Chromium Cycling in Redox-Stratified Basins Challenges δCr Paleoredox Proxy Applications.

Chromium stable isotope composition (δCr) is a promising tracer for redox conditions throughout Earth's history; however, the geochemical controls of δCr have not been assessed in modern redox-stratified basins. We present new chromium (Cr) concentration and δCr data in dissolved, sinking particulate, and sediment samples from the redox-stratified Lake Cadagno (Switzerland), a modern Proterozoic ocean analog. These data demonstrate isotope fractionation during incomplete (non-quantitative) reduction and removal of Cr above the chemocline, driving isotopically light Cr accumulation in euxinic deep waters.

Exopolymeric Substances Control Microbial Community Structure and Function by Contributing to both C and Fe Nutrition in Fe-Limited Southern Ocean Provinces.

Organic ligands such as exopolymeric substances (EPS) are known to form complexes with iron (Fe) and modulate phytoplankton growth. However, the effect of organic ligands on bacterial and viral communities remains largely unknown. Here, we assessed how Fe associated with organic ligands influences phytoplankton, microbial, and viral abundances and their diversity in the Southern Ocean.

Elemental Stoichiometry and Photophysiology Regulation of Synechococcus sp. PCC7002 Under Increasing Severity of Chronic Iron Limitation.

Iron (Fe) is an essential cofactor for many metabolic enzymes of photoautotrophs. Although Fe limits phytoplankton productivity in broad areas of the ocean, phytoplankton have adapted their metabolism and growth to survive in these conditions. Using the euryhaline cyanobacterium Synechococcus sp.

Southern Ocean phytoplankton physiology in a changing climate.

The Southern Ocean (SO) is a major sink for anthropogenic atmospheric carbon dioxide (CO), potentially harbouring even greater potential for additional sequestration of CO through enhanced phytoplankton productivity. In the SO, primary productivity is primarily driven by bottom up processes (physical and chemical conditions) which are spatially and temporally heterogeneous. Due to a paucity of trace metals (such as iron) and high variability in light, much of the SO is characterised by an ecological paradox of high macronutrient concentrations yet uncharacteristically low chlorophyll concentrations.

Nutrient uplift in a cyclonic eddy increases diversity, primary productivity and iron demand of microbial communities relative to a western boundary current.

The intensification of western boundary currents in the global ocean will potentially influence meso-scale eddy generation, and redistribute microbes and their associated ecological and biogeochemical functions. To understand eddy-induced changes in microbial community composition as well as how they control growth, we targeted the East Australian Current (EAC) region to sample microbes in a cyclonic (cold-core) eddy (CCE) and the adjacent EAC. Phototrophic and diazotrophic microbes were more diverse (2-10 times greater Shannon index) in the CCE relative to the EAC, and the cell size distribution in the CCE was dominated (67%) by larger micro-plankton [Formula: see text], as opposed to pico- and nano-sized cells in the EAC.

Iron Biogeochemistry in Aquatic Systems: From Source to Bioavailability.

Iron (Fe) is an essential trace element for several key metabolic processes in phytoplankton; however Fe is present in low concentration in many aquatic systems including vast oceanic regions and large lakes. In these systems, Fe can limit the growth of phytoplankton and atmospheric carbon dioxide biological fixation. Indeed Fe limitation exerts a global impact on the carbon cycle and the imprint of aquatic systems on our climate.

The impact of iron limitation on the physiology of the Antarctic diatom .

Iron availability strongly governs the growth of Southern Ocean phytoplankton. To investigate how iron limitation affects photosynthesis as well as the uptake of carbon and iron in the Antarctic diatom , a combination of chlorophyll fluorescence measurements and radiotracer incubations in the presence and absence of chemical inhibitors was conducted. Iron limitation in led to a decline in growth rates, photochemical efficiency and structural changes in photosystem II (PSII), including a reorganisation of photosynthetic units in PSII and an increase in size of the functional absorption cross section of PSII.

Iron limitation modulates ocean acidification effects on southern ocean phytoplankton communities.

The potential interactive effects of iron (Fe) limitation and Ocean Acidification in the Southern Ocean (SO) are largely unknown. Here we present results of a long-term incubation experiment investigating the combined effects of CO2 and Fe availability on natural phytoplankton assemblages from the Weddell Sea, Antarctica. Active Chl a fluorescence measurements revealed that we successfully cultured phytoplankton under both Fe-depleted and Fe-enriched conditions.

