Methane is the most abundant organic chemical in Earth's atmosphere, and its concentration is increasing with time, as a variety of independent measurements have shown. Photochemical reactions oxidize methane in the atmosphere; through these reactions, methane exerts strong influence over the chemistry of the troposphere and the stratosphere and many species including ozone, hydroxyl radicals, and carbon monoxide. Also, through its infrared absorption spectrum, methane is an important greenhouse gas in the climate system. We describe and enumerate key roles and reactions. Then we focus on two kinds of methane production: microbial and thermogenic. Microbial methanogenesis is described, and key organisms and substrates are identified along with their properties and habitats. Microbial methane oxidation limits the release of methane from certain methanogenic areas. Both aerobic and anaerobic oxidation are described here along with methods to measure rates of methane production and oxidation experimentally. Indicators of the origin of methane, including C and H isotopes, are reviewed. We identify and evaluate several constraints on the budget of atmospheric methane, its sources, sinks and residence time. From these constraints and other data on sources and sinks we construct a list of sources and sinks, identities, and sizes. The quasi-steady state (defined in the text) annual source (or sink) totals about 310(±60) × 1012 mol (500(±95) × 1012 g), but there are many remaining uncertainties in source and sink sizes and several types of data that could lead to stronger constraints and revised estimates in the future. It is particularly difficult to identify enough sources of radiocarbon-free methane.

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Biogeochemical aspects of atmospheric methane

https://www.es.ucsc.edu/~pkoch/EART_229/10-0111%20Fractionation/SUPPLEMENTAL/Hayes%2093%20MarGeo%2013-111.pdf

Factors controlling 13C contents of sedimentary organic compounds: Principles and evidence

The osmoadaptation of most micro-organisms involves the accumulation of K+ ions and one or more of a restricted range of low molecular mass organic solutes, collectively termed ‘compatible solutes’. These solutes are accumulated to high intracellular concentrations, in order to balance the osmotic pressure of the growth medium and maintain cell turgor pressure, which provides the driving force for cell extension growth. In this review, I discuss the alternative roles which compatible solutes may also play as intracellular reserves of carbon, energy and nitrogen, and as more general stress metabolites involved in protection of cells against other environmental stresses including heat, desiccation and freezing. Thus, the evolutionary selection for the accumulation of a specific compatible solute may not depend solely upon its function during osmoadaptation, but also upon the secondary benefits its accumulation provides, such as increased tolerance of other environmental stresses prevalent in the organism’s niche or even anti-herbivory or dispersal functions in the case of dimethylsulfoniopropionate (DMSP). In the second part of the review, I discuss the ecological consequences of the release of compatible solutes to the environment, where they can provide sources of compatible solutes, carbon, nitrogen and energy for other members of the micro-flora. Finally, at the global scale the metabolism of specific compatible solutes (betaines and DMSP) in brackish water, marine and hypersaline environments may influence global climate, due to the production of the trace gases, methane and dimethylsulfide (DMS) and in the case of DMS, also couple the marine and terrestrial sulfur cycles.

Ecological significance of compatible solute accumulation by micro‐organisms: from single cells to global climate

Publisher Summary The determination of the total plankton biomass or biovolume is rapid compared with the enumeration and identification techniques. Many samples can be processed in parallel. These measurements are suitable for mixed samples as well as for samples of selected individual species. There is a wide variety of biovolume and biomass measurements. The effort involved in sample treatment increases within the conventional methods and is generally large in biochemical procedures. The methodological bias decreases stepwise: when the amount of excess and interstitial water is reduced by measuring displacement volume or wet mass instead of settled volume, when body fluids are totally removed during dry mass determination, and when the inorganic substances are subtracted. Volumetric methods are the only choice if the samples are to be used for species identification as well. Volumetric methods are also recommended as a quick-look procedure; that is, on board a ship, and in cases where no microbalance or other sophisticated equipment is available. There are two principles related to the volumetric methods: the direct volume estimation of settled plankton and the measurement of the amount of water displaced by the plankton. The first method is more gentle but less precise.

Biomass and abundance

During three measurement campaigns on the Baltic and North Seas, atmospheric and dissolved methane was determined with an automated gas chromatographic system. Area-weighted mean saturation values in the sea surface waters were 113 ± 5% and 395 ± 82% (Baltic Sea, February and July 1992) and 126 ± 8% (south central North Sea, September 1992). On the bases of our data and a compilation of literature data the global oceanic emissions of methane were reassessed by introducing a concept of regional gas transfer coefficients. Our estimates computed with two different air-sea exchange models lie in the range of 11-18 Tg CH4 yr-1. Despite the fact that shelf areas and estuaries only represent a small part of the world's ocean they contribute about 75% to the global oceanic emissions. We applied a simple, coupled, three-layer model to numerically simulate the time dependent variation of the oceanic flux to the atmosphere. The model calculations indicate that even with increasing tropospheric methane concentration, the ocean will remain a source of atmospheric methane.

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Methane in the Baltic and North Seas and a Reassessment of the Marine Emissions of Methane

The concentrations of methane and 333 to 1800-μm ATP-biomass were determined at 15 stations along a transect from Gibraltar to Africa to Puerto Rico. Near-surface waters at one station in the Western Mediterranean and at one station off the coast of North Africa were highly supersaturated with methane. High concentrations of zooplankton encountered at the same locations offer biological evidence that the production of excess methane in the water column may be associated with the pelagic food chain. A possible cause may be the presence of methane bacteria in the digestive tracts of the herbivorous zooplankton.

C.H. Cheek

Papers

Methane supersaturation and ATP-zooplankton blooms in near-surface waters of the Western Mediterranean and the subtropical North Atlantic Ocean