Tuomas Huokko, TCSMT Postdoctoral Researcher, Molecular Plant Biology
Today’s society is still to a great extent dependent on fossil fuels because we use them plenty to e.g. run our vehicles, heat our homes, provide us with electricity and power different industrial sectors. As is well known, fossil fuels are not infinite energy source and importantly contributing heavily to the accumulation of greenhouse gases. Thus, humankind faces a major challenge to replace these by alternative energy sources which would be carbon-neutral and completely renewable.
Solar energy is the most abundant energy source available on Earth. Cyanobacteria are an exceptional group among prokaryotes due to their ability to perform oxygenic photosynthesis where physical energy from sunlight is converted to chemical energy to reduce atmospheric CO2, having water as an electron source, and releasing O2 as a by‐product. Using synthetic biology photosynthetic cells can be modified to living cell factories to produce e.g. fuels, chemicals and food supplements in a sustainable way with energy from photosynthesis. In addition, some cyanobacterial species can grow under photomixotrophy which is a metabolic state that enables photosynthetic microorganisms to simultaneously perform photosynthesis and utilize imported organic carbon substrates which leads to improved biomass production, and consequently to higher yields of desired valuable compounds when compared to photosynthesis alone. Thus, photomixotrophic growth provides an interesting opportunity for blue and circular bioeconomy through acceleration of biomass production by utilization sugar side streams e.g. from wood industry. Additionally, genetically engineered photosynthetic microorganisms can act as a chassis for the whole cell photobiotransformation enabling sustainable synthesis of organic compounds because several enzymes utilised as photo-biocatalysts use reducing agents deriving from light-driven photosynthetic electron transfer reactions.
However, providing carbohydrates to growth environment of photosynthetic microorganisms creates a situation where they must balance the consumption of organic carbon sources with photosynthesis and carbon fixation which tends to decrease photosynthetic activity. Furthermore, cells have several intertwined glycolytic pathways, where carbohydrates are broken down for use as energy, as well as other metabolic shunts which makes it considerably complicated to genetically engineer the network of these metabolic reactions.
To exploit photomixotrophy as a production condition for solar based biofuels and biochemicals with its full potential, it is necessary to maintain photosynthetic activity as high as possible in the cells. This requires a clear understanding about underlying regulatory mechanisms affecting metabolism specifically under photomixotrophy. Gained knowledge can be then applied to create photosynthetic cell factories which will be capable of taking full advantage from both photosynthesis and sugar catabolism simultaneously leading to increased production rates. Even though there are still several question marks regarding the details of metabolic reaction pathways and production yields in many cases are not high enough for industrial level, bioproduction in photosynthetic microorganisms is a promising strategy to develop sustainable production methods for variety of valuable compounds. Therefore, research covering this field is one of the important factors in the fight against ongoing climate change to pave way for greener future.
