Microbial marvels: could microbes resolve climate change?

Why are microbes crucial in tackling climate change?

Carbon and nitrogen are the building blocks of life. Consider the role that microbes in the processing of nitrogen, where the interconnections between the two largest nitrogen pools on Earth, atmospheric molecular nitrogen and biologically reactive nitrogen, are predominantly controlled by just two microbial processes, nitrogen fixation and denitrification [1].

During the carbon cycle, microbes can transform the huge volumes of carbon that are present in our oceans, atmosphere and biosphere from a contained state into free, active forms, such as carbon dioxide, and vice versa. For instance, a staggering 45% of the carbon that is released by humans is sequestered within the planet's oceans, much of it as a result of carbon fixation by marine microbes [2]. Phytoplankton serve as a prime example of this phenomenon and are credited with creating between 50 and 85% of the oxygen on Earth through the carbon-consuming process of photosynthesis.

Turning our attention to the biosphere, soil matter comprises the largest organic carbon pool in the terrestrial biosphere. Here, microbes are also busy at work breaking down organic plant matter into carbon dioxide and dissolved organic carbon, resulting in the re-release of carbon into the atmosphere as carbon dioxide or its long-term storage respectively [3,4].

As such, microbes can present a major driver of climate change. Those found in our oceans and soil directly contribute to rising temperatures by producing key greenhouse gases: carbon dioxide, methane and nitrous oxide. These three gases are responsible for 98% of increased global warming [4]. The balance between microbial production and consumption of carbon dioxide and other greenhouse gases determines whether a region is categorized as a carbon source or a sink, thereby adding to or lowering the net flow of global emissions [5].

A microbial species that could tip the carbon sink-source balance is Prorocentrum cf. balticum, which is particularly effective at sequestering carbon due to its ability to both photosynthesize and consume other organisms. Within the photosynthesis process this microbe produces a carbon-rich exopolymer to attract and capture its microbial prey for digestion. Elements that aren't digested make the carbon-rich exopolymer heavier, causing it to sink and form part of the ocean's natural biological carbon pump [6].

It is estimated that this microbial species could sink between 0.02 and 0.15 gigatons of carbon every year, a truly substantial amount for a tiny microbe considering that 10 gigatons of carbon dioxide needs to be removed annually until 2050 to meet current climate change targets [6,7]. The downstream effects of not meeting these goals are huge. Rising levels of greenhouse gases will inevitably cause more extreme weather conditions including hurricanes, flooding and droughts. Floods and hurricanes disrupt existing microbial communities and spread microbes further afield, whilst droughts result in food insecurities leading to mass starvation events and malnutrition. Pair this with the fact that microbial infections reproduce more effectively in warmer temperatures and have more deadly effects in individuals with malnutrition, this leads to a disproportionate impact on the rise of microbial diseases and fatalities [8,9].

Unlocking the power of microbes in bioremediation

Microbes not only play a major role in balancing greenhouse gas levels, but their innate biological processes can also be exploited to tackle environmental contamination resulting from natural disasters or human activity. This is achieved through a process termed bioremediation, which involves harnessing the natural power of organisms to detoxify or degrade pollutants from contamination sites (Figure 1). Microbes work by transforming hazardous organic pollutants into compounds such as carbon dioxide, water and methane, without causing adverse effects to the environment [10].

Bioremediation can either be conducted at the site of environmental contamination or contaminated soil/water can be extracted and treated elsewhere. It does not require extensive equipment or many specialized workers to roll out so provides a permanent solution using natural techniques, making it both cheaper and more environmentally friendly than traditional methods [10].

Bioremediation strategies for addressing oil spills

Oil spills have catastrophic effects on wildlife, bring huge financial losses and negatively impacting fishing, tourism and marine agriculture [12]. Microbes can play a crucial role in clearing up oil spills as they have an innate ability to turn complex petroleum hydrocarbons into harmless compounds through the natural process of biodegradation [10]. These degrading microbes are ubiquitous and highly diverse, providing vast potential for microbes to break down oil. So why do humans need to get involved at all?

