‘Copper-starved’ microbes enhance global warming

 Summary

New research provides evidence that a lack of metals in aquatic environments results in higher emissions of nitrous oxide (N2O), a greenhouse gas associated with global warming.

General aspects of the denitrification process

Did you know that nitrous oxide (N2O) has a higher global warming potential (at least 265 times more) than the well-known greenhouse gas (carbon dioxide, CO2)1 mainly produced by the burning of fossil fuels? Since 57% of global N2O emissions are from natural soils and oceans, the global N2O emissions into the atmosphere are mainly regulated by biogeochemical denitrification of nitrate (NO-3) to nitrogen (N2) mainly by microbes in aquatic systems2. Other non-biological denitrification processes (e.g., dissimilatory nitrate reduction to ammonium, DNRA) also occur in nature. Biological denitrification is normally a four-stage process involving the reduction of nitrate (NO-3) to nitrite (NO-2), nitric oxide (NO), nitrous oxide (N2O) and finally nitrogen (N2)3. Thus, incomplete denitrification leads to the emission of N2O into the atmosphere.

Denitrifying microbes

Denitrifying microbes (Source: Encyclopedia Britannica)

A group of metal-containing enzyme proteins (or metalloenzymes) are involved in the denitrification process. In particular, the metalloenzymes mainly contain copper (Cu), molybdenum (Mo), manganese (Mn) and iron (Fe) which are used in various stages during the denitrification process. Of note is the reduction of NO-3 to NO-2 which requires a Fe- and Mo-containing catalyst, the reduction of NO-2   to NO which requires a Fe- or Cu-containing catalyst depending on the microbes involved, the reduction of NO to N2O which requires a Fe-containing catalyst, and the reduction N2O to N2  which requires a Cu-containing catalyst4–6. Thus, limited quantities of Cu could negatively affect the complete conversion of N2O to N2, and result in the accumulation and emission of N2O.

New findings

A recent study published in Geochimica et Cosmochimica Acta based soils and sediments from natural aquatic systems (wetlands and streams) investigated the conversion of N2O to N2 by microbes. In these systems, the Cu content is at or below the Earth's crustal levels (~440 nmol/g). The experiments conducted for this study showed that wetland and stream samples with low Cu content exhibited high N2O accumulation, whereas a marked drop in the N2O content was noted after the addition of trace amounts of Cu. Thus, the study concluded that natural aquatic systems characterised by low Cu content at or below those of the Earth’s crust are characterised by incomplete biogeochemical conversion of N2O to N2, and result in the build-up and the subsequent emission of N2O into the atmosphere.

 

References and further reading

1.           Sovacool, B. K., Griffiths, S., Kim, J. & Bazilian, M. Climate change and industrial F-gases: A critical and systematic review of developments, sociotechnical systems and policy options for reducing synthetic greenhouse gas emissions. Renew. Sustain. Energy Rev 141, (2021).

2.           Makowski, D. N2O increasing faster than expected. Nat. Clim. Chang. 9, 909–910 (2019).

3.           Giannopoulos, G. et al. Trace metal availability affects greenhouse gas emissions and microbial functional group abundance in freshwater wetland sediments. Front. Microbiol. 1–12 (2020).

4.           Bertero, M. G. et al. Insights into the respiratory electron transfer pathway from the structure of nitrate reductase A. Nat. Struct. Mol. Biol. 10, 681–687 (2003).

5.           Nojiri, M. et al. Structure and function of a hexameric copper-containing nitrite reductase. Proc. Natl. Acad. Sci. U. S. A. 104, 4315–4320 (2007).

6.           Brown, K. et al. A novel type of catalytic copper cluster in nitrous oxide reductase. Nat. Struct. Biol. 7, 191–195. (2000).

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