Compound-specific δ(13)C analyses reveal sterol metabolic constraints in an aquatic invertebrate.

RATIONALE: Dietary sterol deficiencies can have severe life history consequences for consumers. Compound-specific stable isotope analysis (CSIA) was applied to the exploration of the sterol metabolic constraints and bioconversion capacities of the amphipod Gammarus roeselii. Evaluating structural sterol requirements has great potential to improve our understanding of the ecological relevance of sterols as limiting nutrients. METHODS: Juvenile G. roeselii were reared on food mixtures consisting of different ratios of the two algae Scenedesmus obliquus (cultivated with (13)C-labeled NaHCO3) and Nannochloropsis limnetica (unlabeled), which have been shown previously to differ in food quality. We measured the sterol content and composition using a gas chromatograph equipped with a flame ionization detector and the δ(13)C values of sterols using compound-specific isotope ratio mass spectrometry to examine potential sterol-mediated nutritional constraints of G. roeselii. RESULTS: In the food mixtures, δ(13)C values of cholesterol, synthesized by N. limnetica, were -25‰ and those of the Δ(7)-phytosterols, chondrillasterol and fungisterol, synthesized by S. obliquus, were 7 and 18‰, respectively. Although the cholesterol concentrations in G. roeselii decreased with increasing proportion of dietary S. obliquus, the δ(13)C values remained constant at -25‰. Lathosterol, which appeared in G. roeselii at high dietary proportions of S. obliquus, had a δ(13)C value of 35‰. CONCLUSIONS: We provide evidence that the the Δ(7)-phytosterols present in S. obliquus cannot be metabolized to cholesterol in G. roeselii, resulting in the accumulation of lathosterol in the animals and potentially in sterol-limited growth. These findings emphasize the advantage of CSIA in revealing the physiological mechanisms associated with nutritional constraints.

Absence of sterols constrains food quality of cyanobacteria for an invasive freshwater bivalve.

The accumulation of cyanobacterial biomass may severely affect the performance of aquatic consumers. Here, we investigated the role of sterols in determining the food quality of cyanobacteria for the invasive clam Corbicula fluminea, which has become a common benthic invertebrate in many freshwater ecosystems throughout the world. In standardized growth experiments, juvenile clams were fed mixtures of different cyanobacteria (Anabaena variabilis, Aphanothece clathrata, Synechococcus elongatus) or sterol-containing eukaryotic algae (Cryptomonas sp., Nannochloropsis limnetica, Scenedesmus obliquus). In addition, the cyanobacterial food was supplemented with different sterols. We provide evidence that somatic growth of C. fluminea on cyanobacterial diets is constrained by the absence of sterols, as indicated by a growth-enhancing effect of sterol supplementation. Thus, our findings contribute to our understanding of the consequences of cyanobacterial mass developments for benthic consumers and highlight the importance of considering sterols as potentially limiting nutrients in aquatic food webs.

Morphological and molecular data show no evidence of the proposed replacement of endemic Pomphorhynchus tereticollis by invasive P. laevis in salmonids in southern Germany.

Changes in parasite communities might result in new host-parasite dynamics and may threaten local fish populations. This phenomenon has been suggested for acanthocephalan parasites in the river Rhine and Danube where the species Pomphorhynchus tereticollis is becoming replaced by the Ponto-Caspian P. laevis. Developing knowledge on morphologic, genetic and behavioural differences between such species is important to follow such changes. However, disagreements on the current phylogeny of these two acanthocephalan species are producing conflicts that is affecting their correct identification. This study is offering a clearer morphological and genetic distinction between these two species. As P. tereticollis is found in rhithral tributaries of the Rhine, it was questioned whether the local salmonid populations were hosts for this species and whether P. laevis was expanding into the Rhine watershed as well. In order to test for this, brown trout, Salmo trutta, and grayling, Thymallus thymallus from South-Western Germany watersheds have been samples and screened for the occurrence of acanthocephalan parasites. For the first time, both species were confirmed to be hosts for P. tereticollis in continental Europe. P. tereticollis was found to be common, whereas P. leavis was found only at a single location in the Danube. This pattern suggest either that the expansion of P. laevis through salmonid hosts into rhithral rivers has not yet occurred, or that not yet ascertained biotic or abiotic features of rhithral rivers hinder P. laevis to spread into these areas.