Biotic and abiotic pathways of phosphorus cycling in minerals and sediments: insights from oxygen isotope ratios in phosphate.

A key question to address in the development of oxygen isotope ratios in phosphate (δ(18)O(p)) as a tracer of biogeochemical cycling of phosphorus in ancient and modern environments is the nature of isotopic signatures associated with uptake and cycling of mineral-bound phosphate by microorganisms. Here, we present experimental results aimed at understanding the biotic and abiotic pathways of P cycling during biological uptake of phosphate sorbed to ferrihydrite and the selective uptake of sedimentary phosphate phases by Escherichia coli and Marinobacter aquaeolei. Results indicate that a significant fraction of ferrihydrite-bound phosphate is biologically available. The fraction of phosphate taken up by E. coli attained an equilibrium isotopic composition in a short time (<50 h) due to efficient O-isotope exchange (between O in PO(4) and O in water; that is, actual breaking and reforming of P-O bonds) (biotic pathway). The difference in isotopic composition between newly equilibrated aqueous and residual sorbed phosphate groups promoted the ion exchange (analogous to isotopic mixing) of intact phosphate ions (abiotic pathway) so that this difference gradually became negligible. In sediment containing different P phases, E. coli extracted loosely sorbed phosphate first, whereas M. aquaeolei preferred Fe-oxide-bound phosphate. The presence of bacteria always imprinted a biotic isotopic signature on the P phase that was taken up and cycled. For example, the δ(18)O(p) value of loosely sorbed phosphate shifted gradually toward equilibrium isotopic composition. The δ(18)O(p) value of Fe-oxide-bound phosphate, however, showed only slight changes initially but, when new Fe-oxides were formed, coprecipitated/occluded phosphate retained δ(18)O values of the aqueous phosphate at the time of formation of new Fe oxides. Concentrations and isotopic compositions of authigenic and detrital phosphates did not change, suggesting that these phosphate phases were not utilized by bacteria. These findings support burgeoning applications of δ(18)O(p) as a tracer of phosphorus cycling in sediments, soils, and aquatic environments and as an indicator of paleo- environmental conditions.

Modeling of biotic and abiotic processes affecting phosphate oxygen isotope ratios in a mineral-water-biota system.

Abiotic and biotic reactions operate side by side in the cycling of phosphorus (P) in the environment, but the relative roles of these two reactions vary both spatially and temporally. In biotic reactions, the uptake and release of P are catalyzed by enzymes and thus change phosphate oxygen isotope ratios, while in abiotic reactions, the absence of hydrolysis-condensation reactions results in no apparent changes in isotope composition, except short-term kinetic isotope effect due solely to preferential ion exchange. Therefore, isotope method could be a promising tool to differentiate relative roles of these two reactions in the environment but the relationship of the dynamic concentration and isotope exchange at the biota-water interface is largely unknown. In this study, we aimed to develop a process-based isotope model underpinning the competition of abiotic (sorption, desorption, and ion exchange) and biotic (uptake, metabolism, and release) reactions during uptake and recycling of ferrihydrite-bound P by E. coli. Our model comprises equations describing the partitioning relationship among different P pools and their corresponding oxygen isotope compositions and is based exclusively on oxygen isotope exchange at multiple sites including mineral surface, aqueous phase, and bacterial cells. The process-based model adequately reproduced the measured concentration and isotope compositions over time. Furthermore, parametric and sensitivity analyses using the model indicated that the rate of biological uptake of P was the major factor controlling the changes of phosphate isotope composition. In conclusion, our model provides new insights into a mechanistic aspect of isotope exchange and could be potentially useful for future efforts to understand the interplay of biotic and abiotic factors on phosphorus cycling in natural environments.

Phytoprotective influence of bacteria on growth and cadmium accumulation in the aquatic plant Lemna minor.

Certain plants are known to accumulate heavy metals, and can be used in remediation of polluted soil or water. Plant-associated bacteria, especially those that are metal tolerant, may enhance the total amount of metal accumulated by the plant, but this process is still unclear. In this study, we investigated metal enhancement vs. exclusion by plants, and the phytoprotective role plant-associated bacteria might provide to plants exposed to heavy metal. We isolated cadmium-tolerant bacteria from the roots of the aquatic plant Lemna minor grown in heavy metal-polluted waters, and tested these isolates for tolerance to cadmium. The efficiency of plants to accumulate heavy metal from their surrounding environment was then tested by comparing L. minor plants grown with added metal tolerant bacteria to plants grown axenically to determine, whether bacteria associated with these plants increase metal accumulation in the plant. Unexpectedly, cadmium tolerance was not seen in all bacterial isolates that had been exposed to cadmium. Axenic plants accumulated slightly more cadmium than plants inoculated with bacterial isolates. Certain isolates promoted root growth, but overall, addition of bacterial strains did not enhance plant cadmium uptake, and in some cases, inhibited cadmium accumulation by plants. This suggests that bacteria serve a phytoprotective role in their relationship with Lemna minor, preventing toxic cadmium from entering plants.

Microbial diversity of boron-rich volcanic hot springs of St. Lucia, Lesser Antilles.

The volcanic Sulphur Springs, St. Lucia, present an extreme environment due to high temperatures, low pH values, and high concentrations of sulfate and boron. St. Lucia offers some unique geochemical characteristics that may shape the microbial communities within the Sulphur Springs area. We chose six pools representing a range of geochemical characteristics for detailed microbial community analyses. Chemical concentrations varied greatly between sites. Microbial diversity was analyzed using 16S rRNA gene clone library analyses. With the exception of one pool with relatively low concentrations of dissolved ions, microbial diversity was very low, with Aquificales sequences dominating bacterial communities at most pools. The archaeal component of all pools was almost exclusively Acidianus spp. and did not vary between sites with different chemical characteristics. In the pool with the highest boron and sulfate concentrations, only archaeal sequences were detected. Compared with other sulfur springs such as those at Yellowstone, the microbial diversity at St. Lucia is very different, but it is similar to that at the nearby Lesser Antilles island of Montserrat. While high elemental concentrations seem to be related to differences in bacterial diversity here, similarities with other Lesser Antilles sites suggest that there may be a biogeographical component as well.

Shifts in rhizoplane communities of aquatic plants after cadmium exposure.

In this study we present the comparative molecular analysis of bacterial communities of the aquatic plant Lemna minor from a contaminated site (RCP) and from a laboratory culture (EPA), as well as each of these with the addition of cadmium. Plants were identified as L. minor by analysis of the rpl16 chloroplast region. Comparative bacterial community studies were based on the analyses of 16S rRNA clone libraries, each containing about 100 clones from the root surfaces of plants. Bacterial communities were compared at three phylogenetic levels of resolution. At the level of bacterial divisions, differences in diversity index scores between treatments, with and without cadmium within the same plant type (EPA or RCP), were small, indicating that cadmium had little effect. When we compared genera within the most dominant group, the beta-proteobacteria, differences between unamended and cadmium-amended libraries were much larger. Bacterial diversity increased upon cadmium addition for both EPA and RCP libraries. Analyses of diversity at the phylotype level showed parallel shifts to more even communities upon cadmium addition; that is, percentage changes in diversity indices due to cadmium addition were the same for either plant type, indicating that contamination history might be independent of disturbance-induced diversity shifts. At finer phylogenetic levels of resolution, the effects of cadmium addition on bacterial communities were very noticeable. This study is a first step in understanding the role of aquatic plant-associated microbial communities in phytoremediation of heavy metals.