Variable carbon isotope fractionation of photosynthetic communities over depth in an open-ocean euphotic zone.

Marine particulate organic carbon (POC) contributes to carbon export, food webs, and sediments, but uncertainties remain in its origins. Globally, variations in stable carbon isotope ratios (δ13C values) of POC between the upper and lower euphotic zones (LEZ) indicate either varying aspects of photosynthetic communities or degradative alteration of POC. During summertime in the subtropical north Atlantic Ocean, we find that δ13C values of the photosynthetic product phytol decreased by 6.3‰ and photosynthetic carbon isotope fractionation (εp) increased by 5.6‰ between the surface and the LEZ-variation as large as that found in the geologic record during major carbon cycle perturbations, but here reflecting vertical variation in δ13C values of photosynthetic communities. We find that simultaneous variations in light intensity and phytoplankton community composition over depth may be important factors not fully accounted for in common models of photosynthetic carbon isotope fractionation. Using additional isotopic and cell count data, we estimate that photosynthetic and non-photosynthetic material (heterotrophs or detritus) contribute relatively constant proportions of POC throughout the euphotic zone but are isotopically more distinct in the LEZ. As a result, the large vertical differences in εp result in significant, but smaller, differences in the δ13C values of total POC across the same depths (2.7‰). Vertical structuring of photosynthetic communities and export potential from the LEZ may vary across current and past ocean ecosystems; thus, LEZ photosynthesis may influence the exported and/or sedimentary δ13C values of both phytol and total organic carbon and affect interpretations of εp over geologic time.

Mercury isotopic evidence for the importance of particles as a source of mercury to marine organisms.

The origin of methylmercury in pelagic fish remains unclear, with many unanswered questions regarding the production and degradation of this neurotoxin in the water column. We used mercury (Hg) stable isotope ratios of marine particles and biota to elucidate the cycling of methylmercury prior to incorporation into the marine food web. The Hg isotopic composition of particles, zooplankton, and fish reveals preferential methylation of Hg within small (< 53 µm) marine particles in the upper 400 m of the North Pacific Ocean. Mass-dependent Hg isotope ratios (δ202Hg) recorded in small particles overlap with previously estimated δ202Hg values for methylmercury sources to Pacific and Atlantic Ocean food webs. Particulate compound specific isotope analysis of amino acids (CSIA-AA) yield δ15N values that indicate more-significant microbial decomposition in small particles compared to larger particles. CSIA-AA and Hg isotope data also suggest that large particles (> 53 µm) collected in the equatorial ocean are distinct from small particles and resemble fecal pellets. Additional evidence for Hg methylation within small particles is provided by a statistical mixing model of even mass-independent (Δ200Hg and Δ204Hg) isotope values, which demonstrates that Hg within near-surface marine organisms (0-150 m) originates from a combination of rainfall and marine particles. In contrast, in meso- and upper bathypelagic organisms (200-1,400 m), the majority of Hg originates from marine particles with little input from wet deposition. The occurrence of methylation within marine particles is supported further by a correlation between Δ200Hg and Δ199Hg values, demonstrating greater overlap in the Hg isotopic composition of marine organisms with marine particles than with total gaseous Hg or wet deposition.

New geochemical tools for investigating resource and energy functions at deep-sea cold seeps using amino acid δ15 N in chemosymbiotic mussels (Bathymodiolus childressi).

In order to reconstruct the ecosystem structure of chemosynthetic environments in the fossil record, geochemical proxies must be developed. Here, we present a suite of novel compound-specific isotope parameters for tracing chemosynthetic production with a focus on understanding nitrogen dynamics in deep-sea cold seep environments. We examined the chemosymbiotic bivalve Bathymodiolus childressi from three geographically distinct seep sites on the NE Atlantic Margin and compared isotope data to non-chemosynthetic littoral mussels to test whether water depth, seep activity, and/or mussel bed size are linked to differences in chemosynthetic production. The bulk isotope analysis of carbon (δ13 C) and nitrogen (δ15 N), and δ15 N values of individual amino acids (δ15 NAA ) in both gill and muscle tissues, as well as δ15 NAA- derived parameters including trophic level (TL), baseline δ15 N value (δ15 NPhe ), and a microbial resynthesis index (ΣV), were used to investigate specific geochemical signatures of chemosynthesis. Our results show that δ15 NAA values provide a number of new proxies for relative reliance on chemosynthesis, including TL, ∑V, and both δ15 N values and molar percentages (Gly/Glu mol% index) of specific AA. Together, these parameters suggested that relative chemoautotrophy is linked to both degree of venting from seeps and mussel bed size. Finally, we tested a Bayesian mixing model using diagnostic AA δ15 N values, showing that percent contribution of chemoautotrophic versus heterotrophic production to seep mussel nutrition can be directly estimated from δ15 NAA values. Our results demonstrate that δ15 NAA analysis can provide a new set of geochemical tools to better understand mixotrophic ecosystem function and energetics, and suggest extension to the study of ancient chemosynthetic environments in the fossil record.

Compound-Specific Isotope Geochemistry in the Ocean.

