New Algae
The obvious idea is to use genetically modified photosynthetic algae to produce fuel and valuable materials in economically relevant quantities.
4.1 Modifying Photosynthesis – Why and How?
From an anthropocentric point of view, there are undesirable limitations and performance limits of biological photosynthesis, at least when it comes to the extraction of fuel and industrial resources. On the one hand, the low average efficiency of solar energy use should be mentioned here (high space requirement), the causes of which are discussed in the previous section. On the other hand, the photosynthetic production of primary (glucose) or secondary photosynthetic products (e.g. B. cellulose of the cell walls or the lignin of the wood), most of which cannot be used directly as technical fuels or chemical recyclables.
Are these limitations immutable, or can science and biotechnology overcome them? Numerous research groups are concerned with this question. The first approach was to proceed with conventional breeding (i.e. spontaneous mutations followed by selection by the breeders) or with expeditions to search for "new" photosynthetic microorganisms (algae or cyanobacteria). Although these directions are still being pursued, most researchers today consider the chances of a breakthrough with these classic approaches to be too small or too long-term in view of the urgent challenges of global climate change. The approach most commonly followed today is targeted, knowledge-based modification using the methods of molecular biology orof modern molecular genetics [1, 2]. Various methods can be used and different directions can be pursued, including the (far from achieved) vision of redesigning a photosynthetic organism from scratch using the methods of synthetic biology. Regardless of which special methods are used, these are approaches that are generally discussed as genetic engineering.
4.2 Use small algae and cyanobacteria
Research on far-reaching modifications of photosynthesis relates almost exclusively to photosynthetic microorganisms, i.e. small algae or cyanobacteria that can multiply quickly (e.g. with one generation per day). A major reason for this focus is that molecular-genetic modification is very well established for many microorganisms and can be implemented with reasonable effort. In higher plants and trees, it is not just the difficulties of molecular genetics in connection with long generation cycles that stand in the way of targeted modification. It is also their highly developed, complex physiology with a multitude of specialized cells. These ensure not only stability and controlled macroscopic growth, but also the regulated transport of the starting materials for photosynthesis (water, CO2) and the photosynthesis products such as glucose. Larger molecular-genetic interventions in the complex interaction networks, which are carried out with the aim of forming other product substances instead of glucose on a larger scale, would hardly be able to lead to the desired result. In addition, tree trunks and plant stalks would simply be ballast in the sense of an efficient modified photosynthesis.
In the case of small, mostly unicellular algae and cyanobacteria in water (with a sufficient supply of gas and the necessary trace elements), the supply of the individual photosynthetic cells with water and CO2 is comparatively uncritical – im This is in contrast to the situation with larger terrestrial plants. Equally important, simple techniques for effectively collecting continuously produced product substances are also conceivable. Gases can be collected comparatively easily. Non-gaseous substances can be separated from the suspension of algae or cyanobacteria via their specific gravity: "Fat floats on top."
The use of algae and cyanobacteria normally requires the cultivation and maintenance of the microorganisms in so-called photobioreactors (Fig. 4.1, not included in this excerpt). By this is meant that the cells live in an aqueous solution within a container that is either completely transparent or has appropriate window openings. In addition to the controlled supply of CO2 and other nutrients, technical measures are generally required to avoid the unwanted sedimentation of the microorganisms, as well as technical solutions for harvesting or "collecting" the product substances. The design of inexpensive photobioreactors represents an unsolved problem. In principle, cultivation in translucent plastic bags floating in the sea is conceivable, but certainly not without specific problems. The photovoltaic conversion of solar energy for the operation of lighting systems with LEDs optimized for this purpose appears energetically nonsensical at first glance, but could still lead to a reasonable overall efficiency with future LED efficiencies of 50% and an optimal light wavelength (approx. 680 nm). In any case, effort and costs for the photobioreactor are an aspect of central importance when it comes to using algae or cyanobacteria for the production of products of modified photosynthesis.
4.3 Efficiency Improvement
The maximum efficiency of solar energy use of typical photosynthetic organisms is limited to about 10% (Sect. 3.4). Can values over 10% also be achieved? Broadening the spectral range of sunlight that can be used for photosynthesis is the only way forward without a complete reorganization."Normal" plants hardly use the green light (hence their color), but also the long-wave part of the red light (Fig. 4.2, not included in this sample). Surprising new results in photosynthesis research could point the way to how this light yield could be increased. In fact, there are cyanobacteria in nature that live in special, unusual environments and can use light with wavelengths of 700-760 nm in addition to blue, green, yellow and red light [3]. Not only is this infrared light practically invisible to the human eye, it also cannot be used by the "standard organisms" involved in photosynthesis. These special cyanobacteria form alternative chlorophyll variants, chlorophyll d (Chl-d) and chlorophyll f (Chl-f), through small chemical variations. The absorption spectra of both are shifted to longer wavelengths, so that they can also absorb light in the near infrared range. These chlorophylls are incorporated into the protein complexes of the photosystems.
Light with longer wavelengths corresponds to lower photon energy. The fact that this energy is nevertheless sufficient to drive the photosynthetic electron transport from water to NADP came as a surprise to photosynthesis researchers and is only partially understood to date. Among other things, it is unclear why not all photosynthetic organisms use the spectral range up to about 760 nm. The Chl-d/f-containing cyanobacteria only live and thrive at unusually low light intensities with a comparatively high proportion of infrared light. So e.g. B. the Chl-d-containing cyanobacteria found on the underside of sea squirts, i.e. simple animal marine organisms. This example shows that the molecular-genetic introduction of the enzymes for Chl-d/f formation could expand the usable spectrum of photosynthesis in a surprisingly simple way. However, this approach will not dramatically increase the maximum efficiency of photosynthesis. However, an increase from around 10% to 11-12% seems possible.
A more significant increase in efficiency requires a radical reorganization of the photosynthetic light reactions [4]. This could be achieved by modifying the antenna pigments and chemical reactions of the two photosystems, PSII and PSI, in such a way that the PSII only uses the blue-green part of the light spectrum and the PSI only the longer-wave part (yellow, red and parts of the infrared range). Then – theoretically – a maximum efficiency of over 20% could be achieved. Chlorophyll would then no longer be the molecule of choice, since chlorophyll always absorbs light in both the blue and red spectral ranges. The transformation of the native photosynthetic apparatus would be so drastic that the incremental change starting from today's photosynthetic organisms would hardly lead to success. One of the goals of synthetic biology is the "construction" of completely new, quasi-biological cells that have no evolutionary precursor in the existing biological cells. However, it is still an open question whether photosynthetic cells capable of surviving and reproducing with an increased maximum efficiency of photosynthesis can ever be achieved in this way.
Literature
1. Banerjee C, Dubey K, Shukla P Metabolic engineering of microalgal based biofuel production: Prospects and challenges. Front. microbiol. 7, 432 (2016)
2. Larkum, A., et al.: Selection, breeding and engineering of microalgae for bioenergy and biofuel production. Trends biotechnology.30 , 198 (2012)
3. Nürnberg, DJ, et al.: Photochemistry beyond the red limit in chlorophyll f-containing photosystems. Science. 360, 1210 (2018)
4. Blankenship, R. E., et al.: Comparing photosynthetic and photovoltaic efficiencies and recognizing the potential for improvement. Science.332 , 805 (2011)
