We conducted continuous-flow experiments on intact sediment cores from Laguna Madre, Sabine Lake, East Matagorda Bay, and Nueces Estuary to evaluate internal nitrogen (N) sources, sinks, and retention mechanisms in Texas estuaries having different salinities. Mean ammonium (NH ) flux ranged from slight uptake (negative values) Nitrogen (N), a key nutrient limiting primary production in coastal ecosystems, can be introduced to coastal waters via tributaries, atmospheric deposition, or groundwater inputs as nitrates (NO or NO ), ammonium (NH ), and or-ganic N, or through N-fixation (Brock 2001;Paerl et al. 2002;Seitzinger et al. 2002). Available N is removed from coastal waters via physical transport, sediment burial, or conversion to gaseous forms, such as N 2 or N 2 O, via denitrification (Seitzinger et al. 1984;Brock 2001). An improved understanding of N dynamics in coastal systems is needed 1 Corresponding author (gardner@utmsi.utexas.edu). 2 Current address: Pusan National University, 30 Jangjeon-dong, Geumjeong-gu, Busan 609-735, Korea. AcknowledgmentsRick Kalke, Matt First, Melody Bernot, and Walter Dodds provided assistance for some of the field collections. Discussions and suggestions about N dynamics in Texas coastal regions provided by Ron Benner, Paul Montagna, and other colleagues are appreciated. We are grateful for the thorough and constructive observations and suggestions provided by two anonymous reviewers.

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Lead (Pb) is a common environmental contaminant found in soils. Unlike other metals, Pb has no biological role, and is potentially toxic to microorganisms. Effects of low (1 ppm) and high (500–2000) levels of lead (Pb) upon the soil microbial community was investigated by the PCR/DGGE analysis of the 16S and nirK gene markers, indicative of general microbial community and denitrifying community, respectively. Community analysis by use of those markers had shown that Pb has detectable effects upon the community diversity even at the lowest concentration tested. Analysis of sample diversity and similarity between the samples suggested that there are several thresholds crossed as metal concentration increase, each causing a substantial change in microbial diversity. Preliminary data obtained in this study suggest that the denitrifying microbial community adapts to elevated levels of Pb by selecting for metal-resistant forms of nitrite reductases.

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A neutrophilic Fe(II)-oxidizing bacterium (FeOB) isolated fromFe-rich freshwater wetland sediments has been phylogenetically and physiologically characterized. The 16S rRNA gene sequence of this organism (designated strain TW2) places it among the Rhodocyclus group within the β-proteobacteria. The closest known relative to strain TW2 is the heterotrophic perchlorate reducer Dechlorosoma suillum, with 94% 16S rRNA gene identity. TW2 grows chemolithoautotrophically with Fe(II) as an electron donor and O 2 as an electron acceptor. Inorganic carbon fixation during growth on Fe(II) was demonstrated via H 14 CO − 3 fixation experiments. The organism can also grow organotrophically with acetate as the sole carbon and energy source, and can utilize acetate as an auxiliary source of fixed carbon which enhances cell yield (2-3-fold) during lithotrophic growth on Fe(II). No other electron donors and no electron acceptors other than O 2 were utilized. The organism's ability to grow with Fe(II) and acetate, along with its limitations with respect to electron acceptor utilization, suggests a specific adaptation to microaerobic niches in redox interfacial environments. The unique metabolism of strain TW2, together with the 16S rRNA sequence data, suggests that this organism represents a novel taxonomic group at the genus level.