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Comparisons of the activities and diversities of ammonia-oxidizing bacteria (AOB) in the root environment of different cultivars of rice (Oryza sativa L.) indicated marked differences despite identical environmental conditions during growth. Gross nitrification rates obtained by the 15 N dilution technique were significantly higher in a modern variety, IR63087-1-17, than in two traditional varieties. Phylogenetic analysis based on the ammonium monooxygenase gene (amoA) identified strains related to Nitrosospira multiformis and Nitrosomonas europaea as the predominant AOB in our experimental rice system. A method was developed to determine the abundance of AOB on root biofilm samples using fluorescently tagged oligonucleotide probes targeting 16S rRNA. The levels of abundance detected suggested an enrichment of AOB on rice roots. We identified 40 to 69% of AOB on roots of IR63087-1-17 as Nitrosomonas spp., while this subpopulation constituted 7 to 23% of AOB on roots of the other cultivars. These results were generally supported by denaturing gradient gel electrophoresis of the amoA gene and analysis of libraries of cloned amoA. In hydroponic culture, oxygen concentration profiles around secondary roots differed significantly among the tested rice varieties, of which IR63087-1-17 showed maximum leakage of oxygen. The results suggest that varietal differences in the composition and activity of root-associated AOB populations may result from microscale differences in O 2 availability.

The impact of bioturbation by the lugworm Arenicola manna on sediment microflora and biochemical activities was investigated in intertidal sediments of the North Sea. Burrow walls and fecal casts were compared with surface and subsurface sediment surrounding the burrows. At the main study site burrow walls contained twice as much organic matter as the sediment surface. Viable counts of aerobic proteolytic and chitinolytic bacteria peaked at external and internal boundary layers (sediment surface and burrow walls). Burrow walls showed maximal bacterial production (incorporation of tritiated thymidine into DNA), maximal microheterotrophic activity (incorporation and mineralization of glucose and acetate) and highest levels of certain hydrolytic enzymes (alkaline phosphatase and sulfatase). Rates of chemoautotrophic COz fixation in both burrow walls and surrounding subsurface sedment were 3 to 4 times higher than at the sediment surface, suggesting a short-cucuited CO2 turnover driven by an elevated catabolism. In a second, more polluted study area, burrow walls were again the main site of microheterotrophic activity, but only minor differences were noted for CO2 fixation. Biogeochemical consequences of bioturbation by A, marina could be understood essentially as a shifting of catabolic and anabolic microbial activity peaks from the top to subsurface layers, where burrow walls showed the most conspicuous effects. Nevertheless, protease peaks in the fecal casts indicated also that this shifting pattern can be reversed for certain parameters.

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