values as a Proxy for Dissolved Inorganic Carbon δ13C Values in a Temperate River

The stable isotope ratio of dissolved inorganic carbon (δ13CDIC) in rivers reflects the dominant vegetation type in the surrounding watershed, rates of chemical weathering, atmospheric CO2 fluxes, and the relative rates of photosynthesis and respiration. Reconstructing past δ13CDIC values may reveal changes in these characteristics before watersheds experience land-use change and/or climate change. This study uses freshwater mussels, Elliptio complanata, and coeval environmental data to assess how high-resolution changes in the oxygen and carbon isotope ratios of shell carbonate (δ18Oshell and δ13Cshell, respectively) can be used as proxies of paleotemperature and paleo-DIC in rivers. To test our hypotheses, we analyzed time-series δ18Oshell and δ13Cshell values, δ13C values in bivalve tissues, and environmental data collected fortnightly from the Neuse River, North Carolina. Shell microsamples milled every 150 μm along the maximum growth axis represent an average of 12 ± 5 days (n = 524; excluding periods of growth cessation of >30 days), which is similar to the environmental data resolution (∼14 days). Serially sampled δ18Oshell and δ13Cshell values did not capture the full range of environmental conditions due to growth cessation during winter shutdown and extreme weather events. Low water temperature and elevated turbidity appear to be significant drivers of growth cessation. Spatial and temporal differences in the amount of metabolic carbon incorporated in the shell (∼0%–44%) likely occur due to variable mussel metabolic rates within and among study sites. Though high-resolution δ13Cshell values did not reflect variations in δ13CDIC values, average δ13Cshell values were indicative of average δ13CDIC values in the Neuse River

15N Values in Crassostrea virginica Shells Provides Early Direct Evidence for Nitrogen Loading to Chesapeake Bay

Crassostrea virginica is one of the most common estuarine bivalves in the United States? east coast and is frequently found in archaeological sites and sub-fossil deposits. Although there have been several sclerochronological studies on stable carbon and oxygen isotopes in the shells of this species, less is known about ?15N values within their shells, which could be a useful paleoenvironmental proxy to assess estuarine nitrogen dynamics. Modern C. virginica samples were collected in Chesapeake Bay for comparison with archaeological shells from nearby sites ranging in age from ~100 to 3,200 years old. Left valves were sampled by milling the hinge area and the resulting powder was analyzed for %N and ?15N values. Comparison of ?15N values between C. virginica shells shows relatively constant values from ~1250 BC to ~1800 AD. After ~1800 AD, there are rapid increases in 15N enrichment in the shells, which continue to increase in value up to the modern shell values. The increase in ?15N values is evidence of early anthropogenic impact in Chesapeake Bay. These results corroborate the observation that coastal nitrogen pollution occurred earlier than the 19th century and support the use of oyster shell ?15N values as a useful environmental proxy

Reconstructing central African hydroclimate over the past century using freshwater bivalve shell geochemistry

