Russell L. Cuhel

The cycling of sulfur in surface seawater of the northeast Pacific

Oceanic dimethylsulfide (DMS) emissions to the atmosphere are potentially important to the Earths radiative balance. Since these emissions are driven by the surface seawater concentration of DMS, it is important to understand the processes controlling the cycling of sulfur in surface seawater. During the third Pacific Sulfur/Stratus Investigation (PSI-3, April 1991) we measured the major sulfur reservoirs (total organic sulfur, total low molecular weight organic sulfur, ester sulfate, protein sulfur, dimethylsulfoniopropionate (DMSP), DMS, dimethylsulfoxide) and quantified many of the processes that cycle sulfur through the upper water column (sulfate assimilation, DMSP consumption, DMS production and consumption, air-sea exchange of DMS, loss of organic sulfur by particulate sinking). Under conditions of low plankton biomass ( 8 μM nitrate), 250 km off the Washington State coast, DMSP and DMS were 22% and 0.9%, respectively, of the total particulate organic sulfur pool. DMS production from the enzymatic cleavage of DMSP accounted for 29% of the total sulfate assimilation. However, only 0.3% of sulfate-S assimilated was released to the atmosphere. From these data it is evident that air-sea exchange is currently only a minor sink in the seawater sulfur cycle and thus there is the potential for much higher DMS emissions under different climatic conditions.

A simple in vitro fluorescence method for biomass measurements in algal growth inhibition tests

Abstract The estimation of biomass concentrations in algal growth inhibition tests from measurements of pigment fluorescence in extracts of 20% sample (final v/v) prepared by direct addition to dimethylsulfoxide/acetone solvent offers several advantages compared to currently used direct or indirect methods. The extraction stops the electron transfer and other processes which interact with chlorophyll fluorescence when measured in vivo. As a result the response is stabilized and the sensitivity improved. The injection method is very fast, has a high potential for automation, allows storage of samples and is suitable for small sample volumes (e.g., 0.2 ml). The typical initial cell density in standard toxicity tests of 104 cells ml−1 of Selenastrum capricornutum was measured precisely with a standard fluorimeter set-up, and 103 cells ml−1 of S. capricornutum was measured reliably with a sensitive fluorimeter. At low levels of toxicity by the model test compound potassium dichromate, the proposed fluorescence method resulted in very similar inhibition figures as obtained with electronic particle counting. At high levels of toxicity, on the other hand, biomass determinations from pigment fluorescence readings were markedly affected by toxicant-induced changes of the algal physiology. The low effect part of a dose response curve is normally that one of major interest, and biomass estimation errors associated with fluorescence measurements on extracts are thus considered acceptable in most situations. When the entire dataset was applied for endpoint estimation by the Weibull model, EC-1 estimates were markedly affected by the curve fitting to data in the high inhibition range, while EC-10 and EC-20 were less and EC-50 almost unaffected. The method is expected to be less suitable for toxicity testing of herbicides specifically inhibiting photosynthesis.

Ecosystem Transformations of the Laurentian Great Lake Michigan by Nonindigenous Biological Invaders

Lake Michigan, a 58,000-km(2) freshwater inland sea, is large enough to have persistent basin-scale circulation yet small enough to enable development of approximately balanced budgets for water, energy, and elements including carbon and silicon. Introduction of nonindigenous species-whether through invasion, intentional stocking, or accidental transplantation-has transformed the lakes ecosystem function and habitat structure. Of the 79 nonindigenous species known to have established reproductive populations in the lake, only a few have brought considerable ecological pressure to bear. Four of these were chosen for this review to exemplify top-down (sea lamprey, Petromyzon marinus), middle-out (alewife, Alosa pseudoharengus), and bottom-up (the dreissenid zebra and quagga mussels, Dreissena polymorpha and Dreissena rostriformis bugensis, respectively) transformations of Lake Michigan ecology, habitability, and ultimately physical environment. Lampreys attacked and extirpated indigenous lake trout, the top predator. Alewives outcompeted native planktivorous fish and curtailed invertebrate populations. Dreissenid mussels-especially quagga mussels, which have had a much greater impact than the preceding zebra mussels-moved ecosystem metabolism basin-wide from water column to bottom dominance and engineered structures throughout the lake. Each of these non indigenous species exerted devastating effects on commercial and sport fisheries through ecosystem structure modification.

Microbial Communities and Chemosynthesis in Yellowstone Lake Sublacustrine Hydrothermal Vent Waters

Five sublacustrine thermal spring locations from 1 to 109 m water depth in Yellowstone Lake were surveyed by 16S ribosomal RNA gene sequencing in relation to their chemical composition and dark CO2 fixation rates. They harbor distinct chemosynthetic bacterial communities, depending on temperature (16–110°C) and electron donor supply (H2S <1 to >100 μM; NH3 <0.5 to >10 μM). Members of the Aquificales, most closely affiliated with the genus Sulfurihydrogenibium, are the most frequently recovered bacterial 16S rRNA gene phylotypes in the hottest samples; the detection of these thermophilic sulfur-oxidizing autotrophs coincided with maximal dark CO2 fixation rates reaching near 9 μM C h−1 at temperatures of 50–60°C. Vents at lower temperatures yielded mostly phylotypes related to the mesophilic gammaproteobacterial sulfur oxidizer Thiovirga. In contrast, cool vent water with low chemosynthetic activity yielded predominantly phylotypes related to freshwater Actinobacterial clusters with a cosmopolitan distribution.

Microbiogeochemical Ecophysiology of Freshwater Hydrothermal Vents in Mary Bay Canyon, Yellowstone Lake, Yellowstone National Park WY

Geothermally derived energy was shown to be definitively involved in elemental cycling and microbiogeochemical ecophysiology of a confined canyon in the caldera portion of Yellowstone Lake. Visual and olfactory evidence from small boats indicated efflux of volatile, redox-labile sulfur compounds on any calm morning. Bulk chemical composition of lakewater was found to be enriched in common geothermal constituents, ΣCO2, Cl−, SiO2, and the oxidized end-product SO4=. Lakewater chemosynthesis in the canyon was not necessarily high, but persistent presence of active chemosynthetic microbial populations was demonstrated in enrichment incubations. Admixtures of lakewater and hydrothermal emanations collected at the orifice of hot vents under this water column contained variable, often substantial enrichment in energy substrates, but samples collected simultaneously only 50 cm above the vent orifice had already become nearly indistinguishable chemically from deep lake water. Ventwaters supported aerobic chemosynthetic activity (dark CO2 fixation) at receiving water temperatures that could be several times the rate of light-saturated surface water photosynthesis on a volumetric basis. Some vents had little activity at 15 °C but sprung into action at 50 °C, indicating populations of obligately thermophilic chemolithoautotrophs. Below the sediment-water interface, pore water chemistry of a >60 °C gravity core displayed strong influence of geothermally altered fluids. Geochemicals ΣCO2, SiO2, and H2S were present at levels far above those attainable through organic matter diagenesis, punctuated by conservative Cl− concentrations of >9 mM, over 60 times that of overlying lakewater. Apparently geothermally altered water was transported through semipermeable subsurface conduits to sediment surface orifices, where emanations were biogeochemically altered to support microbial productivity and mineral encrustation processes. Deep waters of the confined canyon supported stable microbial populations poised to use reduced mineral energy for growth. Chemolithoautotrophic metabolism, either active or potential, was evident throughout the Mary Bay Canyon ecosystem.