by Anna Vietmeier
On ancient Earth, water was the most likely place for life to begin, and water is, arguably, one of the most critical and undervalued resources we have [1, 2]. For life on Earth, water is essential, as it makes up 60% of our bodies as the main component of cells, tissues, and organs . Freshwater resources include lakes, rivers, and groundwater . Unfortunately, in the twenty-first century we are facing issues that stand in the way of fresh water quantity and quality. More than a third of the world’s population does not have access to safe drinking water, which is further exacerbated by the waste and uneven distribution of water resources [1, 4]. One way that water can be wasted is through inefficient irrigation of crops and leaky water pipes . Water quality is impacted by increasing pollution due to growing urban populations and industrial waste discharge .
Chemical water pollutants can be macropollutants (measured at milligrams per liter), or micropollutants (measured at nanogram or microgram per liter concentrations) . Macro scale water pollutants can be added via human activities such as crop fertilization and irrigation and road maintenance in snowy conditions. When fertilizer macropollutants, such as nitrogen and phosphorus, run off into the watershed, they increase the nutrient concentrations in water, which leads to the production of biomass, depletion of oxygen, and toxic algal bloom formations . The use of road salts for road maintenance and excessive irrigation of crops lead to an increase in salt concentrations in water, which inhibits crop growth and makes the water non-drinkable .
Due to the low concentration of micropollutants, it can be difficult to assess their effect as their toxic impacts are typically subtle chronic effects and not acute . Inorganic pollutants in water include heavy metals, which are especially troubling as they naturally cycle through different chemical oxidation states that are subsequently associated with different toxicity levels .
In addition to the pollution of drinking water, we are also experiencing a global energy crisis . Fossil fuels, including oil, gas, coal, and uranium, are projected to be nearly depleted in 55-75 years . The overuse of fossil fuels has also propagated other environmental concerns such as CO2 emission and thermal pollution, both of which increase the rate of climate change . There is a desperate need for alternative fuel sources as fossil fuel concentrations decrease, global demand for energy increases, and the impacts of human driven climate change becomes more evident . The replacement of fossil fuels with other energy sources can counter greenhouse gas production that is the leading cause of climate change . Alternative greener sources of energy production include solar power, hydropower, and wind power. According to new scientific research, microbes may be another source of green energy.
Microbes are microscopic organisms that can be found just about everywhere. Microbes can include fungi, algae, and bacteria. Microbes adapt to their environment, and in doing so, have evolved a multitude of ways to generate energy to drive their metabolism: a multitude of microbial products and byproducts. Microbes are capable of generating energy, treating contaminated waters, and producing products due to their electron transport and carbon metabolism capabilities . Microbial biomass and microbial biological waste can be used as renewable energy/fuels .
Due to the metabolic diversity of microbes, they are capable of producing biofuels from a variety of substrates or reactants . Biofuel may be a sustainable carbon-neutral energy source that would be compatible with current engine technology . A wide range of bacteria have been found capable of creating a range of biofuels (Figure 1) . For instance, bacteria can easily convert sugars into ethanol. Cyanobacteria and microalgae can photosynthetically reduce atmospheric CO2 into biofuels, while methanotrophs can use methane to produce methanol . Microbial production of ethanol has been reviewed extensively, with the most commonly used organism being Saccharomyces cerevisiae, Zymomonas mobilis, and Escherichia coli . Microbial pathways have been found that can generate molecules that are similar or identical to those found in gasoline . Use of lignocellulosic biomass (wood, agricultural, or forestry wastes) to generate biofuel via microbes involves the conversion of monosaccharides to target molecules . With increased research in regards to the microbial physiology and pathways involved in producing biofuels, microbial engineering can be used to advance the uses of microbes to generate biofuels .
