Parasite effects on host's trophic and isotopic niches.

Wild animals are usually infected with parasites that can alter their hosts' trophic niches in food webs as can be seen from stable isotope analyses of infected versus uninfected individuals. The mechanisms influencing these effects of parasites on host isotopic values are not fully understood. Here, we develop a conceptual model to describe how the alteration of the resource intake or the internal resource use of hosts by parasites can lead to differences of trophic and isotopic niches of infected versus uninfected individuals and ultimately alter resource flows through food webs. We therefore highlight that stable isotope studies inferring trophic positions of wild organisms in food webs would benefit from routine identification of their infection status.

Reconstructing the diet, trophic level and migration pattern of mysticete whales based on baleen isotopic composition.

Baleen from mysticete whales is a well-preserved proteinaceous material that can be used to identify migrations and feeding habits for species whose migration pathways are unknown. Analysis of δ13C and δ15N values from bulk baleen have been used to infer migration patterns for individuals. However, this approach has fallen short of identifying migrations between regions as it is difficult to determine variations in isotopic shifts without temporal sampling of prey items. Here, we apply analysis of δ15N values of amino acids to five baleen plates belonging to three species, revealing novel insights on trophic position, metabolic state and migration between regions. Humpback and minke whales had higher reconstructed trophic levels than fin whales (3.7-3.8 versus 3-3.2, respectively) as expected due to different feeding specialization. Isotopic niche areas between baleen minima and maxima were well separated, indicating regional resource use for individuals during migration that aligned with isotopic gradients in Atlantic Ocean particulate organic matter. Phenylanine δ15N values confirmed regional separation between the niche areas for two fin whales as migrations occurred and elevated glycine and threonine δ15N values suggested physiological changes due to fasting. Simultaneous resolution of trophic level and physiological changes allow for identification of regional migrations in mysticetes.

Combining isotopic analysis of bulk-skin and individual amino acids to investigate the trophic position and foraging areas of multiple cetacean species in the western South Atlantic.

We investigated the trophic structure and habitat use of ten cetacean species occurring in the oceanic waters of the western South Atlantic using naturally-occurring stable isotopes. We analysed δ15N in individual amino acids (AA) to estimate cetacean trophic position (TP) and to evaluate the spatial differences in baseline δ15N (source AAs). We adjusted cetacean bulk-skin δ13C and δ15N for the effect of trophic level using their estimated TPs, obtaining δ13CAdjusted and δ15NAdjusted, respectively. These values were applied to estimate the overlap in the niche areas of cetacean baseline sources. Our analyses showed spatial segregation between Steno bredanensis and the remaining odontocetes, and the high δ15N in this species reflects its occurrence in neritic waters of the southern region. The highest TPs were observed in Physeter macrocephalus, Stenella attenuata and Globicephala melas, while the lowest TPs were reported for S. longirostris, S. clymene and Orcinus orca. Overall, source AA-δ15N showed similar patterns as those of baseline-δ15N (zooplankton) and were higher in species sampled in the southernmost region of the study area (e.g., Delphinus delphis). Isotopic niche areas estimated using δ13CAdjusted and δ15NAdjusted suggested high overlap in foraging area between S. frontalis and Tursiops truncatus, with the latter occupying a higher TP. Our analyses of δ15N in AAs provide a unique insight into the trophic ecology, forage areas and spatial segregation in resource use among these cetacean populations. Additionally, our work provides AA-δ15N baseline for future studies on the trophic ecology and habitat use of marine organisms in the western South Atlantic.

Isotopic discrimination in helminths infecting coral reef fishes depends on parasite group, habitat within host, and host stable isotope value.

Stable isotopes of carbon and nitrogen characterize trophic relationships in predator-prey relationships, with clear differences between consumer and diet (discrimination factor Δ13C and Δ15N). However, parasite-host isotopic relationships remain unclear, with Δ13C and Δ15N remaining incompletely characterized, especially for helminths. In this study, we used stable isotopes to determine discrimination factors for 13 parasite-host pairings of helminths in coral reef fish. Differences in Δ15N values grouped according to parasite groups and habitat within the host with positive Δ15N values observed for trematodes and nematodes from the digestive tract and variable Δ15N values observed for cestodes and nematodes from the general cavity. Furthermore, Δ13C values showed more complex patterns with no effect of parasite group or habitat within host. A negative relationship was observed between Δ15N and host δ15N values among different host-parasite pairings as well as within 7 out of the 13 pairings, indicating that host metabolic processing affects host-parasite discrimination values. In contrast, no relationships were observed for Δ13C values. Our results indicate that parasite group, habitat within host, and host stable isotope value drive Δ15N of helminths in coral reef fish while their effect on Δ13C is more idiosyncratic. These results call for use of taxon- or species-specific and scaled framework for bulk stable isotopes in the trophic ecology of parasites.

