Microbes as sources of sustainable biofuels

Considering the prevalence, durability, and efficiency of microbes, the notion of utilising the microscopic organisms as a means of generating sustainable energy sounds too good to be true. In a new issue on "microbial ecology and sustainable energy" in the journal Nature Reviews Microbiology, the Bruce Rittman and his fellow Biodesign Institute researchers outline paths where bacteria are the best hope in producing renewable energy in large quantities without damaging the environment or competing with our food supply.

Dr. Rittman took time out to speak with Scientist Live about the exciting research being conducted at the institute.

By what means/mechanisms can microbes play a role in producing usable, sustainable energy?

They can do it in two ways that are complementary. In the first way, communities of anaerobic bacteria convert the energy value (contained in electrons) of biomass into socially useful bioenergy forms: methane (natural gas), hydrogen gas, or electricity. The biomass can be wastes from agriculture, animals, industry (like food), and humans (sewage). In this case, the capturing of the bioenergy also is a means to remove the pollution in the waste material. In addition, the biomass could be made especially for being a bioenergy source, which leads to the second way. In the second way, photosynthetic microorganisms capture sunlight energy through photosynthesis to make more of themselves. The microorganisms can be algae or photosynthetic bacteria, also called cyanobacteria for their blue-green colour. In our team, we use cyanobacteria. Some of the photosynthetic microbes contain a high proportion of lipids, or oils, that can be used a a feedstock for liquid fuels, such as biodiesel. The non-lipid portions constitute biomass for feeding into the first way for energy conversion.

What microbes can be utilised? Are some more effective than others?

For the photosynthetic systems, we think that certain microbe types are more efficient for capturing sunlight energy and converting it to high-value energy material, such as lipids, at high rate. Thus, we are focusing on cyanobacteria that are very good at forming lipids and growing very fast. For the other systems, we do not rely on one type of microorganism, but on communities of microorganisms that work synergistically to convert the biomass to either methane, hydrogen, or electricity. In some cases (mainly for making methane), we know the key members of the community pretty well.

For the other systems, we are still in the discovery stage, and this is the point of our NRM perspective piece.

Can you provide examples of some microbes, what they could produce, and how they produce it? How do the raw products become usable energy?

The key example from our work is the cyanobacterium Synechocystis, which has all the good characteristics we seek and also has a fully sequenced genome, which allows us to track it and its activity, as well as make some genetic improvements so that it does the desired job more effectively (e.g., make a lot of lipids).

For methanogenic systems, Methanosaeta is well know for its ability to convert acetate to methane with high efficiency. Of course, Methanosaeta never works alone in a real bioenergy system, but has synergistic relationships with a range of fermenting bacteria.

Microbes are very small. What quantities would be needed to produce enough energy to be of use?

The small size of microorganisms is a major part of their big advantage for use in bioenergy situations. Small size means that they are simple, can grow very fast, and have low "overhead."

Thus, their ability to capture sunlight energy and make biomass is at least 100-fold greater than can be achieved by plants. Also, they do not need to grow in arable soil and compete with food supply.

Also, small size means that they can exploit any small opportunity to gain some energy and grow. In ecology, we call this a niche.

This niche exploitation is what makes them effective in the energy-conversion application. Big bulky organisms cannot exploit such niches and are useless. Only microorganism can do what we need.

Where would the energy be produced and what facilities/technologies are necessary?

Of course, it depends on the application. For the bioenergy conversion processes, we are wise to locate them near to the biomass source, like next to a pig farm or a photobioenergy system.

For photobioenergy, we would like to locate them outside where it is sunny and we have land not used for other important purposes. Desert and plains are good. It is ideal to locate them near a power plant so that we can use the CO2 as a CO2 source.

Are there any ecological dangers involved and how can they be safeguarded against?

Ecological dangers should be much smaller than an alternatives one might imagine. However, we cannot be lax about considering possible risks. For the biomass-conversion processes, we are using naturally occurring microorganisms that we manage. For the photosynthetic systems, we can use natural or "improved" versions of the phototrophs. In the latter case, we will study the fate of the modified organisms in the natural environment. All the systems we use are closed and contained. So, they will not get be spread about freely. The biggest problem probably would be if the reactors broke. We will provide containment in case of a leak or break.

How has metagenomics eased the research process?

Metagenomics is too new to have had an impact up to now. Part of our research plan is to exploit metagenomics as a "mining" tool to look for especially capable microorganisms in nature or in our reactors.

Specifically, what pre- and post-genomic tools can be used and how?

We use pre-genomics routinely to track key species and to finger-print the community to look for new, unindentified species that seem to be important in our systems. We need to know "who is there" and if it changes due to alterations we make in our system or for some other reason. If we have key species, we need to be certain that they are present in adequate numbers. Post-genomics is just beginning to be used in a major way. We can use it to understand what the key microorganisms "are doing." We call this "structure and function" in the lingo of microbial ecology. Structure refers to "who is there," and function refers to "what they are doing." Pre-genomics mainly addresses structure, while post-genomics mainly addresses function. The differentiation is not 100% that simple, but for the most part, this is the difference. We are much more capable today with pre-genomics, but post-genomics is advancing rapidly due to high-throughput technologies, such as mass spectrometry to identify proteins (i.e., proteomics).

What microbe or group of microbes hold the most immediate promise?

All of them are important and promising. We will need all the systems. However, I give special note to the photosynthetic microorganisms - in particular Synechocystis - which offers the potential to produce what I call new biomass in massive amounts, enough to give us a chance to replace fossil fuels with renewable fuels in the incredibly large amounts that human society now consumes. Not that over 80% of all energy used by human society is fossil fuel. I attach a recent paper that I wrote that explains this aspect. The attached paper is a perfect complement to the NRM perspective piece.

Finally, how far off is microbial sustainable energy?

We are making and using methane in pretty large amounts now and have been doing it for 150 years. So, we need to expand and improve this approach, but it is available today. For the other bioconversion processes, I would say that we are moving fast and can be at incipient commercial scale in 3 - 5 years (with sufficient investment). The photosynthetic system also is about 5 years from incipient commercial application. Of course, it will take many years to ramp up so that we can replace most to the fossil-fuel industries.

In the meantime, society is wise to emphasise efficiency and conservation so that the challenge does not grow more than it is already.


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