What is a photosynthetic organism
Modified Photosynthesis: New Algae and Cyanoacteria
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 fuels and industrial materials. On the one hand, there is the low average efficiency of solar energy use (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 photosynthesis products (e.g. cellulose of the cell walls or the lignin of wood) should be mentioned, which mostly cannot be used directly as technical fuels or chemical valuable materials.
Are these limitations irreversible, or can science and biotechnology overcome them? Numerous research groups are concerned with this question. The initial 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 will continue to be pursued, the possibilities for a breakthrough with these classic approaches seem to most researchers to be too few or too long-term undertaking in view of the urgent challenges of global climate change. The most common approach today is targeted, knowledge-based modification using the methods of molecular biology or modern molecular genetics [1, 2]. Various methods can be used and different directions can be pursued, including the (still far from achieved) vision of completely redesigning a photosynthetic organism 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
The 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 the molecular-genetic change for many microorganisms is very well established and can be implemented with reasonable effort. In higher plants and trees, it is not only difficulties in molecular genetics in connection with long generation cycles that stand in the way of targeted modification. It is also their sophisticated, complex physiology with a multitude of specialized cells. These not only ensure stability and controlled macroscopic growth, but also the regulated transport of the starting materials for photosynthesis (water, CO2) as well as photosynthesis products such as glucose. Here, major molecular genetic interventions in the complex interaction networks, which are carried out with the aim of producing other product substances on a larger scale instead of glucose, would hardly be able to lead to the desired result. Tree trunks and plant stems would simply be ballast in the sense of efficient modified photosynthesis.
With small, mostly unicellular algae and cyanobacteria in water (with sufficient gas supply and the necessary trace elements), the supply of the individual photosynthetic cells with water and CO2 comparatively uncritical - in contrast, for example, to the situation with larger land plants. It is equally important that 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 by 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 sample). This means that the cells live in an aqueous solution inside a container that is either completely transparent or has suitable window openings. In addition to the controlled supply of CO2 and other nutrients, technical measures are generally required to avoid undesired sedimentation of the microorganisms, as well as technical solutions for harvesting or "collecting" the product substances. The design of inexpensive photobioreactors represents an as yet unsolved problem. The cultivation in translucent plastic bags floating in the sea is conceivable in principle, 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 with future LED efficiencies of 50% and optimal light wavelength (approx. 680 nm) it could still lead to a reasonable overall efficiency. In any case, effort and costs for the photobioreactor are an aspect of central importance when it comes to the use of algae or cyanobacteria for the production of products of modified photosynthesis.
4.3 Improving efficiency
The maximum efficiency of the use of solar energy by typical photosynthetic organisms is limited to approx. 10% (Section 3.4). Can values above 10% also be achieved? Without a complete reorganization, a broadening of the spectral range of sunlight, which can be used for photosynthesis, is the only way. Of the "normal" plants, it is mainly the green (hence their color) but also the long-wave part of the red light that is hardly used (Fig. 4.2, not included in this sample). Surprising new results from photosynthesis research could point the way to how this light output could be increased. Because there are actually cyanobacteria in nature that live in special, unusual environments and, in addition to blue, green, yellow and red light, can also use light with wavelengths of 700–760 nm . This infrared light is not only practically invisible to the human eye, but also cannot be used by the "standard organisms" involved in photosynthesis. These special cyanobacteria form, through small chemical variations, alternative chlorophyll variants, chlorophyll d (Chl-d) and chlorophyll f (Chl-f). In both cases, the absorption spectra are shifted to longer wavelengths so that they can also absorb light in the near infrared range. These chlorophylls are built into the protein complexes of the photosystems.
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