Centennial records of climatic and hydrological data are scarce on the African continent. Freshwater bivalve shells have the potential to record climate-related proxies, from which reconstructions of river discharge (Q) or water isotope variations over long time periods would be possible. The Oubangui River, the largest right-bank tributary of the Congo River, is one of the few African rivers for which long-term Q records are available. This, together with the availability of museum-archived shells, makes it an ideal location to study changes in hydroclimate in central Africa over the past century and to validate the potential of proxies recorded in freshwater shells. We analysed the carbon and oxygen isotope composition (δ13C, δ18O) across the growth axis of museum-archived (collected between 1891 and ∼1952) and contemporary (live-collected in 2011 and 2013) Chambardia wissmanni shells from the Oubangui River, covering sections of the past ∼130 years. Shell isotopes show a clear baseline shift. Both δ13C and δ18O exhibit pronounced cyclicity in recent and historical shells, but recent shells showed a much wider range in δ18O values and a narrower range in δ13C values compared to historical specimens. In the Oubangui River, high Q is represented as low water δ18O (δ18Ow) values, while low Q is represented as high δ18Ow values, and these correlations are expected to be expressed in shell δ18O (δ18Oshell) values similarly. The historical δ18Oshell values covered only the lower part of the range measured in recent shells, which suggests a major change in the low-flow conditions of the Oubangui River between the 1950s and 2010s. Reconstructed Q values, based on the logarithmic relationship established between recent water δ18O values and measured Q, tended to overestimate the low flow values in the past, suggesting a different δ18Ow–Q relationship in the earlier part of the 20th century. Thus, while the freshwater shell δ18O patterns are consistent with the documented long-term Q changes in the Oubangui, the shells show that the most pronounced changes in hydroclimate over the past century are expressed in low-flow sections of the hydrograph and likely result from a combination of changes in the rainfall regime, aquifer recharge, and/or vegetation cover in the upper catchment. These results are consistent with the suggestion that dry periods in the upper Congo basin have become more extreme in recent times and highlight that freshwater shells could offer a valuable archive to study recent changes in catchments where no long-term empirical hydrological or climatological data are available.</p

Assessing δ15N values in the carbonate-bound organic matrix and periostracum of bivalve shells as environmental archives

Though previous studies demonstrate the utility of nitrogen and carbon stable isotope ratios (δ15N and δ13C, respectively) in bivalve soft tissues as biogeochemical proxies, it is necessary to develop alternative proxies for environmental reconstructions when soft tissues are unavailable, such as with fossils or in museum-archived specimens. This study assesses the reliability of the δ15N values of carbonate-bound organic matrix (δ15NCBOM) and periostracum (δ15Nperiostracum) in bivalve shells as recorders of the δ15N values of particulate nitrogen (δ15NPN) by comparing the δ15NCBOM and δ15Nperiostracum values of live-collected freshwater mussels (Elliptio complanata) and estuarine clams (Rangia cuneata) to the δ15N values of particulate nitrogen (δ15NPN) in the water column. The δ15NCBOM and δ15Nperiostracum values in both species were within the range of the δ15NPN values that have been corrected for trophic-level enrichment. Thus, our findings illustrate that δ15NCBOM and δ15Nperiostracum values reliably record δ15NPN values in rivers and estuaries. The significant positive correlation between δ15NCBOM and δ15Nperiostracum values in both species indicates that they may be used in a similar manner to record δ15NPN values. The δ15N values in E. complanata muscle, mantle, and gill tissues were enriched by about +3.4‰ compared to δ15NPN from the water column, which suggests that they are primary consumers that reflect baseline trophic levels. On the other hand, δ15N values in the soft tissues of R. cuneata have trophic-level enrichment consistent with both primary and secondary consumption. Therefore, variations in the δ15N values of tissues in R. cuneata may be related to trophic-level shifts and/or changes in N sources. Differences between the δ15N values of soft tissue, CBOM, and periostracum in E. complanata and R. cuneata can be attributed to asynchronous growth, metabolic rate, and organic molecule composition. The δ15NCBOM values vary along a freshwater-estuarine gradient because of land-use change and differences in the trophic level of the compared species. The δ15NCBOM values between neighboring sites reflect influences from biosolid application and treated wastewater discharge. While δ15NCBOM values did not differentiate between sites dominated by urban and forested land-cover, δ15NCBOM values were highest at the site with the highest agricultural land-use. These results demonstrate the potential of δ15NCBOM values in bivalve shells to record long-term changes in watershed land use

An international intercomparison of stable carbon isotope composition measurements of dissolved inorganic carbon in seawater

