Algae sulfate reduction

Sulfur fulfills many diverse roles in lakes. As the sixth most abundant element in biomass, it is required as a major nutrient by all organisms. For most algae, S is abundant in the form of sulfate in the water column however, in dilute glacial lakes in Alaska (I) and in some central African lakes (2) low concentrations of sulfate may limit primary production. Sulfur also serves the dual role of electron acceptor for respiration and, in reduced forms, source of energy for chemolithotrophic secondary production. Net sulfate reduction can account for 10-80% of anaerobic carbon oxidation in lakes (3-5), and hence this process is important in carbon and energy flow. Sulfate reduction, whether associated with uptake of sulfate and incorpo-... 

Sulfate Reduction. Dissimilatory sulfate reduction, anaerobic respiration with sulfate as the terminal electron acceptor, is performed by relatively few genera of bacteria (84). Many bacteria and algae are able to... 

Not all measures of salinity convey the same degree of salinity. For example, compare Orca Basin, the Great Salt Lake, the Dead Sea, and Basque Lake (Table 5.1). All four of these waters contain about the same salinity % [25.1-26.4% salt (wt/wt)]. Note, however, that Basque Lake has a much more favorable (for life) aw (0.919) compared with Orca Basin (0.774), Great Salt Lake (0.776), and, especially, the Dead Sea (0.690). The impact of salts on life depends on the anions and cations and their charges and molecular weight. Bacterial sulfate reduction occurs with salt concentrations up to 24% (Oren 1988), but chloride salt solutions at such concentrations deals much more harshly with life. Only the most halophilic organisms can live in the Dead Sea (Table 4.2). The Dead Sea was called dead because it was only in 1936 that life forms (e.g., bacteria, algae, yeast) were first isolated from this hypersaline water (Ventosa et al. 1999). 

More direct biological channels also seem promising as sources. Land plants release H2S, but the process has not been considered for marine algae ( ). Intermittent deep sulfide maxima could be connected with anoxic microenvironments recently located in marine snow. These organic particulates accumulate in the pycnocline and offer potential sites for contrary redox reactions such as dissimilatory sulfate reduction (34). 

The most important metabolic reaction is the assimilation of sulfur into organic forms which ultimately require the reduction of oxidized sulfur to the oxidation level of sulfide. This reduction is effected by the majority of microorganisms (bacteria, algae, fungi) and plants and, because of its abundance, sulfate is the dominant precursor of reduced sulfur. Pathways of assimilatory sulfate reduction are discussed briefly in Chapter 6.2 and depicted in Fig. 6.2.1 (p. 317). 

The extent of H2S release by plants as the result of sulfate reduction is another unknown flux in the sulfur cycle. Relatively rapid reduction of sulfate and thiosulfate to HjS by a thermophilic blue-green alga Synechococ-cus lividus isolated from a thermal spring in Yellowstone has been reported (Sheridan, 1966 Sheridan and Castenholz, 1968). Wilson et al. (1977) described light-dependent emission of H2S from leaves of a variety of plants at a maximum rate of 8 n mol min g (fresh-wei t) which they judged to be comparable to the activity associated with sulfate uptake. However, the emission was not a steady phenomenon and increased markedly with stresses of root injury, increases in light intensity and increased bisulfite or sulfate ion concentrations. Emissions with bisulfite solutions were higher than with sulfate solutions. 

Assimilatory sulfate reduction Bacteria, fungi, algae, and plants... 

In eukaryotic plants the former reaction is presumed to be involved in the synthesis of sulfolipids and sulfate esters. In cyanobacteria, the two reactions are thought to be involved in assimilatory sulfate reduction. The APS kinase of the green alga Chlamydomonas (Schwenn and Schriek, 1984) and the PAPS sulfotransferase of the cyanobacterium Synechococcus (Schmidt and Christen, 1978) are subject to light modulation via a thioredoxin-mediated system similar to that reported for several other light-mediated enzymes (Buchanan, 1980). However, the significance of these r ulatory mechanisms with respect to APS metabolism is not currently understood. 

Seston Sulfur. Much of the sulfur that is immobilized by assimilatoiy uptake or dissimilatory reduction is oxidized and reenters the water column as sulfate. We have approached the question of seston-S recycling by comparing carbon/sulfur (C/S) ratios in seston and sediment and by following the fate of 35S in labeled algae added to laboratory sediment-water microcosms. 

A. I. Usov and M. Ya. Elashvili, Polysaccharides of algae. 44. Investigation of sulfated galactan from Laurencia nipponica Yamada (Rhodophyta, Rhodomelaceae) using partial reductive hydrolysis, Bot. Mar., 34 (1991) 553-560. 

Desulfation of organic sulfate esters with the release of sulfate ion is identified with enzymes such as glycosulfatase in the red algae, Porphyra umbilicus (Rees, 1961). However organic esters may be reduced to liberate methanethiol or dimethyl sulfide. Young and Maw (1958) reported that the fungus, Schizophyllum commune, synthesises dimethyl sulfide and methyl sulfide by the following successive reduction and methylation reactions ... 

Our work started with the assumption that when, 3 billion years ago, the blue-green algae started to make oxygen, a unicellular form of life started to concert the oxidation of iron with the o.xidation of saturated hydrocarbons. It is agreed that after I billion years of life under reductive conditions, the world was full of saturated hydrocarbons, Fe compounds and metallic iron, as well as, an abundance of hydrogen sulfide, since life was using the reduction of sulfate to sulfide as an energy source. 

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