Compound-specific isotope analysis encompasses a variety of methods for examining the naturally occurring isotope ratios of individual organic molecules. In marine environments, these methods have revealed heterogeneous sources and alteration processes that underlie the more commonly measured isotope ratios of bulk materials, as well as revealing signatures of marine metabolisms that may otherwise be impossible to isolate. Recently, compound-specific isotopic techniques have improved the reconstruction of metazoan diets and revealed a new potential of metazoan biomass as an archive of paleoecological information. Despite six decades of practice and a diversity of applications, the use of compound-specific isotopic techniques remains uncommon in marine studies. This review examines broad theoretical motivations behind compound-specific isotopic approaches, some applications to studies of marine carbon cycling and trophic relationships, and methodological limitations. In coming years, improvements in analytical efficiency and molecular or intramolecular specificity may transform compound-specific isotope analysis into a tool that can be applied more broadly and help to build global oceanographic data sets.

Diet of the prehistoric population of Rapa Nui (Easter Island, Chile) shows environmental adaptation and resilience.

OBJECTIVES: The Rapa Nui "ecocide" narrative questions whether the prehistoric population caused an avoidable ecological disaster through rapid deforestation and over-exploitation of natural resources. The objective of this study was to characterize prehistoric human diets to shed light on human adaptability and land use in an island environment with limited resources. MATERIALS AND METHODS: Materials for this study included human, faunal, and botanical remains from the archaeological sites Anakena and Ahu Tepeu on Rapa Nui, dating from c. 1400 AD to the historic period, and modern reference material. We used bulk carbon and nitrogen isotope analyses and amino acid compound specific isotope analyses (AA-CSIA) of collagen isolated from prehistoric human and faunal bone, to assess the use of marine versus terrestrial resources and to investigate the underlying baseline values. Similar isotope analyses of archaeological and modern botanical and marine samples were used to characterize the local environment. RESULTS: Results of carbon and nitrogen AA-CSIA independently show that around half the protein in diets from the humans measured came from marine sources; markedly higher than previous estimates. We also observed higher δ15 N values in human collagen than could be expected from the local environment. DISCUSSION: Our results suggest highly elevated δ15 N values could only have come from consumption of crops grown in substantially manipulated soils. These findings strongly suggest that the prehistoric population adapted and exhibited astute environmental awareness in a harsh environment with nutrient poor soils. Our results also have implications for evaluating marine reservoir corrections of radiocarbon dates.

Export of submicron particulate organic matter to mesopelagic depth in an oligotrophic gyre.

Sixty percent of the world ocean by area is contained in oligotrophic gyres [Longhurst A (1995) Prog Oceanog 36:77-16], the biomass of which is dominated by picophytoplankton, including cyanobacteria and picoeukaryotic algae, as well as picoheterotrophs. Despite their recognized importance in carbon cycling in the surface ocean, the role of small cells and their detrital remains in the transfer of particulate organic matter (POM) to the deep ocean remains disputed. Because oligotrophic marine conditions are projected to expand under current climate trends, a better understanding of the role of small particles in the global carbon cycle is a timely goal. Here we use the lipid profiles, radiocarbon, and stable carbon isotopic signatures of lipids from the North Pacific Subtropical Gyre to show that in the surface ocean, lipids from submicron POM (here called extra-small POM) are distinct from larger classes of suspended POM. Remarkably, this distinct extra-small POM signature dominates the total lipids collected at mesopelagic depth, suggesting that the lipid component of mesopelagic POM primarily contains the exported remains of small particles. Transfer of submicron material to mesopelagic depths in this location is consistent with model results that claim the biological origin of exported carbon should be proportional to the distribution of cell types in the surface community, irrespective of cell size [Richardson TL, Jackson GA (2007) Science 315:838-840]. Our data suggest that the submicron component of exported POM is an important contributor to the global biological pump, especially in oligotrophic waters.

Diversity of hopanoids and squalene-hopene cyclases across a tropical land-sea gradient.

Bacterial hopanoids are ubiquitous in Earth surface environments. They hold promise as environmental and ecological biomarkers, if the phylogeny and physiological drivers of hopanoid biosynthesis can be linked with the distribution of hopanoids observed across a breadth of samples. Here we survey the diversity of hopanoid cyclases from a land-sea gradient across the island of San Salvador, in the easternmost part of the Bahamas. The distribution of lipids was determined for the same sites, for the first time overlaying quantification of bacteriohopanepolyols with sqhC phylogeny. The results are similar to previous reports: environmental sqhCs average < 65% translated amino acid identity to their closest named relatives, and sequences from putative Proteobacteria dominate. Additionally, a new and apparently ubiquitous group of marine hopanoid producers is identified; it has no identifiable close relatives. The greatest diversity of hopanoid lipids occurs in soil, but hopanoids represent a minor fraction of total soil-derived lipids. Marine samples contain fewer identifiable hopanoids, but they are more abundant as a fraction of the total extractable lipids. In soil, the dominant compounds are 35-aminobacteriohopane-32,33,34-triol and adenosylhopane. In an upper estuarine sample, bacteriohopanetetrol and 32,35-anhydrobacteriohopanetetrol dominate; while in lower estuarine and open marine samples, the most abundant are bacteriohopanetetrol and bacteriohopaneribonolactone. Cyclitol ethers are trace components in the soil, absent in the estuary, and of moderate abundance in the open marine setting, suggesting a dominant marine source. Conversely, aminotriol and aminotetrol decrease in abundance or disappear completely from land to ocean, while 2-methyldiplopterol shows the opposite trend. Small quantities of 2-methylbacteriohopanepolyols are detectable in all samples. The overall hopanoid distributions may correlate to the major phylogenetic families of hopanoid producers or to the environments in which they are found.