We report results of an intercomparison of stable carbon isotope ratio measurements in seawater dissolved inorganic carbon (δ 13C‐DIC) which involved 16 participating laboratories from various parts of the world. The intercomparison involved distribution of samples of a Certified Reference Material for seawater DIC concentration and alkalinity and a preserved sample of deep seawater collected at 4000 m in the northeastern Atlantic Ocean. The between‐lab standard deviation of reported uncorrected values measured with diverse analytical, detection, and calibration methods was 0.11‰ (1σ ). The multi‐lab average δ 13C‐DIC value reported for the deep seawater sample was consistent within 0.1‰ with historical measured values for the same water mass. Application of a correction procedure based on a consensus value for the distributed reference material, improved the between‐lab standard deviation to 0.06‰. The magnitude of the corrections were similar to those used to correct independent data sets using crossover comparisons, where deep water analyses from different cruises are compared at nearby locations. Our results demonstrate that the accuracy/uncertainty target proposed by the Global Ocean Observing System (±0.05‰) is attainable, but only if an aqueous phase reference material for δ 13C‐DIC is made available and used by the measurement community. Our results imply that existing Certified Reference Materials used for seawater DIC and alkalinity quality control are suitable for this purpose, if a “Certified” or internally consistent “consensus” value for δ 13C‐DIC can be assigned to various batches.publishedVersio

Responses of Soil Fungi to Logging and Oil Palm Agriculture in Southeast Asian Tropical Forests

© 2014, Springer Science+Business Media New York. Human land use alters soil microbial composition and function in a variety of systems, although few comparable studies have been done in tropical forests and tropical agricultural production areas. Logging and the expansion of oil palm agriculture are two of the most significant drivers of tropical deforestation, and the latter is most prevalent in Southeast Asia. The aim of this study was to compare soil fungal communities from three sites in Malaysia that represent three of the most dominant land-use types in the Southeast Asia tropics: a primary forest, a regenerating forest that had been selectively logged 50 years previously, and a 25-year-old oil palm plantation. Soil cores were collected from three replicate plots at each site, and fungal communities were sequenced using the Illumina platform. Extracellular enzyme assays were assessed as a proxy for soil microbial function. We found that fungal communities were distinct across all sites, although fungal composition in the regenerating forest was more similar to the primary forest than either forest community was to the oil palm site. Ectomycorrhizal fungi, which are important associates of the dominant Dipterocarpaceae tree family in this region, were compositionally distinct across forests, but were nearly absent from oil palm soils. Extracellular enzyme assays indicated that the soil ecosystem in oil palm plantations experienced altered nutrient cycling dynamics, but there were few differences between regenerating and primary forest soils. Together, these results show that logging and the replacement of primary forest with oil palm plantations alter fungal community and function, although forests regenerating from logging had more similarities with primary forests in terms of fungal composition and nutrient cycling potential. Since oil palm agriculture is currently the mostly rapidly expanding equatorial crop and logging is pervasive across tropical ecosystems, these findings may have broad applicability

Alteration of the bZIP60/IRE1 Pathway Affects Plant Response to ER Stress in Arabidopsis thaliana

The Unfolded Protein Response (UPR) is elicited under cellular and environmental stress conditions that disrupt protein folding in the endoplasmic reticulum (ER). Through the transcriptional induction of genes encoding ER resident chaperones and proteins involved in folding, the pathway contributes to alleviating ER stress by increasing the folding capacity in the ER. Similarly to other eukaryotic systems, one arm of the UPR in Arabidopsis is set off by a non-conventional splicing event mediated by ribonuclease kinase IRE1b. The enzyme specifically targets mature bZIP60 RNA for cleavage, which results in a novel splice variant encoding a nuclear localized transcription factor. Although it is clear that this molecular switch widely affects the transcriptome, its exact role in overall plant response to stress has not been established and mutant approaches have not provided much insight. In this study, we took a transgenic approach to manipulate the pathway in positive and negative fashions. Our data show that the ER-resident chaperone BiP accumulates differentially depending on the level of activation of the pathway. In addition, phenotypes of the transgenic lines suggest that BiP accumulation is positively correlated with plant tolerance to chronic ER stress