Insights in how cyanobacteria use ‘leftover’ light in shaded environments

Currently, photosynthesis in plants only converts few percent of the incoming light into chemical energy. This low yield is partly because only a portion of the solar spectrum, corresponding roughly to visible light (wavelengths below 700 nm), can be absorbed and used to power photosynthesis. Roberta Croce, professor of biophysics of photosynthesis of Vrije Universiteit Amsterdam, and her group are trying to increase the absorption of far-red light, corresponding to wavelengths above 700 nanometres.

07/13/2020 | 2:32 PM

The ultimate goal of this research is to introduce a chlorophyll variant from cyanobacteria - which are very good at that specific absorption - into the plants so that they can use far-red light. This is particularly relevant for crops because only far-red light penetrates into the crop field, meaning that mostly far-red light is available for the leaves in the shade. A preliminary study validating this strategy was published July 13 in Nature Plants.

Photosynthesis is the process that transforms light energy into biomass (food and fuel) and in this way it sustains all life on Earth. Cyanobacteria are bacteria that perform photosynthesis.  Also plants perform photosynthesis because they contain an organelle very similar to a cyanobacterium (actually it is a cyanobacterium that was “eaten” by the plant long ago).

Cyanobacteria, similar to algae and plants, use pigments called chlorophylls to absorb light and power the electron transfer reactions that initiate photosynthesis, like a sort of natural solar cell. Chlorophyll captures sunlight and converts the solar energy into chemical energy, which is then used for photosynthesis. Not all solar photons, however, carry enough energy to drive these reactions, and the efficiency of photosynthesis substantially drops in the infra-red. This is why oxygenic photosynthesis mostly employs a particular chlorophyll, chlorophyll a. Indeed this pigment, which is responsible for the green coloration of plants, absorbs only visible light (the colours of the rainbow, corresponding to photon wavelengths of 400-700 nm; note that longer photon wavelengths imply lower photon energy). As a result, the infra-red (above 700 nm) wavelengths, representing over 50% of all solar photons reaching the Earth surface, remain largely unutilized. This restriction can be particularly limiting under the shade of a dense plant canopy, where most visible light is absorbed by the upper leaves and the available light is highly enriched in infra-red photons. Therefore, pushing the light-harvesting capacity beyond these natural spectral limits seems promising for increasing biomass yields.

Chlorophyll f
A strategy for making use of the infra-red photons is to select pigments absorbing at longer wavelengths (or, equivalently, lower energies) than chlorophyll a. In this respect, a lesson from nature comes from some recently discovered cyanobacterial species that populate deep shaded environments. These species can adapt to use the longer wavelength “leftover” light by remodelling their photosynthetic apparatus and producing a different pigment, chlorophyll f. Though chlorophyll f makes these cyanobacteria capable of capturing photons with wavelengths between 700-800 nm, very little was known so far about its impact on the photosynthetic efficiency. This information is essential if we wish to introduce this pigment into other organisms, such as plants and algae, to expand their light-harvesting capacity.

By studying the ultra-fast processes that follow light absorption by the chlorophylls (typically on timescales of fractions of billionth of seconds) the researchers found that the presence of chlorophyll f could sometimes reduce the efficiency of photosynthetic reactions. However, the use of chlorophyll f remains beneficial in shaded environments that are highly enriched in low-energy photons, where chlorophyll a would hardly absorb any light. This finding proves that the use of chlorophyll f is a viable strategy for extending the absorption spectrum of engineered crops. The analyses also shed light on the structural and energetic reasons behind the reduced performances of chlorophyll f-containing cyanobacteria. This represents a starting point for the rational design of new photosynthetic units that is required to achieve the desired high biomass yields.