The contrasting effects of the invasive Marenzelleria viridis and the native Nereis diversicolor on benthic metabolism, partitioning of reaction pathways and distribution of inorganic porewater (C and N) solutes in homogenized sandy sediment were investigated experimentally over a period of 1 mo. The 2 species were studied separately and in combination to observe possible effects and interactions. Benthic O2 uptake and total CO2 (TCO2) release were affected similarly by M. viridis, N. diversicolor and the two in combination, with roughly a doubling after 1 to 2 wk compared to defaunated sediment. Sulfate reduction after 1 mo, on the other hand, was more than twice as high in sediment inhabited by M. viridis alone than in any other treatment, even when combined with N. diversicolor. Denitrification estimated from benthic TCO2 release, porewater reaction stoichiometry and nutrient fluxes was largely unaffected by the presence of fauna. Accordingly, the partitioning of reaction pathways after 1 mo revealed that M. viridis stimulated sulfate reduction at the expense of aerobic respiration. Most of the oxygen uptake in M. viridis sediment was apparently due to enhanced oxidation resulting from an upward drifting front of sulfide as indicated by low redox and the appearance of Beggiatoa sp. near the surface. Porewater solute profiles showed that M. viridis was capable of stronger and deeper irrigation than N. diversicolor despite ∼10 times higher burrow ventilation by the latter species. This effect was caused by percolation of return water in the deep (>20 cm) I- or J-shaped burrows of M. viridis compared to the flushing of the more shallow (6 to 8 cm) U- shaped burrows of N. diversicolor. A replacement of the native N. diversicolor with the invasive M. viridis as the dominant burrow-dwelling polychaete in shallow coastal sediments will probably affect the biogeochemical functioning and ecological stability of the ecosystem. Among other things, organisms tolerant to sulfide are likely to be favored at the expense of more sensitive species. © Inter-Research 2011.

The structural characteristics and microprofiles of dissolved oxygen and redox potential were examined in benthic cyanobacterial mats, which developed in a nutrient-rich river. The mats were highly dynamic both in their structure and relative composition, and showed large spatial heterogeneity. The mats differed in the contribution of Oscillatoria limosa, which was higher in the free-floating than in the attached mats. Maximum dissolved oxygen concentration at saturating irradiances occurred within the first 2 mm in the attached mat, but was smaller and deeper in the free-floating mat. The free-floating mats showed heterogeneous profiles inside the same mat due to varying thickness in nearby zones of the mats. In these mats, some black micropatches (high proportion of Oscillatoria) showed anoxic conditions under low irradiances, whereas other brown micropatches (lower proportion of Oscillatoria) never became anoxic. After several hours in the dark, a negative redox potential (- 400 mV) was detected inside the mat in the black micropatch, but redox potential was always positive in the brownish micropatches. The dissolved oxygen and redox profiles indicate that potential conditions for the production of geosmin could occur in the black micropatches. There, diffusion constraints within the mat could be associated with resource depletion. High peptidase activities in the black micropatch indicated a high demand for inorganic nitrogen. These mats with prevalence of Oscillatoria functioned as "hot spots" of limited diffusion, which probably caused low nutrient availability, defining the appropriate conditions for the production of geosmin.

The concomitant occurrence of molecular hydrogen (H 2) and organic acids along the alimentary canal of the earthworm is indicative of ongoing fermentation during gut passage. Fermentative H 2 production is catalyzed by [FeFe]-hydrogenases and group 4 [NiFe]-hydrogenases in obligate anaerobes (e.g., Clostridiales) and facultative aerobes (e.g., Enterobacteriaceae), respectively, functional groups that might respond differently to contrasting redox conditions. Thus, the objectives of this study were to assess the redox potentials of the alimentary canal of Lumbricus terrestris and analyze the hydrogenase transcript diversities of H 2 producers in glucose-supplemented gut content microcosms. Although redox potentials in the core of the alimentary canal were variable on an individual worm basis, average redox potentials were similar. The lowest redox potentials occurred in the foregut and midgut regions, averaging 40 and 110 mV, respectively. Correlation plots between hydrogenase amino acid sequences and 16S rRNA gene sequences indicated that closely related hydrogenases belonged to closely related taxa, whereas distantly related hydrogenases did not necessarily belong to distantly related taxa. Of 178 [FeFe]-hydrogenase gene transcripts, 177 clustered in 12 Clostridiales-affiliated operational taxonomic units, the majority of which were indicative of heretofore unknown hydrogenases. Of 86 group 4 [NiFe]-hydrogenase gene transcripts, 79% and 21% were affiliated with organisms in the Enterobacteriaceae and Aeromonadaceae, respectively. The collective results (i) suggest that fermenters must cope with variable and moderately oxidative redox conditions along the alimentary canal, (ii) demonstrate that heretofore undetected hydrogenases are present in the earthworm gut, and (iii) corroborate previous findings implicating Clostridiaceae and Enterobacteriaceae as active fermentative taxa in earthworm gut content. © 2011, American Society for Microbiology.