Stephen E. MacAvoy

Links between chemosynthetic production and mobile predators on the Louisiana continental slope: stable carbon isotopes of specific fatty acids

Abstract The Gulf of Mexico supports chemoautotrophic communities associated with hydrocarbon seeps. The chemoautotrophic symbiont-containing metazoans are dominated by mussels and tube worms that harbor bacteria which utilize reduced carbon (CH 4 ) and sulfur (H 2 S) compounds as an energy source. In the deep sea, nutritional input from photosynthetic production is scarce, and chemoautotrophic production may be a significant source of nutrients to mobile, benthic predators. Large and significant differences exist between the stable carbon isotope signatures of chemosynthetic and photosynthetic primary production. This isotopic difference makes it possible to determine the importance of each type of primary production to heterotrophs because heterotrophs incorporate the isotope signature of their food. Here, we examine the carbon isotope signatures of specific fatty acids in heterotrophic predators caught both within and approximately 2 km from known chemosynthetic communities. Heterotroph fatty acid signatures were compared to those observed in chemoautotrophic symbiont-containing fauna in order to evaluate degree of usage. Most organisms had at least a 6‰ range in their fatty acids. This reflected patterns of de novo synthesis, with essential and precursor fatty acids being 13 C enriched (in heterotrophs) and highly unsaturated fatty acids being 13 C depleted. The fatty acid δ 13 C values show that heterotrophs had a wide range in their utilization of chemosynthetic production. Bathynomus giganteus (giant isopod), captured off-site, did not utilize chemosynthetic production. Its nonessential fatty acids ranged from −26.3‰ to −18.8‰ (δ 13 C), reflecting kinetic isotope effects during de novo synthesis. Essential omega-3 fatty acids, which are directly incorporated from diet, originated from photosynthetic production (δ 13 C=−16.2‰). Sclerasterias cf. tanneri (starfish), captured on-site, relied predominantly on chemosynthetic production. Its fatty acid δ 13 C values ranged from −30.5‰ to −42.2‰, reflecting chemosynthetic production. Other predators such as Eptatretus sp. (hagfish) and Rochina crassa (spider crab) derived variable percentages of their fatty acid pool from chemosynthetic production, estimated to be 38% and 5%, respectively. Bulk analysis of δ 15 N and δ 34 S show chemosynthetic production usage patterns generally consistent with the specific fatty acid δ 13 C data.

Chemoautotrophic Production Incorporated by Heterotrophs in Gulf of Mexico Hydrocarbon Seeps: An Examination of Mobile Benthic Predators and Seep Residents

Abstract The continental slope of the Gulf of Mexico supports dense aggregations of tubeworms and mussels that have symbiotic chemoautotrophic bacteria. Associated with these communities are numerous heterotrophic fauna and free-living bacteria. Here we examine the stable C, N, and S isotope compositions of fauna from two chemoautotrophic communities to identify isotope ranges of chemoautotrophic primary production and determine the usage of that primary production by heterotrophic invertebrates. The range in isotope values of the chemoautotrophic production is different between sites. A brine seep (GC233) dominated by mussels symbiotic with methanotrophic bacteria has 13C and 15N depleted nutrient sources (−50 to −65‰ and −9 to −12‰, respectively), indicating methanotrophy using biogenic methane and suggesting ammonium as the dominant nitrogen source. However, those same sources were 34S-enriched (6‰–11‰), as indicated by resident heterotrophs (Munidopsis sp., Methanoaricia dendrobranchiata Blake 2000, Alvinocaris stactophila Williams 1988, Phascolosoma turnerae Rice 1985), indicating that thiotrophy was a minor chemosynthetic method at the site. A site dominated by tube worms and mussels (GC234) has two isotopically distinct sources of carbon, one between −24 and −30‰ and another of approximately −40‰, as indicated by the resident heterotrophs. Resident heterotrophs at GC 234 had δ15N and δ34S values from 1‰ to 5‰ and −10‰ to 6‰, respectively. These isotope values suggest a mix of thiotrophy and methanotrophy (largely from thiotrophic sources) at the site. We estimate that hagfish (Eptatretus sp.) captured approximately 2 km from the communities derived at least 10% of their carbon from chemoautotrophic sources because of low δ13C values. In contrast, giant isopods (Bathynomus giganteus Milne Edwards, 1879), captured with the hagfish show negligible incorporation of chemosynthetic production.

