Two more potent biogenic greenhouse gases—methane (CH 4 ) and nitrous oxide
(N 2 O)—are even more directly relevant to microbial activities, especially in agriculture. Methane is the second most important biogenic greenhouse gas because of
its sheer quantity (10% of US greenhouse gas emission in 2017) and the fact that
methane is around 25 times more potent than carbon dioxide in warming potential
per equal mass. Methane has multiple man-made sources: land-use change, the
livestock industry, and landfills. However, methane is produced from those environments actually by archaea populations belonging to Euryarchaeota phylum,
which are abundant in anaerobic environments like wetlands, marine sediments,
the rumens of ruminants, and termite guts. Rice is a major crop that grows in artificial
wetlands called rice fields, and annual methane production from rice fields is
estimated to be 20%–25% of total biogenic methane production. With increased
meat consumption due to an increasing human population and their quality of life,
methane production from ruminants is estimated to be about 20%.
Nitrous oxide is the least abundant (6% of US greenhouse gas emission in 2017)
but most potent (~300 times to carbon dioxide) greenhouse gas and is mostly
produced from two microbial processes: nitrification and denitrification. Nitrification
is the process of oxidizing ammonia (NH 3 ) to nitrite (NO 2
À
) to nitrate (NO 3
À
), and
denitrification is anaerobic respiration of organic matter using nitrate as an electron
acceptor. In low oxygen conditions, ammonia-oxidizing bacteria and ammoniaoxidizing archaea use nitrite as an electron acceptor instead of oxygen—thus leading
to the reduction to nitrous oxide (NO 2
À
! N 2 O). Nitrous oxide is an intermediate in
the denitrification process, and low oxygen and low organic matter seem to favor the
nitrous oxide production pathway. As nitrogen is often naturally limited in terrestrial
ecosystems, agricultural fields experience an excessive amount of anthropogenically
associated nitrogen input as chemical fertilizer or organic forms (e.g., manure). The
natural soil microbial processes from agricultural fields then introduce more nitrous
oxide into the atmosphere. Increased nitrogen input also enhances overall soil
microbial respiration through a stoichiometric response between carbon and nitrogen
contents, thus increasing the release of soil carbon into carbon dioxide as well.
11.3 Consequences
Rapid environmental events, like those associated with the current climate change
trends, force affected organisms to take one of these responses: migration, acclimation, adaptation, or extinction. Individual organisms and populations of organisms
would make use of one or more of those responses based on their physiology and
genetics. For example, vegetation is more prone to acclimation or adaptation over
time, if not extinction, as migration usually is not a realistic nor effective response
compared to the rapid movement available for insects or animals. Microbes are
known for their hyperdiversity in taxonomic, phylogenetic, and functional dimensions, so they have an advantage in making use of any of these responses, which
enables them to thrive in certain environmental events. Nevertheless, these response
11 Microbes’ Many Roles in Climate Change: Contribution, Consequence, Mitigation,. . . 189
(N 2 O)—are even more directly relevant to microbial activities, especially in agriculture. Methane is the second most important biogenic greenhouse gas because of
its sheer quantity (10% of US greenhouse gas emission in 2017) and the fact that
methane is around 25 times more potent than carbon dioxide in warming potential
per equal mass. Methane has multiple man-made sources: land-use change, the
livestock industry, and landfills. However, methane is produced from those environments actually by archaea populations belonging to Euryarchaeota phylum,
which are abundant in anaerobic environments like wetlands, marine sediments,
the rumens of ruminants, and termite guts. Rice is a major crop that grows in artificial
wetlands called rice fields, and annual methane production from rice fields is
estimated to be 20%–25% of total biogenic methane production. With increased
meat consumption due to an increasing human population and their quality of life,
methane production from ruminants is estimated to be about 20%.
Nitrous oxide is the least abundant (6% of US greenhouse gas emission in 2017)
but most potent (~300 times to carbon dioxide) greenhouse gas and is mostly
produced from two microbial processes: nitrification and denitrification. Nitrification
is the process of oxidizing ammonia (NH 3 ) to nitrite (NO 2
À
) to nitrate (NO 3
À
), and
denitrification is anaerobic respiration of organic matter using nitrate as an electron
acceptor. In low oxygen conditions, ammonia-oxidizing bacteria and ammoniaoxidizing archaea use nitrite as an electron acceptor instead of oxygen—thus leading
to the reduction to nitrous oxide (NO 2
À
! N 2 O). Nitrous oxide is an intermediate in
the denitrification process, and low oxygen and low organic matter seem to favor the
nitrous oxide production pathway. As nitrogen is often naturally limited in terrestrial
ecosystems, agricultural fields experience an excessive amount of anthropogenically
associated nitrogen input as chemical fertilizer or organic forms (e.g., manure). The
natural soil microbial processes from agricultural fields then introduce more nitrous
oxide into the atmosphere. Increased nitrogen input also enhances overall soil
microbial respiration through a stoichiometric response between carbon and nitrogen
contents, thus increasing the release of soil carbon into carbon dioxide as well.
11.3 Consequences
Rapid environmental events, like those associated with the current climate change
trends, force affected organisms to take one of these responses: migration, acclimation, adaptation, or extinction. Individual organisms and populations of organisms
would make use of one or more of those responses based on their physiology and
genetics. For example, vegetation is more prone to acclimation or adaptation over
time, if not extinction, as migration usually is not a realistic nor effective response
compared to the rapid movement available for insects or animals. Microbes are
known for their hyperdiversity in taxonomic, phylogenetic, and functional dimensions, so they have an advantage in making use of any of these responses, which
enables them to thrive in certain environmental events. Nevertheless, these response
11 Microbes’ Many Roles in Climate Change: Contribution, Consequence, Mitigation,. . . 189