Effects of artificial light and ionizing radiation on the nutritive value of photosynthetic bacteria used for waste recycling in space (PhD)

Photosynthetic bacteria, using light energy instead of chemical energy, are highly valuable for engineered biodegradation and biosynthesis processes.

Photosynthetic bacteria have been of interest for advanced waste recycling in space, as part of the 'Micro Ecological Life Support System Alternative' (MELiSSA) (http://ecls.esa.int/ecls/?p=melissa). MELiSSA is a biological life support system, targeting food production as well as complete air and water regeneration , via organic waste recycling by the combined activity of different microorganisms and plants; including the photosynthetic bacteria Rhodospirillum rubrum and Arthrospira sp.

But photosynthetic bacteria have also many applications on Earth. The red photosynthetic bacterium Rhodospirillum rubrum can, without using oxygen, recycle organic waste into valuable biomass. R. rubrum extracts are reported to lower cholesterol in animals (Patent PCT/NL2003/000884). The green cyanobacterium Arthrospira sp. is a typical 'plant like' bacterium that can produce oxygen from carbon dioxide, while producing edible biomass. Arthrospira (sold as Spirulina) is rich in proteins (essential aminoacids) and antioxidants (β-carotenes), and has potential blood pressure and cholesterol lowering, anti tumor, prebiotic and radioprotection effects. In addition, cyanobacteria (microalgea) are also used for biofuel production. In previous studies in our lab, the full genetic potential of these 2 photosynthetic bacteria was determined, i.e. there full DNA was sequenced and surveyed.  

To increase the efficiency and control of the bacterial photosynthesis processes, it is aimed to culture these bacteria in engineered photobioreactors. Within the photobioreactors, these red and green photosynthetic bacteria are however exposed to light (photon fluxes)  conditions, very different from their original natural lake habitat. The photosynthetic bacteria do not grown under day-night regime, but are continuously exposed to artificial light. The cellular function and the genetic adaptation of photosynthetic bacteria under these continuous photobioreactor light radiation of specific wavelenghts is presently unknown and needs to be determined. In addition, the impact of changing light fluxes (low or high light fluxes, due to increasing or lowered cell densities) on the biochemical content and nutritive value of the biomass that is produced, is to be characterized.

In addition, preliminary experiments in SCK•CEN have shown that these photosynthetic bacteria show a very different susceptibility to ionizing radiation, with R. rubrum being very sensitive and Arthrospira sp. potentially very resistant to gamma ionizing radiation. It is known that exposure to ionizing radiation, very much like exposure to high light intensities, can have a severe impact on bacterial physiology. It can be specifically damaging for the bacterial photosynthesis apparatus (photooxidation) and electron transfer processes in the cell. The role of pigments or efficient DNA and protein repair mechanisms have been suggested, but it is not exactly known how some photosynthetic bacteria can protect or repair their photosynthesis systems from radiation damage, while others cannot. So far there is no explanation. To understand these processes, and to elucidate if and how light and ionizing radiation susceptibility is correlated, for photosynthetic bacteria, is novel. The impact of high doses of light and/or ionizing radiation conditions on the biochemical content and nutritive value of the photosynthetic bacteria is to be characterised. It could open ways for more efficient bioreactor applications. In addition, the exposure to high doses of light and/or ionizing radiation could be a tool to select for genetic variants that can withstand higher doses of light, and thus could be more efficient in the targeted biodegradation or biosynthesis processes.

In this project it is aimed to investigate the effects of light and ionizing radiation on the cellular response and nutritive value of bacteria Rhodospirillum rubrum and Arthrospira platensis. It is aimed to characterize in detail the genetic and metabolic adaptation of the photosynthetic bacteria Rhodospirillum rubrum and Arthrospira platensis under long-term axcenic cultivation in continuous photobioreactor artifical light conditions and radiation exposure.

Cultures of the test bacteria will be exposed to different doses of light and ionizing radiation. After exposure, their photosynthetic efficiency and growth kinetics will be investigated. In addition, their response will be profiled by full DNA, RNA and protein content analysis. Different molecular tools, such as DNA amplification and sequencing, RNA microarray, and protein mass spectrometry analysis, will be used. Biochemical analysis of the cellular content (amino acids, fatty acids, etc.) and metabolome (i.e. all the sugars, proteins and lipids) will be performed. This global approach will give a complete view on the changes in microbial physiology and metabolism due to continuous light and ionizing radiation. 

In addition, strains will be exposed in several cycles, to potentially enrich for genetic variants that are metabolically more stable or more beneficial (e.g. strains with increased photosynthetic activity or increased resistance to radiation). Genetic systems preventing or reporting certain genetic or metabolic rearrangements could be constructed. Marker genes or enzymes, highly responding to light or radiation stress, could be identified and exploited as valuable as biosensors.

This study will enhance our comprehension of microbial photosynthetic activity and genetic adaptation during long time culturing under chronic light in bioreactor conditions and radiation exposure. It will increase our knowledge on the correlation between light sensitivity and radiation resistance in bacteria. It will support the further development of the MELiSSA system, to support human life in space exploration. In addition, the results obtained may also lead to optimization of the industrial processes and applications on Earth of R. rubrum as animal food supplement or Arthrospira as human food supplement, or potentially as biofuel producers.