Microorganisms are vital in mediating the earth’s biogeochemical cycles; yet, despite our rapidly increasing ability to explore complex environmental microbial communities, the relationship between microbial community structure and ecosystem processes remains poorly understood. Here, we address a fundamental and unanswered question in microbial ecology: ‘When do we need to understand microbial community structure to accurately predict function?’ We present a statistical analysis investigating the value of environmental data and microbial community structure independently and in combination for explaining rates of carbon and nitrogen cycling processes within 82 global datasets. Environmental variables were the strongest predictors of process rates but left 44% of variation unexplained on average, suggesting the potential for microbial data to increase model accuracy. Although only 29% of our datasets were significantly improved by adding information on microbial community structure, we observed improvement in models of processes mediated by narrow phylogenetic guilds via functional gene data, and conversely, improvement in models of facultative microbial processes via community diversity metrics. Our results also suggest that microbial diversity can strengthen predictions of respiration rates beyond microbial biomass parameters, as 53% of models were improved by incorporating both sets of predictors compared to 35% by microbial biomass alone. Our analysis represents the first comprehensive analysis of research examining links between microbial community structure and ecosystem function. Taken together, our results indicate that a greater understanding of microbial communities informed by ecological principles may enhance our ability to predict ecosystem process rates relative to assessments based on environmental variables and microbial physiology.

The assimilation efficiency (AE), efflux rate, and release budget of Cd, Cr(III), Se(IV), and Zn by a freshwater zooplankton Daphnia magna were measured under different food concentrations. The AEs of trace elements by Daphnia on two algal diets (Chlamydomonas reinhardtii and Scenedesmus obliquus) were 30-77% for Cd, 8-44% for Cr, 24-58% for Se, and 7-66% for Zn at food concentrations ranging from 0.136 to 7.50 mg carbon L Ϫ1 . Metal AEs increased significantly with decreasing food concentrations, with a maximum increase of 5.5 and 4.0ϫ for Zn and Cr, respectively. AEs were generally on the order of Cd Ͼ Se Ͼ Zn Ͼ Cr. Efflux rate constants, determined during 7 d depuration after 8 d of exposure to metals in the dissolved phase or dietary phase, were 0.012-0.216 d Ϫ1, with the highest efflux for Zn, followed by Cr Ͼ Se Ͼ Cd. The relative contribution of different routes of metal loss to the overall metal loss was also quantitatively assessed during the 7-d depuration period. Metals differed substantially in their routes of release from Daphnia. In general, metal excretion into the dissolved phase was the most important route for metal loss. Molting represented nearly 50-70% and 20-70% of daily metal efflux for Cd and Zn, respectively, following aqueous exposure within the first 4 d but was Ͻ20 and Ͻ30%, respectively, following food exposure. Release by offspring production contributed substantially to Se efflux by the animals. Up to 44-67% and 16-47% of Se was lost from the animals through reproductive allocation on a daily basis following uptake from the aqueous and dietary phases, respectively. The major routes of Cr efflux were by excretion and feces egestion. Our study suggested that trace metal assimilation and regeneration in Daphnia may play an important role in the biogeochemical fates of metals in lake systems.

We investigated microbial methylmercury (CH(3)Hg) production in sediments from the South River (SR), VA, an ecosystem contaminated with industrial mercury (Hg). Potential Hg methylation rates in samples collected at nine sites were low in late spring and significantly higher in late summer. Demethylation of (14)CH(3)Hg was dominated by (14)CH(4) production in spring, but switched to producing mostly (14)CO(2) in the summer. Fine-grained sediments originating from the erosion of river banks had the highest CH(3)Hg concentrations and were potential hot spots for both methylation and demethylation activities. Sequencing of 16S rRNA genes of cDNA recovered from sediment RNA extracts indicated that at least three groups of sulfate-reducing bacteria (SRB) and one group of iron-reducing bacteria (IRB), potential Hg methylators, were active in SR sediments. SRB were confirmed as a methylating guild by amendment experiments showing significant sulfate stimulation and molybdate inhibition of methylation in SR sediments. The addition of low levels of amorphous iron(III) oxyhydroxide significantly stimulated methylation rates, suggesting a role for IRB in CH(3)Hg synthesis. Overall, our studies suggest that coexisting SRB and IRB populations in river sediments contribute to Hg methylation, possibly by temporally and spatially separated processes.

c Methylmercury (MeHg), a neurotoxic substance that accumulates in aquatic food chains and poses a risk to human health, is synthesized by anaerobic microorganisms in the environment. To date, mercury (Hg) methylation has been attributed to sulfateand iron-reducing bacteria (SRB and IRB, respectively). Here we report that a methanogen, Methanospirillum hungatei JF-1, methylated Hg in a sulfide-free medium at comparable rates, but with higher yields, than those observed for some SRB and IRB. Phylogenetic analyses showed that the concatenated orthologs of the Hg methylation proteins HgcA and HgcB from M. hungatei are closely related to those from known SRB and IRB methylators and that they cluster together with proteins from eight other methanogens, suggesting that these methanogens may also methylate Hg. Because all nine methanogens with HgcA and HgcB orthologs belong to the class Methanomicrobia, constituting the late-evolving methanogenic lineage, methanogenic Hg methylation could not be considered an ancient metabolic trait. Our results identify methanogens as a new guild of Hg-methylating microbes with a potentially important role in mineral-poor (sulfate-and iron-limited) anoxic freshwater environments. M ercury (Hg) is a global environmental contaminant whose concentration in the biosphere is increasing as a result of industrial activity. Mercury enters the biosphere mostly in its inorganic form, Hg(II), but public health concerns are focused primarily on the neurotoxic substance monomethylmercury (MeHg). Since MeHg is environmentally persistent and is biomagnified in aquatic food webs (1), in situ methylation reactions critically affect the ecosystem and human health consequences of Hg contamination.Methylation of Hg takes place in anoxic environments and is attributed largely to anaerobic microbes (2). To date, bacteria affiliated with the Deltaproteobacteria, including sulfate-and ironreducing bacteria (SRB and IRB, respectively) have been identified as Hg methylators in environmental incubations and in pure cultures (3-5). Indeed, coexisting SRB and IRB may simultaneously contribute to MeHg production in river sediments (6). Evidence that other anaerobic microorganisms are capable of methylating Hg has remained elusive for decades. Methanogens were initially proposed to be Hg methylators as a conclusion of experiments showing MeHg synthesis in cell extracts of Methanobacterium bryantii (7-9). This idea was rejected after later studies identified SRB (3) and IRB (4-6) as the principal Hg methylators in salt marsh and freshwater sediments and failed to show methylation by methanogens in pure cultures (10). However, Hamelin et al. (11) recently showed that Hg methylation in periphyton collected from a freshwater lake could be attributed to methanogens, since methylation was abolished by the addition of 2-bromoethane sulfonic acid, a specific inhibitor of methanogenesis, and was stimulated 45-fold over methylation in unamended controls by the addition of molybdate. A corresponding change in the active-commu...