Increasing atmospheric levels of greenhouse gases (GHGs) and short-lived climate pollutants (SLCPs) through human activities is fueling climate change, posing a threat to our very existence. These pollutants act as Earth’s thermal cocoon, warming up the planet by trapping the sun’s heat in the atmosphere instead of letting it escape back to space. Global temperature increase is expected to exceed 2°C by 2050, if annual GHG and SLCP emissions continue to rise.
Climate change is driving extreme weather occurrences, rising sea levels through melting glaciers, natural disasters, as well as infectious diseases outbreaks which are becoming more commonplace. There is an urgent need for global measures to reduce human-driven GHG and SLCP emissions, if we want to prevent further temperature rises and solve the climate crisis.
Thankfully, many microorganisms can consume GHGs and transform them into energy which they need for survival, growth, and reproduction. Scientists are developing microbial technologies as part of ongoing efforts to limit GHG emissions and the extent of future climate change.
Microbes can capture and turn carbon dioxide into useful products
Carbon dioxide (CO2) – the most common GHG and therefore the largest contributor to climate change – can stay in the atmosphere for 300 to 1000 years! It is naturally emitted into Earth’s atmosphere via the Carbon Cycle which includes processes like fermentation, respiration, decomposition, volcanism (both underwater and on land), and rock erosion.
However, human activities have increased the levels of CO2 in the atmosphere primarily through the extraction and combustion of fossil fuels (oil, coal, natural gas) for energy or transportation purposes, large-scale wildfires, deforestation, and wastewater treatment.
Biological carbon sequestration – the removal and storage of atmospheric CO2 by photosynthetic plants and microorganisms – is one way to reduce carbon dioxide emissions. Microalgae are aquatic plant-like microorganisms that have evolved an amazing ability to siphon 10-50 times more CO2 than any land-dwelling plants. They convert CO2 into lipids and carbohydrates which can be turned into biofuels and other commercially viable products.
These days, microalgae are grown outdoors in open ponds or indoors in closed bioreactors where they sequester CO2 while simultaneously producing a multitude of products used for pharmaceuticals, cosmetics, nutraceuticals (food supplements), and biofuels.
Photobioreactors (PBR) are transparent illuminated vessels designed for growing photosynthetic organisms like algae. PBRs provide an environment where algal growth can be monitored and manipulated.
Wastewater treatment facilities produce CO2 in addition to other GHGs like methane and nitrous oxide through microbial decomposition of waste matter. Bioelectrochemical systems, such as Microbial Electrolytic Carbon Capture (MECC) and Microbial Electrosynthesis (MES), are energy-efficient modern technologies that can change waste treatment to a carbon-neutral or even carbon-negative process. These systems use microbes to convert CO2 produced from waste treatment process into value-added products like alcohols and lipids (for MES), and bio-hydrogen (for MECC).
Methane-devouring microbes could be key to effectively managing methane emissions
Methane is a powerful short-lived pollutant that persists in the atmosphere for up to 12 years, but it absorbs about 30 to 80 times more heat than CO2. Besides its high global warming potential, methane also reacts in the atmosphere to form tropospheric or ground level ozone. Tropospheric ozone is a GHG and a harmful air pollutant that has damaging effects on the respiratory system. While some methane emissions come from natural sources, more than half of all emissions are due to human activities.
The largest sources of human-driven emissions are ruminant livestock flatulence, intentional venting and leakages of fossil fuels, as well as landfills and wastewater treatment facilities where microbes produce methane through the decomposition of waste materials.
Wetlands (swamps, mudflats, bogs, marshes) are natural sources of methane contributing to about 40% of total methane emission. These ecosystems typically have waterlogged soils, creating oxygen-poor conditions ideal for methane production.
Wetland methane is formed when certain microbes called methanogens break down or decompose plant and animal materials in the absence of oxygen. Other natural sources of methane include rice paddies, termites, the oceans, and geological structures (e.g., mud volcanoes).
Methane-devouring microorganisms called methanotrophs can consume methane, converting the gas first into methanol with the aid of a special enzyme called methane monooxygenase, and then to formaldehyde via methanol dehydrogenase. Formaldehyde is transformed into formate (with the help of formaldehyde dehydrogenase), and finally into CO2 (via formate dehydrogenase). Methanotrophs like Methylococcus capsulatus and Methylomonas methanica are found in many environments including soils, sediments, lakes, landfills, and wetlands. Scientists estimate that 50-90% of global wetland methane emissions is consumed by these microbes before it escapes into the atmosphere.
Landfill methane is captured in wells, processed, and used to produce electricity and heat. However, Microbial Methane Oxidation Systems (MMOS) are considered low-cost alternatives for limiting landfill emissions when methane capture is logistically or economically unviable. MMOs like the bio-covers, bio-filters, and bio-windows are designed to enhance the activity of methanotrophs naturally present in landfills.
