Trace metal requirements for microbial enzymes involved in the production and consumption of methane and nitrous oxide.

Jennifer B Glass,Victoria J Orphan

doi:10.3389/fmicb.2012.00061

Abstract

Fluxes of greenhouse gases to the atmosphere are heavily influenced by microbiological activity. Microbial enzymes involved in the production and consumption of greenhouse gases often contain metal cofactors. While extensive research has examined the influence of Fe bioavailability on microbial CO2 cycling, fewer studies have explored metal requirements for microbial production and consumption of the second- and third-most abundant greenhouse gases, methane (CH4), and nitrous oxide (N2O). Here we review the current state of biochemical, physiological, and environmental research on transition metal requirements for microbial CH4 and N2O cycling. Methanogenic archaea require large amounts of Fe, Ni, and Co (and some Mo/W and Zn). Low bioavailability of Fe, Ni, and Co limits methanogenesis in pure and mixed cultures and environmental studies. Anaerobic methane oxidation by anaerobic methanotrophic archaea (ANME) likely occurs via reverse methanogenesis since ANME possess most of the enzymes in the methanogenic pathway. Aerobic CH4 oxidation uses Cu or Fe for the first step depending on Cu availability, and additional Fe, Cu, and Mo for later steps. N2O production via classical anaerobic denitrification is primarily Fe-based, whereas aerobic pathways (nitrifier denitrification and archaeal ammonia oxidation) require Cu in addition to, or possibly in place of, Fe. Genes encoding the Cu-containing N2O reductase, the only known enzyme capable of microbial N2O conversion to N2, have only been found in classical denitrifiers. Accumulation of N2O due to low Cu has been observed in pure cultures and a lake ecosystem, but not in marine systems. Future research is needed on metalloenzymes involved in the production of N2O by enrichment cultures of ammonia oxidizing archaea, biological mechanisms for scavenging scarce metals, and possible links between metal bioavailability and greenhouse gas fluxes in anaerobic environments where metals may be limiting due to sulfide-metal scavenging.

Highlights

With increasing concern about the future impacts of global climate change, a detailed understanding of the sources and sinks of greenhouse gases is essential to mitigating their environmental impact
SUMMARY Extensive research has been focused on the importance of Cu for particulate methane monooxygenase, resulting in a detailed understanding of physiological and biochemical aspects of Cu in pure cultures of MOB
Few studies have investigated additional metal requirements, Fe, and small amounts of Mo and Zn, for aerobic and intra-aerobic methanotrophy, clearly Fe in particular is important for these metabolisms (Figure 2) and cell surface Cu acquisition mechanisms (Karlsen et al, 2011)

Summary

INTRODUCTION

With increasing concern about the future impacts of global climate change, a detailed understanding of the sources and sinks of greenhouse gases is essential to mitigating their environmental impact. N2O has a very long residence time in the atmosphere (120 years) and reacts with atomic O to form nitric oxide (NO), which is involved in ozone destruction (Montzka et al, 2011). Both humans and microbes play important, and often interwoven, roles in the production of CH4 and N2O. At the heart of the pathways that generate these gases are enzymes that catalyze redox reactions Many of these enzymes contain transition metals as cofactors for electron transport or as catalytic centers at active sites. Important transition metals in the pathway of CH4 production (methanogenesis) and anaerobic methane oxidation include Fe, Ni, Co, Mo/W, and Zn, whereas www.frontiersin.org

Glass and Orphan

Metal Substrate

Peat bog waters

Soil pore water Oxic lake water

Findings

CONSUMPTION OF NITROUS OXIDE