Production of reduced trace gases by oceanic bacteria

Abstract

The reduced trace gases, N2O, H2, and CH4, are dissolved in oceanic water and are often found to be supersaturated with respect to the atmosphere. The origins of the gases appear to be biological production and a common feature of the biogeochemical cycles of the three gases is that each gas can be produced by anaerobic bacteria. This research was designed to determine the presence or absence of such bacteria in oceanic waters, measure gas production rates, and enumerate certain species of gas producers by an immunofluorescent technique. Seawater samples were collected from several oceanic areas and, in some cases, suspended particles were concentrated and size fractionated. The seawater or concentrate was incubated anaerobically and gas production was monitored. Antisera against three species of H2-producer bacteria were used to enumerate the organisms in preserved seawater samples for comparison with dissolved H2 concentrations and H2 production rates. Gas production measurements indicated N2O- and H2-producers to be ubiquitous in the regions sampled. Concentration of suspended particulates did not result in increased gas production, suggesting that most of the gas-producer cells were free in the water column. Although the gas-producers must have been in an aerobic environment in the water column, they were quickly able to switch to anaerobic metabolism and produce the reduced gases when placed in the experimental anaerobic environment. Maximum gas production potential was up to 1.41 x 10^5 and 1.20 x 10^7 nl l^1 d^-1 for N2O and H2, respectively. The immunofluorescent technique proved useful in enumerating H2-producer cells, whose numbers ranged from 0 to 644 ml^-1. The number of cells was not, however, directly related to either dissolved H2 or H2 production rates. CH4 was not detected in any samples, probably because methanogenic bacteria that might have been present on suspended particles were killed by exposure to oxygen during the sample collection and concentration procedure.