These systems typically consist of an upper layer of waste materials (like compost or sewage sludge), along with a lower gas distribution layer which supplies the upper layer with methane gas formed underneath. The waste materials in the upper layer serve as substrate on which methanotrophs can grow, which accelerates their consumption or oxidization of methane.
Microorganisms as sources of renewable energy
Biofuels from microbes
The energy sector emits the largest amount of GHGs due to its use of fossil fuels in vehicles, and for heat and electricity generation. The best way to curb climate change is by replacing traditional fuel sources with cleaner renewables like biofuels and bioelectricity. Through photosynthesis, algae accumulate large quantities of carbohydrates and lipids in their cells. Once harvested, algal biomass can be burned to generate heat and electricity, or converted into biofuels through several processes.
Bioelectricity from microbes
Certain bacteria called exoelectrogens have an astonishing ability to generate electricity by transferring electrons from inside their cells to metals in their surroundings, while decomposing organic materials. This electron transfer process is called extracellular respiration and it takes place in the absence of oxygen. Exoelectrogens, often fondly called “metal breathers”, include Geobacter metallireducens and Shewanella oneidensis. Their unique trait is being used in the Microbial Fuel Cell (MFC), a Biolectrochemical system (BES) that produces bioelectricity from wastes.
At present, MFCs are only able to generate enough electricity to power small devices such as LEDs, wireless environmental monitoring sensors, small fans, and membrane bioreactors for wastewater treatment. However, various power management systems (e.g., the low voltage booster multiplier) are currently being developed to boost MFC power generation. Despite its current limitations, the MFC system remains a promising technology for generating alternative renewable energy.
Microbes could keep nitrous oxide emissions in check
Commonly called “laughing gas”, Nitrous oxide (N2O), is 300 times more powerful than carbon dioxide in trapping the sun’s heat. This potent GHG is involved in the creation of ground-level ozone while also being a potent stratospheric ozone-destroying agent.
Stratospheric ozone, often referred to as “Earth’s sunscreen”, shields all living organisms from the sun’s harmful ultraviolet (UV) rays. On the other hand, ground-level or tropospheric ozone is a pollutant formed when nitrous oxide and volatile organic compounds react in the presence of sunlight.
Like carbon dioxide, nitrous oxide is long-lived remaining in the atmosphere for about 110 years. Farming practices such as the handling and storage of manure, and the use of ammonia-based fertilizers are major sources of nitrous oxide emission.
Ammonia is converted to nitrous oxide by soil microorganisms through the process of nitrification followed by denitrification. At the final step of denitrification, nitrous oxide is transformed into harmless nitrogen gas by microbes like Pseudomonas stutzeri and Bradyrhizobium japonicum that have the enzyme, nitrous oxide reductase. This enzyme along with its encoding gene, the nosZ gene, have been the focus of more than 500 studies done in the last five years. Researchers hope that gaining a better understanding of these microorganisms can pave the way for new nitrous oxide abatement technologies.
Volatile Organic Compounds (VOCs) are chemicals that turn into vapour at room temperature. They are emitted by car engines and are also present in commonly used products such as paints, adhesives, dyes, cleaning products, air fresheners, perfumes and deodorants, wood varnishes, insect repellent, carpets, and refrigerating units.
Ice-dwelling microbes could help remove soot particles from glaciers
Apart from its potentially fatal impact on human health, soot or black carbon is a notorious climate-warming agent that is driving the melting of glaciers in the Arctic and parts of Antarctica. Unlike CO2, soot is short-lived, staying in the atmosphere for four days to three weeks, but it has 460-1,500 times the warming power of CO2!
Soot consists of small carbon particles formed by incomplete burning of wood or fossil fuels at low temperatures or in an atmosphere with insufficient oxygen. These particles settle on and darken the surfaces of snow and glaciers transforming them from excellent reflectors of sunlight to heat absorbers, causing them to melt faster.
Soot particles emitted by airplane engines also contribute to climate change. Contrails – those white plumes that trail after airplanes across the sky – are formed when water vapour freezes around aviation soot particles. Contrails can spread out and develop into cirrus clouds which are high-flying ice clouds that tend to trap the sun’s heat.
“Cirrus clouds, El Calafate, Argentina”, by Dimitry B., licensed under CC BY 2.0
Soot was previously thought to be non-biodegradable until a recent study showed that it can be degraded by some ice-dwelling microbe, namely Bacillus and Arthrobacter.
Climate change has led to more frequent large-scale wildfires, spewing out even more soot into the atmosphere. Future studies on soot biodegradation could provide invaluable information that can help save the planet’s ice sheets.
Concluding Remarks
Microorganisms have been around for about 3.5 billion years, thriving in harsh environments where no other life forms can survive, and adapting to both natural and human-driven changes to the planet. Microbes are important in regulating climate change through their consumption of GHGs (e.g., CO2, methane, and nitrous oxide), but they also produce these gases while decomposing organic substances. Improving our understanding of these incredibly resilient organisms may be key to mitigating climate change.
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