C1 BACTERIA IN THE WATER COLUMN OF CHESAPEAKE BAY, USA .1. DISTRIBUTION OF SUBPOPULATIONS OF O2-TOLERANT, OBLIGATELY ANAEROBIC, METHYLOTROPHIC METHANOGENS THAT OCCUR IN MICRONICHES REDUCED BY THEIR BACTERIAL CONSORTS

Sub-populations of O2-tolerant, obligately anaerobic methanogens were enriched with monomethylamine (MMA) under 3 different regimens: Regime n 1, where oxygenated seawater was reduced by the aerobic bacterial co nsorts of the methanogens in sealed gas-permeable polycarbonate flasks that apparently prevented the accumulation of hydrogen sulphide while allowing the production of traces of ethane and ethylene; Regimen 2, like 1, but with bacterial reduction in glass serum bottles that permi tted the accumulation of hydrogen sulphide formed by sulphate-reductio n but not the C2 hydrocarbons; and Regimen 3, an anaerobic medium chem ically reduced by sodium sulphide and cysteine that is used to culture methanogens from anoxic sediment. In Regimens 1 and 2, methanogenic b acterial consortia (MBC) were initiated by MMA-oxidizing bacteria that formed reduced microzones in which MMA could be cleaved by methylotro phic methanogens to form methane, which was apparently oxidized by aer obic methanotrophs almost as quickly as it was produced until dissolve d oxygen was exhausted. The process was accelerated under Regimen 2, w hose enrichments were used for the most probable number estimation of methanogenic particulates which showed that they accumulate in the pyc nocline. These methanogenic particulates were quite sensitive to filte r concentration. The distribution of the 3 sub-populations of methanog ens is then shown at 10 stations along the density gradient of the est uary where water samples were obtained from the surf ace layer, pycnoc line and bottom layer of water. The results were similar for each laye r, where Regimen 2 consistently produced twice the number of methanoge nic enrichments as obtained with Regimen 1 or 2. In contrast, there wa s a marked difference in the distribution of the methanogen population s down the density gradient of the estuary. Enrichment in Regimen 3, w hich was very successful up the estuary, decreased in effectiveness wi th increasing density. Enrichments in Regimen 2 were quite effective t hroughout the transect of stratified waters, while those in Regimen 1 had a narrower distribution. I conclude that O2-tolerant methanogens t hat occur throughout the water column and peak in the pycnocline grow in fragile microniches that are reduced by bacteria that either consum e oxygen, produce hydrogen sulphide, or both. A 'top-down working hypo thesis is presented that could explain the diversity, nature and distr ibution of the bacterial components of the methanogernic enrichments o f the water column, and how the methanogens in anoxic sediments lackin g aerobic bacterial consorts may be selected from them. I also postula te that the methanogens living in bacterially reduced microniches may be unique both physiologically and taxonomically, and may have redox p otential (Eh) requirements less strict than methanogens from anoxic H2 S-rich sediments.