Microbial Energy Production
Bioelectrochemical cells, cells that produce a chemical electrical gradient, have gained interest for their applications in bioenergy, bioremediation, and bioelectronics . Bioelectrochemical cells generate bioenergy from organic biomass in wastewaters . Metabolic potential of microbes through electron transfer in microbial metabolism has a direct relationship to redox reactions that are used to remove contaminants from solution and generate electricity . Microbes have the ability to respire (breathe) metal by using the electrons from environmental metals to power their electron transport chain for energy production . Bacteria that are capable of carrying electrons beyond their cell walls are capable of interacting electrically with their environment . Electrons can either be removed or applied from an external circuit or source . Electrons that move beyond a cell wall can be transferred via soluble mediators or with solid surfaces . To aid the movement of electrons out of the microbial cell and into the environment, microbes have evolved pili, also known as nanowires,with metallic-like conductivity . Nanowires can be used for centimeters long range electron transport, which on a microbial scale is a large distance [9, 10]. Nanowires can also move electrons to and from other species through interspecies electron transfer and insoluble electron acceptors . Microbial fuel cells are devices that generate electricity from microbial catabolism that utilize organic compounds . Microbial fuel cells use integrated anode and cathode chambers . Electrons generated during microbial oxidation of organic compounds are transferred to the anode of the microbial fuel cell, thereby generating an electrical current and energy (Figure 2) .
The remediation for contaminated drinking water may help in releasing some of the burden associated with the energy crisis. Several emerging microbial electrochemical technologies utilize interactions between electrodes and bacteria for the production of energy, treatment of waste and wastewater, bioremediation, and the production of valuable products . Microbial fuel cells are an alternative energy source that have an excellent ability to produce electricity while removing pollutants . Microbial fuel cells are promising for sustainable energy production and simultaneous removal of heavy metals from soil and water . Bioremediation is one of the most favored sustainable means of treating contaminated sites, as it is renewable and cost-effective to treat contaminated soils and sediments .
One of the challenges for producing biofuels is using microbial factories to generate a large amount of fuel on a comparatively low budget with a greater efficiency as compared to conventional fossil fuels . We have a current need for microbial fuel cells that can be scaled to support widespread and cost-effective applications and a better understanding of the physical properties of potential biofuel molecules . Some biofuel properties still need a better understanding of their sustainability and the purification process . Continued research on metabolic engineering for the production of bulk chemicals and biofuels can hopefully aid in the large-scale use of biofuels and bioenergy in the future .
1. Wang, M., et al., Rural industries and water pollution in China. J Environ Manage, 2008. 86(4): p. 648-59.
2. Nielsen, F.H., Evolutionary events culminating in specific minerals becoming essential for life. Eur J Nutr, 2000. 39: p. 62-66.
3. Jequier, E. and F. Constant, Water as an essential nutrient: the physiological basis of hydration. Eur J Clin Nutr, 2010. 64(2): p. 115-23.
4. Schwarzenbach, R.P., et al., Global Water Pollution and Human Health. Annual Review of Environment and Resources, 2010. 35(1): p. 109-136.
5. Fang, C. and V. Achal, The Potential of Microbial Fuel Cells for Remediation of Heavy Metals from Soil and Water-Review of Application. Microorganisms, 2019. 7(12).
6. Kumar, R. and P. Kumar, Future Microbial Applications for Bioenergy Production: A Perspective. Front Microbiol, 2017. 8: p. 450.
7. Kracke, F., I. Vassilev, and J.O. Kromer, Microbial electron transport and energy conservation – the foundation for optimizing bioelectrochemical systems. Front Microbiol, 2015. 6: p. 575.
8. Fortman, J.L., et al., Biofuel alternatives to ethanol: pumping the microbial well. Trends Biotechnol, 2008. 26(7): p. 375-81.
9. Lovley, D.R. and N.S. Malvankar, Seeing is believing: novel imaging techniques help clarify microbial nanowire structure and function. Environ Microbiol, 2015. 17(7): p. 2209-15.
10. Yuan, Y., et al., Centimeter-Long Microbial Electron Transport for Bioremediation Applications. Trends Biotechnol, 2020.
11. Hou, H., et al., Microfabricated microbial fuel cell arrays reveal electrochemically active microbes. PLoS One, 2009. 4(8): p. e6570.
Non-Peer Reviewed Images