No abstract

The warming trend in the northern part of the Western Antarctic Peninsula (WAP) has led to a decrease in perennial and summer sea ice, an increase in heat content over the shelf, and lower phytoplankton biomass, which could affect the prey quality of krill and fish that are utilized by apex predators. We compared prey quality metrics, including elemental (C, N) content; total, neutral, and polar lipid content; and energy densities of known penguin prey items including krill (Euphausia superba, Thysanoessa macrura, and E. crystallorophias) and fish (silverfish Pleuragramma antarcticum and the myctophid Electrona antarctica) along the WAP latitudinal gradient each January from 2009 to 2011. E. antarctica had the highest prey quality in terms of lipid content and energy density, followed by T. macrura and P. antarcticum, then E. crystallorophias and E. superba. For all species, variations in C and N content were most strongly correlated with the animals' neutral lipid content, in that animals with the larger neutral lipid stores had significantly higher C and lower N content. Across all sexes and maturity stages, E. superba in the southern study region had ca. 20% higher total lipid content than E. superba in the north, and a stepwise multiple linear regression analysis showed that latitude, sexual differences, and upper water column temperatures best explained this regional difference in lipid content. This regional variability in E. superba prey quality could affect the ability of apex predators that rely on E. superba to meet their energetic demands, and should be considered in future modeling efforts.

International audienceThe kinetics and efficiency of sterol production and bioconversion of phytosterols in two heterotrophic protists Oxyrrhis marina and Gyrodinium dominans were examined by feeding them two different algal species (Rhodomonas salina and Dunaliella tertiolecta) differing in sterol profiles. R. salina contains predominantly brassicasterol (≅99%) and <2% cholesterol. The major sterols in D. tertiolecta are ergosterol (45-49%), 7-dehydroporiferasterol (29-31%) and fungisterol (21-26%). O. marina fed R. salina metabolized dietary brassicasterol to produce 22-dehydrocholesterol and cholesterol. O. marina fed D. tertiolecta metabolized dietary sterols to produce cholesterol, 22-dehydrocholesterol, brassicasterol and stigmasterol. G. dominans fed either R. salina or D. tertiolecta metabolized dietary sterols to make cholesterol, brassicasterol and a series of unknown sterols. When protists were fed R. salina, which contains cholesterol, the levels of cholesterol were increased to a magnitude of nearly 5- to 30-fold at the phytoplankton-heterotrophic protist interface, equivalent to a production of 172.5 ± 16.2 and 987.7 ± 377.7 ng cholesterol per mg R. salina carbon consumed by O. marina and G. dominans, respectively. When protists were fed D. tertiolecta, which contains no cholesterol, a net production of cholesterol by the protists ranged from 123.2 ± 30.6 to 871.8 ± 130.8 ng per mg algal C consumed. Cholesterol is not only the dominant sterol, but a critical precursor for many physiologically functional biochemicals in higher animal. As intermediates, these heterotrophic protists increase the amount of cholesterol at the phytoplankton-zooplankton interface available to higher trophic levels relative to zooplankton feeding on algae directly

We investigated the capability and species-specific differences in long-chain n-3 essential fatty acid (LCn-3EFA), sterol, and steroidal ketone production of 6 heterotrophic protists: 3 thecate dinoflagellates (Cryptoperidiniopsis brodyi, Pfiesteria piscicida, and Luciella masanensis), 1 athecate dinoflagellate (Amphidinium longum), 1 herbivorous ciliate (Strombidinopsis sp.), and 1 bacterivorous ciliate (Uronema sp.) by feeding them algae (Rhodomonas salina or Dunaliella tertiolecta) or bacteria. The 3 thecate species did not convert algal sterols to other usual and common sterols. Instead, they produced sterols and steroidal ketones, such as dinosterol, dinostanol, dinosterone, and dinostanone, usually found in autotrophic dinoflagellates when fed R. salina or D. tertiolecta, both of which do not contain them. The A. longum, Strombidinopsis sp., and Uronema sp. did not bioconvert dietary sterols to other sterols or produce sterols and steroidal ketones. Pfiesteria piscicida and L. masanensis grown on the LCn-3EFA-deficient alga D. tertiolecta and Uronema sp. were capable of producing the long-chain n-3 polyunsaturated fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are essential for organisms at higher trophic levels. The bacterial prey of Uronema sp. lacked EPA, DHA, and LCn-3EFA precursors. Although the nutritional values of the sterols and steroidal ketones produced by the 3 thecate dinoflagellates are not known, the contribution of EPA and DHA by 2 of them and the bacterivorous ciliate are noteworthy. To further understand the intermediate roles of heterotrophic protists and their essential nutrient contribution in planktonic food webs, it is necessary to examine more species, particularly those newly discovered and isolated.KEY WORDS: Heterotrophic dinoflagellates and ciliates · Sterols and steroidal ketones · Algae · Long-chain n-3 fatty acids · Eicosapentaenoic acid · Docosahexaenoic acid

We evaluated annual and regional variation in the dietary niche of Pygoscelis penguins including the sea ice-obligate Adélie penguin (Pygoscelis adeliae), and sea ice-intolerant chinstrap (Pygoscelis antarcticus) and gentoo (Pygoscelis papua) penguins, three species that nest throughout the western Antarctic Peninsula (AP) to test the sea ice trophic interaction hypothesis, which posits that penguin breeding populations with divergent trends, i.e., declining or increasing, are reliant on differing food webs. Our study relies on values of naturally occurring carbon (13C/12C, δ13C) and nitrogen (15N/14N, δ15N) stable isotopes as integrated proxies of penguin food webs measured over three years at three different breeding colonies. At Anvers Island in the north, where reductions in sea ice and changes in breeding population trends among sympatric sea ice-obligate (Adélie) and sea ice-intolerant (chinstrap and gentoo) penguins have been most notable, our analyses show that all three species of Pygoscelis penguins became more similar isotopically over the reproductive period. By late chick-rearing at Anvers Island, crèched chicks at 5-weeks-old for all species occupied similar trophic positions. Isotopic mixing models indicated that the proportions of prey provisioned by adult penguins to 5-week-old chicks at Anvers Island were generally similar across species within years, consisting primarily of Antarctic krill (Euphausia superba). Crèched Adélie chicks had higher δ13C and δ15N values at Avian and Charcot Islands, southern breeding colonies where sea ice is more prominent and populations of Adélie penguins have increased or remain stable. Trophic position increased with latitude, while the proportions of prey provisioned by Adélie penguin adults to chicks at southern breeding colonies included species typical of high Antarctic marine food webs, especially crystal krill (Euphausia crystallorophias). A Bayesian metric for dietary niche width, standard ellipse area (SEA-B), indicated that Pygoscelis penguins with greater population changes in the north had more variability in dietary niche width than stable populations further south. Our results lend insight on marine food web drivers of Pygoscelis penguin reproduction at the regional scale and question the long-standing paradigm that Antarctic krill are the only food web component critical to penguin reproductive survival in this region of the Southern Ocean.