Résumé: Size-based approaches are paramount tools for the study of marine food webs. Here, we investigated the relationship between zooplankton body size, stable isotope composition and trophic level (TL) along a large-scale onshore-offshore gradient in the western tropical Atlantic. Samples were obtained on the Brazilian continental shelf, slope and in oceanic waters (off Fernando de Noronha archipelago and Rocas Atoll) in September and October 2015. Zooplankton was sieved into five size fractions. Zooplankton was dominated by copepods, except for the largest (> 2000 mu m) size fraction, that showed a high biovolume of chaetognaths, decapods, and fish larvae. Maximum zooplankton abundance and biovolume was found at the continental slope. POM showed consistently lower delta C-13 than zooplankton, indicating a selective use of C-13-rich primary food sources by zooplankton. Particulate organic matter (POM) was more C-13-enriched in shelf areas (average: -22.8, -23.6 and -24.3% at the shelf, slope and oceanic islands, respectively), probably due to the higher abundance of diatoms nearshore. POM had delta N-15 values between 2.5 and 6.9% (average: 4.0, 4.9 and 4.2% at the shelf, slope and oceanic islands, respectively). Zooplankton delta N-15 and TL increased with body size. The delta N-15 of the 200-500 mu m size fraction was used as baseline for TL estimation. Oceanic areas (average baseline delta N-15 = 5.8% +/- 0.52, n = 14) showed a higher baseline delta N-15 than the shelf (average = 3.9% +/- 0.69, n = 9) and the slope areas (average = 3.1% +/- 0.93, n = 9). In spite of differing baselines, the delta N-15 data produced a consistent pattern of log-linear increase in TL with increasing size, in all areas. The choice of input trophic enrichment factor (TEF) values only slightly changed the log10 (body size) vs TL slopes, but this choice had a considerable effect on the estimates of predator/prey size ratio (PPSR) and predator/prey mass ratio (PPMR). Using a TEF above 2.3 leads to unrealistic PPSR and PPMR estimates. Overall average slope was 0.59 +/- 0.08 TL mu m(-1) with TEF = 2.3 and 0.42 +/- 0.07 TL mu m(-1) with TEF = 3.2.

Résumé: We designed and tested a set of specific primers for specific PCR amplification of the biotin carboxylase subunit gene (accC) of the Acetyl CoA carboxylase (ACCase) enzyme. The primer set yielded a PCR product of c. 460 bp that was suitable for denaturing gradient gel electrophoresis (DGGE) fingerprinting followed by direct sequencing of excised DGGE bands and sequence analysis. Optimization of PCR conditions for selective amplification was carried out with pure cultures of different bacteria and archaea, and laboratory enrichments. Next, fingerprinting comparisons were done in several aerobic and anaerobic freshwater planktonic samples. The DGGE fingerprints showed between 2 and 19 bands in the different samples, and the primer set provided specific amplification in both pure cultures and natural samples. Most of the samples had sequences grouped with bacterial accC, hypothetically related to the anaplerotic fixation of inorganic carbon. Some other samples, however, yielded accC gene sequences that clustered with Crenarchaeota and were related to the 3-hydroxypropionate/4-hydroxybutyrate cycle of autotrophic crenarchaeota. Such samples came from oligotrophic high mountain lakes and the hypolimnia of a sulfide-rich lake, where crenarchaeotal populations had been previously reported by 16S rRNA surveys. This study provided a fast tool to look for presence of accC genes in natural environments as potential marker for studies of carbon dioxide assimilation in the dark. After further refinement for better specificity against archaea, the new and novel primers could be very helpful to establish a target for crenarchaeota with implications for our understanding of archaeal carbon biogeochemistry.

Résumé: N-2-fixing cyanobacteria represent a major source of new nitrogen and carbon for marine microbial communities, but little is known about their ecological interactions with associated microbiota. In this study we investigated the interactions between the unicellular N-2-fixing cyanobacterium Cyanothece sp. Miami BG043511 and its associated free-living chemotrophic bacteria at different concentrations of nitrate and dissolved organic carbon and different temperatures. High temperature strongly stimulated the growth of Cyanothece, but had less effect on the growth and community composition of the chemotrophic bacteria. Conversely, nitrate and carbon addition did not significantly increase the abundance of Cyanothece, but strongly affected the abundance and species composition of the associated chemotrophic bacteria. In nitrate-free medium the associated bacterial community was co-dominated by the putative diazotroph Mesorhizobium and the putative aerobic anoxygenic phototroph Erythrobacter and after addition of organic carbon also by the Flavobacterium Muricauda. Addition of nitrate shifted the composition toward co-dominance by Erythrobacter and the Gammaproteobacterium Marinobacter. Our results indicate that Cyanothece modified the species composition of its associated bacteria through a combination of competition and facilitation. Furthermore, within the bacterial community, niche differentiation appeared to play an important role, contributing to the coexistence of a variety of different functional groups. An important implication of these findings is that changes in nitrogen and carbon availability due to, e.g., eutrophication and climate change are likely to have a major impact on the species composition of the bacterial community associated with N-2-fixing cyanobacteria.