GEOLOGY AND MICROBIOLOGY OF SULFATE-REDUCING ALKALINE WETLANDS, WESTERN NEBRASKA, USA

Wetland soils at Facus Springs, Morrill Co., Nebraska (FS) and in the Nebraska Sand Hills at Hiers Lakes, Sheridan Co. (HL), and Smith Lake, Garden Co. (SL) show evidence for intense microbial activity, most notably the reduction of sulfate from shallow groundwaters. Horizons of sulfate reduction (SR) vary in thickness from 0.5 cm to over 130 cm. Typically, SR horizons lie just below the soil surface and persist as long as the soil is saturated. At FS, however, the SR front in wetland soils migrated downward by as much 40 cm during the 2000 drought.

FS is characterized by loamy to sandy soils and lies within a tributary valley of the North Platte River that ends blindly against dune-mantled bedrock uplands, from which the FS wetlands receive groundwater discharge. At FS, several ponds within only 1.7 km of each other vary in conductivity from 1.4 to 53 mS and in pH from 8.7 to 11.6; these ponds also range from permanent to semi-permanent (present in 3/4 study years) to ephemeral (present for a few weeks only). HL and SL wetlands have sandy soils, contained water in 4/4 study years, and are linked to the subregional Sand Hills groundwater flow system. Nonetheless, individual ponds at these sites vary greatly in conductivity: 5.4-85 mS across 2.4 km at HL and 0.7-44 mS across 2.3 km at SL. Water pH at HL and SL varies from 9.4-11.6 at HL and 9.3-10.1 at SL.

Alkaline conditions and high levels of dissolved ions at all three sites greatly influence the total biomass, composition, turnover rates, and stress levels of/in their associated microbial communities. As Na⁺ and SO₄^2- concentrations increase, total microbial biomass and eukaryote:prokaryote ratios decrease. The proportion of sulfate reducing bacteria relative to total community biomass varies little between sites, an indication of the prominence of sulfur-cycle processes. Bacterial turnover rates vary between sites, but tend to increase as total biomass decreases and alkalinity and conductivity increase. Furthermore, as solute concentration increases, the synthesis of trans fatty acids by Gram negative bacteria increases (a typical membrane-permeability-decreasing adaptation in stressful environments). Novel microbes from the sites have been detected by DGGE analysis, including two Archaea and one Gram positive bacterium related to Dehalococcoides ethenogenes.