STABLE ISOTOPE VARIATION AMONG THE MUSSEL BATHYMODIOLUS CHILDRESSI AND ASSOCIATED HETEROTROPHIC FAUNA AT FOUR COLD-SEEP COMMUNITIES IN THE GULF OF MEXICO

Abstract Efforts to determine the utilization of Gulf of Mexico (GOM) chemosynthetic production by benthic predators have relied on stable isotope differences between photosynthetic and chemosynthetic production. Whereas the photosynthetic δ13C value in GOM surface waters is relatively uniform, chemosynthetic production may differ in different areas depending on prevalence of thiotrophy versus methanotrophy and inorganic carbon source. In this paper we compare the δ13C and δ15N signatures of the symbiont-containing mussel, Bathymodiolus childressi Gustafson, 1998, from four different chemosynthetic sites to test the hypothesis that methanotrophic production results in significant differences among them. Bathymodiolus childressi from two areas characterized by brine seepage and biogenic methane (GC425 and GC233) had very low δ15N (−3.7‰ and −16.6‰) and δ13C (−57.5‰ and −63.5‰) relative to areas with substantial thiotrophic production (GC234 and GC185). Bathymodiolus childressi from each chemosynthetic community had significantly different δ13C, and three of the sites also had distinct δ15N values. The δ13C and δ15N signatures of hagfish (Eptatretus sp.) and giant isopods (Bathynomus giganteus) captured from two sites showed little or no chemosyntheic usage. Squat lobsters (Munidopsis sp.) showed heavy incorporation of chemosyntheic production, but did not directly consume B. childressi.

The intraspecies relationship between tissue turnover and metabolic rate in rats

Ecologists interested in studying fluctuating relationships between consumers and nutrient sources are increasingly involved in modeling the rate at which consumers incorporate dietary components. In mammals a correlation between resting metabolic rate (RMR) and tissue turnover may exist across a range of species. Less is known about the variation of tissue turnover rate within a species, and how that correlates with RMR. Here we examine two strains of rats (Rattus norvegicus) with different RMR to test whether variation in RMR is positively correlated with tissue turnover rate within a species. If RMR, a relatively simple measurement, can be correlated with tissue turnover, then this relationship could be used to better interpret ecological functions, including impact of migratory or seasonally available nutrient sources. Here, the changing isotope signature in rat whole blood was modeled using a modified exponential decay equation and a reaction progress variable model. The modeled rate of turnover, metabolic rate (O2 consumed), and mass were then compared between strains of rats. The mass and RMRs (conditions during which RMRs were determined modified from the ideal, as outlined in the Methods) were significantly different between strains, but half-life and the metabolic tissue replacement component of turnover (as opposed to turnover from mass gain) were not. No significant correlation was found between RMR and metabolic tissue replacement between the strains. Results suggest that within a species showing a range of RMRs, blood tissue turnover should not vary significantly.

Nutrients, oxygen dynamics, stable isotopes and fatty acid concentrations of a freshwater tidal system, Washington, D.C.

The Anacostia River in Washington, D.C., USA is an urban waterway contaminated with PAHs, PCBs, metals and sewage. Although several studies have examined the heavy metal geochemistry within the river, no studies have examined basic biogeochemical processes within the Anacostia river system. This study examines nutrients, bacterial biomarkers, organic material, and carbon, nitrogen and sulfur sources in the system. High biological oxygen demand and low nitrogen (0.33-0.56 mg L(-1)) and phosphorus (0.014-0.021 mg L(-1)) concentrations were observed in three areas of the river. Downstream sites had higher nutrient concentrations and dissolved organic matter (up to 13.7 mg L(-1)). Odd-chain length and branched fatty acids (FAs) in the sediments indicated bacterial sources, but long chain FAs indicative of terrestrial primary production were also abundant in some sediments. Sediment carbon stable isotope analyses showed a mix of autochthonous and allochthonous derived materials, but most carbon was derived from terrestrial sources (-23.3 to -31.7 per thousand). Sediment nitrogen stable isotopes ranged from -5.4 to 5.6 per thousand, showing nitrate uptake by plants and also recycling of nitrogen within the river. Sulfur sources were generally between 3 and -5 per thousand, reflecting local sulfate sources and anaerobic sulfate reduction.