Diverse microbial communities and numerous energy-yielding activities occur in deeply buried sediments of the eastern Pacific Ocean. Distributions of metabolic activities often deviate from the standard model. Rates of activities, cell concentrations, and populations of cultured bacteria vary consistently from one subseafloor environment to another. Net rates of major activities principally rely on electron acceptors and electron donors from the photosynthetic surface world. At open-ocean sites, nitrate and oxygen are supplied to the deepest sedimentary communities through the underlying basaltic aquifer. In turn, these sedimentary communities may supply dissolved electron donors and nutrients to the underlying crustal biosphere.

Deposition of nanomaterials onto surfaces is a key process governing their transport, fate, and reactivity in aquatic systems. We evaluated the transport and deposition behavior of carboxyl functionalized single-walled carbon nanotubes (SWNTs) in a well-defined porous medium composed of clean quartz sand over a range of solution chemistries. Our results showthat increasing solution ionic strength or addition of calcium ions result in increased SWNT deposition (filtration). This observation is consistent with conventional colloid deposition theories, thereby suggesting that physicochemical filtration plays an important role in SWNT transport. However, the relatively insignificant change of SWNT filtration at low ionic strengths (< or = 3.0 mM KCl) and the incomplete breakthrough of SWNTs in deionized water (C/Co = 0.90) indicate that physical straining also plays a role in the capture of SWNTs within the packed sand column. It is proposed that SWNT shape and structure, particularly the very large aspect ratio and its highly bundled (aggregated) state in aqueous solutions, contribute considerably to straining in flow through porous media. We conclude that both physicochemical filtration and straining play a role at low (< 3.0 mM) ionic strength, while physicochemical filtration is the dominant mechanism of SWNT filtration at higher ionic strengths. Our results further show that deposited SWNTs are mobilized (released) from the quartz sand upon introduction of low ionic strength solution following deposition experiments with monovalent salt (KCl). In contrast, SWNTs deposited in the presence of calcium ions were not released upon introduction of low ionic strength solution to the packed column, even when humic acid was present in solution during SWNT deposition.

ABSTRACT. Geochemical cycling of phosphorus (P) in aquatic environments is carried out almost exclusively by biota and involves reactions that are catalyzed by enzymes. Oxygen isotope effects accompanying phosphoenzymatic reactions have been determined in controlled laboratory experiments in order to elucidate processes underlying biogeochemical cycling of P, and to identify possible reaction pathways for P-compounds in nature. Phosphate oxygen isotope effects are distinct for specific enzymatic reaction mechanisms measured in microbial culture experiments and in cell-free systems. P 16 O 4 is taken up preferentially from inorganic phosphate (P i ) in the growth medium by intact E. coli cells, producing a kinetic fractionation in the extracellular dissolved P i pool. Inorganic pyrophosphatase is the intracellular enzyme that catalyzes the temperature-dependent equilibrium oxygen isotope fractionations between phosphates and water in biological systems, and imprints an equilibrium isotope signature on P i that is turned over or cycled by intact cells. Alkaline phosphatase, a key enzyme involved in extracellular P i regeneration in aquatic systems, catalyzes hydrolysis of phosphomonoesters, reactions that are accompanied by kinetic fractionations and disequilibrium (inheritance) isotope effects in released P i . Comparison of laboratory determined enzyme-specific isotopic fractionations with those observed in microbial culture experiments and in natural aquatic systems, provide new insights into processes controlling P cycling and the relations between P availability and the cycling of N and C. Isotopic signatures associated with specific cellular processes and phosphoenzyme reaction pathways may be useful in assessing P status and for tracing P turnover. Definition of Symbols and Abbreviations

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