Abundant porewater Mn(III) is a major component of the sedimentary redox system.

Soluble manganese(III) [Mn(III)] can potentially serve as both oxidant and reductant in one-electron-transfer reactions with other redox species. In near-surface sediment porewater, it is often overlooked as a major component of Mn cycling. Applying a spectrophotometric kinetic method to hemipelagic sediments from the Laurentian Trough (Quebec, Canada), we found that soluble Mn(III), likely stabilized by organic or inorganic ligands, accounts for up to 90% of the total dissolved Mn pool. Vertical profiles of dissolved oxygen and dissolved and solid Mn suggest that soluble Mn(III) is primarily produced via oxidation of Mn(II) diffusing upwards from anoxic sediments with lesser contributions from biotic and abiotic reductive dissolution of MnO2. The conceptual model of the sedimentary redox cycle should therefore explicitly include dissolved Mn(III).

Photoferrotrophs thrive in an Archean Ocean analogue.

Considerable discussion surrounds the potential role of anoxygenic phototrophic Fe(II)-oxidizing bacteria in both the genesis of Banded Iron Formations (BIFs) and early marine productivity. However, anoxygenic phototrophs have yet to be identified in modern environments with comparable chemistry and physical structure to the ancient Fe(II)-rich (ferruginous) oceans from which BIFs deposited. Lake Matano, Indonesia, the eighth deepest lake in the world, is such an environment. Here, sulfate is scarce (<20 micromol x liter(-1)), and it is completely removed by sulfate reduction within the deep, Fe(II)-rich chemocline. The sulfide produced is efficiently scavenged by the formation and precipitation of FeS, thereby maintaining very low sulfide concentrations within the chemocline and the deep ferruginous bottom waters. Low productivity in the surface water allows sunlight to penetrate to the >100-m-deep chemocline. Within this sulfide-poor, Fe(II)-rich, illuminated chemocline, we find a populous assemblage of anoxygenic phototrophic green sulfur bacteria (GSB). These GSB represent a large component of the Lake Matano phototrophic community, and bacteriochlorophyll e, a pigment produced by low-light-adapted GSB, is nearly as abundant as chlorophyll a in the lake's euphotic surface waters. The dearth of sulfide in the chemocline requires that the GSB are sustained by phototrophic oxidation of Fe(II), which is in abundant supply. By analogy, we propose that similar microbial communities, including populations of sulfate reducers and photoferrotrophic GSB, likely populated the chemoclines of ancient ferruginous oceans, driving the genesis of BIFs and fueling early marine productivity.

Root-induced cycling of lead in salt marsh sediments.

A gold-mercury amalgam microelectrode was used in situ to measure Pb(II) by anodic stripping voltammetry and O2, Fe(II), Mn(II), and HS- by square-wave voltammetry in sediment pore water in a Haliomione portulacoides stand in a Tagus estuary salt marsh. The measurements were made in spring, summer, and fall, and were supplemented with analysis of Pb in solid phases and stable isotope analysis of Pb. In spring, the pore water was anoxic, Fe(II) reached concentrations as high as 1700 micromol/L, and Pb(II) was undetectable (<0.1 micromol/L). However, in summer, the pore water was oxic, Fe(II) was undetectable, and Pb(II) was present throughout the 20 cm deep root zone in concentrations reaching 6 micromol/L. In fall, low levels of O2 and Pb(II) were detected in the upper half of the root zone, and low concentrations of Fe(II) were detected in the lower half. The annual cycle of Pb is controlled by the growth and decay of roots. Roots deliver oxygen, which oxidizes lead-bearing solid phases and releases Pb(II) to the sediment pore water. Iron oxides, which form in the rhizosphere when Fe(II) is oxidized, are apparently not efficient sorbents for Pb(II) under the organic-rich conditions in this sediment. This allows Pb(II) to remain soluble and available for uptake by the roots. In fall and winter,when roots decay and the oxygen flux to the sediment stops, Pb is released from the decaying roots and returned to and precipitated in the anoxic sediment, likely as a sulfide. On an annual basis more than 20% of the total mass of Pb in the root zone cycles between root tissue and inorganic sediment phases. Depending on location, anthropogenic Pb constitutes 30-90% of total Pb in Tagus Estuary salt marshes.