While microbes can certainly switch harmful components of oil spills into safe compounds, when left to their own devices this process can take a very long time [13]. To help optimize microbial function, there is the option of biostimulation. This technique works on the premise of removing nutrient limitations, thereby increasing the activities of indigenous microbes by delivering oxygen, nutrients and moisture to unsaturated vadose zones in the form of fertilizer, ultimately increasing the rate that oil is removed [11].

The 1989 Exxon Valdex tanker spill, which involved 11 million gallons of crude oil being poured onto Alaska's coastline, was both one of the largest environmental disasters in US history and marks the greatest use of bioremediation. A colossal 48,600 kg of nitrogen fertilizer was applied to 2237 separate shorelines from 1989 to 1991. The biostimulation process involved applying a slow-release fertilizer (Customblen 28-8-0) followed by an oleophilic fertilizer (Inipol EAP22) to contaminated surfaces. Ecological monitoring, including frequent water effluent toxicity tests, were simultaneously employed to check if the biostimulating fertilizers were causing additional adverse effects. In 1989, analysis of sediment samples showcased the powerful effects of bioremediation – approximately 25–30% of the total hydrocarbon in the oil on shorelines had been lost within the first days to weeks after the spill [14].

As highlighted above, in warm, airy environments microbes are effective at cleaning up oil spills but in the deepest parts of the oceans or in muddy banks where nutrients and oxygen levels are low, anerobic microbes take much longer to reproduce and degrade oil particles [11,13]. Twenty years after the Exxon Valdez spill, oil still remained on some beaches due to low concentrations of oxygen. To overcome this, researchers have been injecting hydrogen peroxide (as a source of oxygen) as well as nitrate and phosphate solutions into the more impermeable layers of beaches. This increased the rate of oil degradation from <1% per year to between 13 and 70%, as measured by the oil concentration in sediment and groundwater samples [15].

An emerging technique in this area is nano-enhanced bioremediation, which can be split into two main types. The first nano-based remediation method involves the use of engineered nanoparticles that can form oil-water Pickering emulsions. A Pickering emulsion prevents oil droplets from merging into large masses, making it easier for indigenous microbes to degrade the oil as both the bioavailability and surface area of the oil is increased. Second, researchers are investigating the absorption of oil through magnetic nanosorbents. However, these methods are still at a lab-based level and further research needs to be taken to assess the environmental and biological effects of nano-based formulations on marine environments as well as for cost comparisons to be made against established methods [11].

There is also the option of adding specialized engineered ‘super-microbes’ to the mix, a technique termed bioaugmentation [10]. These super-microbes are grown in the laboratory and, in theory, should speed up the bioremediation process. However, to date, there is no credible evidence that lab-grown microbes are better at degrading oil than indigenous microbial communities [16,17]. Therefore, researchers are predominately focusing their efforts on creating the optimum environment for indigenous microbes.

Conclusion

As has been noted, microbes are both major drivers and remediators of climate change due to their crucial role in the biogeochemical processing of carbon and nitrogen, which are essential for life on Earth.

Microbes' crucial role in the biogeochemical processing of carbon and nitrogen, combined with a rapid generation time – sometimes just a few hours – makes them an ideal model by which to study the effects of climate change on biological systems and global biogeochemical cycles. Current models include horizontally stratified microbial communities, termed microbial mats, that can be exposed to numerous physical-chemical conditions mimicking climate change [18]. This allows for predictions to be made about the effects of climate change on all types of living matter and could help unlock novel methods for reducing greenhouse gases [19].

For a long time, the role of microbes and the views of microbiologists in climate change conversations have been overlooked. Microbes were not often considered relevant for climate-change studies and were disregarded in evidence-based policy development [6]. To attempt to bridge this gap, a multi-institutional group of researchers founded the Microbiologists' Warning to Humanity, a branch of the Scientist's Warning Movement to facilitate the infusion of microbial research into climate change [20,21].

If action is not taken, rising temperatures will inevitably increase the frequency and extremity of microbial diseases and natural disasters [9]. To achieve a sustainable future, we must understand both how microbes contribute to climate change and how they are in turn affected by it [19]. The actions that humans make can directly determine where microbes consume or produce greenhouse gases, placing us in a favorable position. Microbes must be at the forefront of our minds when devising strategies to combat climate change as they could be the most powerful weapon to combat global warming.