Characterization of spatial patterns of functional microbial communities could facilitate the understanding of the relationships between the ecology of microbial communities, the biogeochemical processes they perform and the corresponding ecosystem functions. Because of the important role the ammonia-oxidizing bacteria (AOB) and archaea (AOA) have in nitrogen cycling and nitrate leaching, we explored the spatial distribution of their activity, abundance and community composition across a 44-ha large farm divided into an organic and an integrated farming system. The spatial patterns were mapped by geostatistical modeling and correlations to soil properties and ecosystem functioning in terms of nitrate leaching were determined. All measured community components for both AOB and AOA exhibited spatial patterns at the hectare scale. The patchy patterns of community structures did not reflect the farming systems, but the AOB community was weakly related to differences in soil pH and moisture, whereas the AOA community to differences in soil pH and clay content. Soil properties related differently to the size of the communities, with soil organic carbon and total nitrogen correlating positively to AOB abundance, while clay content and pH showed a negative correlation to AOA abundance. Contrasting spatial patterns were observed for the abundance distributions of the two groups indicating that the AOB and AOA may occupy different niches in agro-ecosystems. In addition, the two communities correlated differently to community and ecosystem functions. Our results suggest that the AOA, not the AOB, were contributing to nitrate leaching at the site by providing substrate for the nitrite oxidizers.

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Nitrogenase activity (NA) in shallow-water (, 1 m) sediments was investigated at 60 randomly selected sites along a 150 km stretch on the brackish-water Swedish west coast, without targeting any specific type of sediments, such as microbial mats. Benthic nitrogen (N) fixation and diazotrophs (nifH genes) were found at all sites, regardless of the presence of cyanobacterial or microbial mats. The majority of sites showed N fixation rates between 0.03 and 1 mmol N m 22 d 21 . These rates were similar to those of benthic denitrification previously measured in the area. Maximum rates up to 3.4 mmol N m 22 d 21 were measured. A structural equation model was used to investigate direct and indirect effects of biogeochemical and physical factors on NA. Number of nifH genes had the largest direct positive influence on NA, whereas increasing wave exposure had an indirect negative effect on NA through its influence on the diazotrophic abundance. Increased salinity, previously been shown to suppress NA in coastal waters, was found to directly stimulate benthic N fixation, likely by generating favorable conditions for diazotrophic sulfate-reducing bacteria. Our field data confirmed previously observed negative effects of dissolved inorganic nitrogen on NA, which have so far mainly been experimentally studied. Both NA rates and the number of nifH genes correlated positively with pore-water dissolved inorganic phosphorus concentrations. These findings show that the potential for N fixation in illuminated sediments can be considerable, stretching beyond cyanobacterial mats, being controlled by complex interactions between biotic and abiotic factors.

Four specific pathogen-free, two to three-month-old, laboratory-raised beagles were infected with Strongyloides stercoralis and monitored by thrice-weekly faecal examinations. Larvae were not detected in the faeces after the 11th week. During the 14th week, daily oral prednisolone (2.2 mg/kg) was given. This led to weak recrudescence of the infection. During the 21st week, the daily dose of prednisolone was doubled and thereafter two of the four dogs developed severe hyperinfective strongyloidiasis with their adult worm populations markedly exceeding the number of larvae inoculated, as did one dog which remained asymptomatic. One dog died during the 15th week, i.e., before there was time for a marked increase in the parasite population. Three of the four dogs provided evidence of disseminated infection with adult worms occurring in ectopic sites. Parasite-specific cellular and humoral immune responses were monitored weekly. All dogs developed a significant rise in specific IgM titres lasting until the fifth week. From the fourth week, all dogs had detectable parasite-specific IgG antibody levels that persisted throughout the experiment in spite of the immunosuppression. Strongyloides-induced lymphocytic responses in vitro were demonstrated in three dogs from the second to the fifth week of infection, but returned to pre-infection levels thereafter. All animals were necropsied. Relevant pathological lesions were found in the small intestine, the colon and the lungs. Larvae were seen in such ectopic sites as mesenteric nodes and prostate. The observations provide further evidence that canine S. stercoralis infections can be manipulated to reproduce with great fidelity the parasitological and clinico-pathological events occurring in human disseminated strongyloidiasis.

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