Purple Bacteria: A Sustainable Protein Source and Beyond

In recent years, there has been a growing interest in new alternative protein sources for human and animal nutrition that will reduce the consumption of animal protein. By 2050, the world is projected to produce 1.250 million tonnes of meat and dairy annually to meet the current global demand for these animal products (Ritala et al., 2017). In addition to plant-based foods, one of the possibilities of using microorganisms as a source of protein is the so-called single cell protein (SCP). These microorganisms, rich in proteins, vitamins, and trace elements, hold promise as a protein source for both animal and human nutrition.

A completely new direction is the biotechnological use of phototrophic purple bacteria (PPB), which include sulphur and non-sulphur bacteria, especially from the group of proteobacteria (Cao et al., 2020). Typical representatives are Rhodobacter spheroides, Rhodobacter capsulatus, Rhodopseudomonas palustris or Afifela marina (Zhang et al., 2019; Spanoghe et al., 2021). PPB are one of the most thermodynamically efficient organisms on Earth, and this high adaptability of their metabolism makes them particularly suitable for the development of biotechnological applications for the circular economy (Puyol et al., 2017). The average SCP content is 40–60%, with the highest production reaching 90%. These proteins are nutritionally very valuable due to their essential amino acid content (Peng et al., 2022). Moreover, PPB biomass contains much lower amounts of nucleic acids compared to other producing microorganisms (Wada et al., 2022). Common nucleic acid concentrations for bacterial SCP are reported to be between 10–16%, fungal SCP up to 10%, and only around 5% for SCP from algae and PPB (Garimella et al., 2017; Bratosin et al. 2012).

Resource recovery from bio-waste treatment is currently a very active area of research and PPBs play an important role in this socio-economic revolution, while their potential is not yet fully exploited. In the last decade, considerable efforts have been made to study PPBs in detail, focusing on the production of microbial proteins, bioplastics, and culture systems have been introduced to produce biohydrogen as well as other bioactive compounds such as PHAs, pigments such as carotenoids and bacteriochlorophylls, 5-aminolevulinic acid, coenzyme Q10, and others (Peng et al., 2022). The adaptability of PPB is currently being applied in the development of new biotechnological processes to treat and convert wastewater or agro-industrial waste into clean water and potentially more valuable products (Capson-Tojo et al., 2020). In summary, purple bacteria play a crucial role as phototrophs, eliminating toxic hydrogen sulphide and converting organic matter through their autotrophic metabolism, especially non-fermentable organic compounds. (Madigan and Jung, 2009).

To produce PPB, whey can be used as an inexpensive source of nutritionally important substances usable by microorganisms as a fermentation medium. Whey is a by-product of the dairy industry, which represents a significant source of environmental pollution due to its large quantity and high organic load (Koutinas et al., 2009). Therefore, in the last decades, considerable efforts have been made to find new ways of utilizing whey as a rich source of valuable nutrients in human and animal diets. Although the biomass of purple bacteria represents a promising source of protein, there is still insufficient information on their properties and thus on the safety of their use in human nutrition. Defining the functional properties and health effects of PPB biomass is still completely lacking or only marginally mentioned in the available literature, e.g., the potential probiotic effect of selected purple bacteria species has been suggested by few studies (Chumpol et al., 2017; Zhang et al., 2019), however, this area requires more in-depth research, as well as the study of other biomass properties such as antioxidant, immunomodulatory, prebiotic, and antimicrobial properties. However, scientific data regarding their safety (e.g. cytotoxicity, resistance to ATB, etc.) will be necessary for future use in human nutrition, using different in vitro and in vivo models.

Bratosin, B.C., Darjan, S., & Vodnar, D.C. (2021). Single Cell Protein: A Potential Substitute in Human and Animal Nutrition. Sustainability, 13(17), 9284.

Cao, K., Zhi, R., & Zhang, G. (2020). Photosynthetic bacteria wastewater treatment with the production of value-added products: A review. Bioresource Technology, 299, 122648.

Capson-Tojo, G., Batstone, D.J., Grassino, M., Vlaeminck, S.E., Puyol, D., Verstraete, W., & Lema, J.M. (2020). Purple phototrophic bacteria for resource recovery: Challenges and opportunities. Biotechnology Advances, 107567.

Garimella, S., Kudle, K.R., Kasoju, A., & Merugu, R. (2017). Current Status on Single Cell Protein (SCP) Production from Photosynthetic Purple Non Sulphur Bacteria. Journal of Chemical and Pharmaceutical Sciences, 10(2), 915–922. ISSN: 0974–2115.

Chumpol, S., Kantachote, D., Nitoda, T., & Kanzaki, H. (2017). The roles of probiotic purple nonsulfur bacteria to control water quality and prevent acute hepatopancreatic necrosis disease (AHPND) for enhancement growth with higher survival in white shrimp (Litopenaeus vannamei) during cultivation. Aquaculture, 473, 327–336.

Koutinas, A.A., Papapostolou, H., Dimitrellou, D., Kopsahelis, N., Katechaki, E., Bekatorou, A., & Bosnea, L.A. (2009). Whey valorisation: A complete and novel technology development for dairy industry starter culture production. Bioresource Technology, 100, 3734–3739.

Madigan, M.T., & Jung, D.O. (2009). An overview of purple bacteria: systematics, physiology, and habitats. In: Hunter, C.N.; Daldal, F.; Thurnauer, M.C.; Beatty, J.T (Eds.), The purple phototrophic bacteria. Advances in Photosynthesis and Respiration, 28, Springer, Dordrecht.

Peng, L., Lou, W., Xu, Y., Yu, S., Liang, Ch., Alloul, A., Song, K., & Vlaeminck, S.E. (2022). Regulating light, oxygen and volatile fatty acids to boost the productivity of purple bacteria biomass, protein and co-enzyme Q10. Science of the Total Environment, 822, 20, 153489.

Puyol, D., Batstone, D.J., Hulsen, T., Astals, S., Peces, M., & Kromer, J.O. (2017). Resource recovery from wastewater by biological technologies: opportunities, challenges, and prospects. Frontiers in Microbiology, 7, 2106.

Ritala, A., Hakkinen, S.T., Toivari, M., & Wiebe, G. (2017). Single Cell protein — State of the art, Industrial Landscape and patents 2001–2016. Frontiers in Microbiology.

Wada, O.Z., Vincent, A.S., & Mackey, H.R. (2022). Single-cell protein production from purple non-sulphur bacteria-based wastewater treatment. Rev Environ Sci Biotechnol, 21, 931–956.

Zhang, P., Sun, F., Cheng, X., Li, X., Mu, H., Wang, S., Geng, H., & Duan, J. (2019). Preparation and biological activities of an extracellular polysaccharide from Rhodopseudomonas palustris. International Journal of Biological Macromolecules, 15(131), 933–940.