A shift in the pool of retained microphytobenthos nitrogen under enhanced nutrient availability.

Sediment microbial communities are an important sink for both organic and inorganic nitrogen (N), with microphytobenthos (MPB) biomass making the largest contribution to short-term N-assimilation and retention. Coastal waters are increasingly subject to anthropogenic nutrient enrichment, but the effect of nutrient enrichment on microbial assimilation, processing, and fate of MPB-derived N (MPB-N) remains poorly characterised. In this study, an MPB-dominated microbial community was labeled in situ with a pulse of 15NH4+-N. Laboratory core incubations of this labeled sediment under increasing nutrient concentrations (NH4+ and PO43-: ambient, 2 × ambient, 5 × ambient, and 10 × ambient) were used to investigate changes in the processing and flux pathways of the 15N-labeled MPB-N across 10.5 d under nutrient enrichment. Short-term retention of MPB-N by MPB was stimulated by nutrient addition, with higher 15N in MPB in the nutrient amended treatments (71-93%) than in the ambient treatment (38%) at 0.5 d After 10.5 d, the nutrient amended treatments had increased turnover of MPB-N out of MPB biomass into an uncharacterised pool of sediment ON (45-75%). Increased turnover of MPB-N likely resulted from decreased recycling of MPB-N between MPB and heterotrophic bacteria as inorganic nutrients were preferentially used as an N source and remineralisation of sediment ON decreased. Decreased breakdown of sediment ON reduced the efflux of MPB-N via DON in the amended (3.9-5.2%) versus the ambient treatment (10.9%). Exports of MPB-N to the water column were relatively small, accounting for a maximum of 14% of 15N exported from the sediment, and were predominantly exported DON and N2 (denitrification). Overall, there was considerable retention of MPB-N over 10.5 d, but increased nutrient loading shifted N from MPB biomass into other sediment ON.

Shining Light on Priming in Euphotic Sediments: Nutrient Enrichment Stimulates Export of Stored Organic Matter.

Estuarine sediments are important sites for the interception, processing, and retention of organic matter, prior to its export to the coastal oceans. Stimulated microbial co-metabolism (priming) potentially increases export of refractory organic matter through increased production of hydrolytic enzymes. Using the microphytobenthos community to directly introduce a pulse of labile carbon into sediment, we traced a priming effect and assessed the decomposition and export of preexisting organic matter. We show enhanced efflux of preexisting carbon from intertidal sediments enriched with water column nutrients. Nutrient enrichment increased production of labile microphytobenthos carbon, which stimulated degradation of previously unavailable organic matter and led to increased liberation of "old" (6855 ± 120 years BP) refractory carbon as dissolved organic carbon (DOC). These enhanced DOC effluxes occurred at a scale that decreases estimates for global organic carbon burial in coastal systems and should be considered as an impact of eutrophication on estuarine carbon budgets.

Practical considerations for improved reliability and precision during determination of δ15 N values in amino acids using a single combined oxidation-reduction reactor.

RATIONALE: There has been increased interest in the measurement of δ15 N values in amino acids (AAs) to gain simultaneous insight into both trophic relationships and the composition of biogeochemical sources used by producers at the base of the food web. A new combustion reactor design in gas chromatography/combustion isotope ratio mass spectrometry (GC/C-irMS) equipment has brought to light variable outcomes in performance, highlighting the need for better information about best practices for new systems. METHODS: Precision for δ15 N values in amino acids using the single combined oxidation-reduction reactor is improved across a sequence of analyses if the reactor is oxidized for a substantial period (2 h) and subsequently maintained throughout the sequence with 12-17 s seed oxidation before each run during GC/C-irMS. A five-point calibration curve using amino acids with a range of δ15 N values from -2.4‰ to +61.5‰ was used in combination with a 13-15 amino acid mixture to consistently normalize measurements to internationally calibrated reference materials. RESULTS: Combining this oxidation method with normalization techniques using both internal and external standards provided a reliable throughput of ~25 samples per week. It allowed for a reproducible level of precision of <±0.5‰, n = 10 within a derivatized standard mixture across each sequence and an average sample precision of ±0.27‰ n = 3, which is lower than the analytical precision typically associated with δ15 N values for amino acid analysis (<±1‰). CONCLUSIONS: A few practical considerations regarding oxidation and conditioning of the combustion reactor allow for increased sequence capacity with the single combined oxidation-reduction reactor. These considerations combined with normalization techniques result in a higher throughput and reduced analytical error during the measurement of δ15 N values in amino acids.