Exploring the Link between Micronutrients and Phytoplankton in the Southern Ocean during the 2007 Austral Summer.

Bottle assays and large-scale fertilization experiments have demonstrated that, in the Southern Ocean, iron often controls the biomass and the biodiversity of primary producers. To grow, phytoplankton need numerous other trace metals (micronutrients) required for the activity of key enzymes and other intracellular functions. However, little is known of the potential these other trace elements have to limit the growth of phytoplankton in the Southern Ocean.

PHYTOPLANKTON SELENIUM REQUIREMENTS: THE CASE FOR SPECIES ISOLATED FROM TEMPERATE AND POLAR REGIONS OF THE SOUTHERN HEMISPHERE(1).

A series of laboratory culture experiments was used to investigate the effect of selenium (Se, 0-10 nM) on the growth, cellular volume, photophysiology, and pigments of two temperate and four polar oceanic phytoplankton species [coccolithophore Emiliania huxleyi (Lohmann) W. W. Hay et H.

Saccharides enhance iron bioavailability to Southern Ocean phytoplankton.

Iron limits primary productivity in vast regions of the ocean. Given that marine phytoplankton contribute up to 40% of global biological carbon fixation, it is important to understand what parameters control the availability of iron (iron bioavailability) to these organisms. Most studies on iron bioavailability have focused on the role of siderophores; however, eukaryotic phytoplankton do not produce or release siderophores.

A novel in situ tool for the exposure and analysis of microorganisms in natural aquatic systems.

To evaluate the effects of contaminants or nutrient limitation in natural waters, it is often desirable to perform controlled exposures of organisms. While in situ exposures are routine for caged organisms or macrophytes, they are extremely difficult to perform for microorganisms, mainly due to difficulties in designing an exposure device that isolates the cells while allowing rapid equilibration with the external media. In this paper, a stirred underwater biouptake system (SUBS) based on the diffusion of chemicals across a semipermeable membrane housing a controlled population of microorganisms is reported.

Application of the biotic ligand model to explain potassium interaction with thallium uptake and toxicity to plankton.

Competitive interaction between TI(I) and K was successfully predicted by the biotic ligand model (BLM) for the microalga Chlorella sp. (Chlorophyta; University of Toronto Culture Collection strain 522) during 96-h toxicity tests. Because of a greater affinity of T1(I) (log K = 7.

Bioavailability of iron sensed by a phytoplanktonic Fe-bioreporter.

This study describes a short-term (12 h) evaluation of iron (Fe) bioavailability to an Fe-dependent cyanobacterial bioreporter derived from Synechococcus PCC 7942. Several synthetic ligands with variable conditional stability constants for Fe(lll) (K* of 10(19.8) to 10(30.

Impact of zinc acclimation on bioaccumulation and homeostasis in Chlorella kesslerii.

Growth curves, cellular Zn contents and cellular protein expression were examined for the green alga, Chlorella kesslerii, as a function of different Zn growth regimes (growth in 16 pM, 1.7 nM or 1.6 microM calculated Zn2+).

Some fundamental (and often overlooked) considerations underlying the free ion activity and biotic ligand models.

Trace metal bioavailability is often evaluated on the basis of steady-state models such as the free ion activity model (FIAM) and the biotic ligand model (BLM). Some of the assumptions underlying these models were verified by examining Pb and Zn uptake by the green microalga Chlorella kesslerii. Transporter bound metal ([M-Rcell]) and free ion concentrations ([M(Z+)]) were related to experimentally determined uptake fluxes (Jint).

Failure of the biotic ligand and free-ion activity models to explain zinc bioaccumulation by Chlorella kesslerii.

Zinc accumulation by Chlorella kesslerii (Chlorophycee) was studied for [Zn2+] ranging from 4 pM to 1 mM. A first-order uptake flux as predicted by the free ion activity model (FIAM) and the biotic ligand model (BLM) was not observed. Furthermore, when algae were preconditioned in slightly limiting (10 pM) versus optimal (1 nM) free zinc concentrations, the internalization flux increased and was nearly constant over the range of [Zn2+] examined.