The pressing need for renewable resources has resulted in growing efforts to develop “sustainable cell factory” platforms for the production of valuable biocompounds. One extremely strong and tough biomaterial, made by nature, is spider silk. Spiders produce amazingly strong and lightweight threads called draglines that are made from silk proteins. The silk could be used to manufacture a number of useful materials, given a large enough quantity. But getting enough of the protein is difficult because only a small amount is produced by each spider.

In a new study, a research team from the RIKEN Center for Sustainable Resource Science (CSRS) produced spider silk using a marine photosynthetic purple bacterium, Rhodovulum sulfidophilum. This study could open a new era in which photosynthetic biofactories stably output the bulk of spider silk.

The work is published in Communications Biology in the paper, “A marine photosynthetic microbial cell factory as a platform for spider silk production.

“Spider silk has the potential to be used in the manufacture of high-performance and durable materials such as tear-resistant clothing, automobile parts, and aerospace components,” explained Choon Pin Foong, PhD, researcher at CSRS and first author on the study. “Its biocompatibility makes it safe for use in biomedical applications such as drug delivery systems, implant devices, and scaffolds for tissue engineering.” Because only a trace amount can be obtained from one spider, and because breeding large numbers of spiders is difficult, attempts have been made to produce artificial spider silk in a variety of species.

Photosynthetic microorganisms such as cyanobacteria, purple bacteria, and microalgae have attracted great interest as promising platforms for economical and sustainable production of bioenergy, biochemicals, and biopolymers.

The CSRS team focused on R. sulfidophilum which has qualities that make it ideal for establishing a sustainable biofactory. It grows in seawater, requires carbon dioxide and nitrogen in the atmosphere, and uses solar energy—all of which are abundant and inexhaustible. R. sulfidophilum grow by utilizing abundant and renewable nonfood bioresources such as seawater, sunlight, and gaseous CO2 and N2, thus making this photosynthetic microbial cell factory a promising green and sustainable production platform for proteins and biopolymers, including spider silks.

The authors explained that spun silk fibers are mainly composed of multiple types of major ampullate spidroin (MaSp) such as MaSp1 and MaSp2, and that MaSp is produced in the major ampullate gland of spiders.

In this work, the team demonstrates heterotrophic production of MaSp in R. sulfidophilum under both photoheterotrophic and photoautotrophic growth conditions.

The researchers genetically engineered the bacterium to produce MaSp1 protein, the main component of the Nephila spider dragline which is thought to play an important role in the strength of the spider silk. Optimization of the gene sequence that they inserted into the bacterium’s genome was able to maximize the amount of silk that could be produced. They also found that a simple recipe of artificial seawater, bicarbonate salt, nitrogen gas, yeast extract, and irradiation with near-infrared light, allows R. sulfidophilum to produce the silk protein efficiently. Further observations confirmed that the surface and internal structures of the fibers produced in the bacteria were very similar to those produced naturally by spiders.

“Our current study shows the initial proof of concept for producing spider silk in photosynthetic bacteria. We are now working to mass produce spider-silk dragline proteins at higher molecular weights in our photosynthetic system,” Numata said. “The photosynthetic microbial cell factories, which produce biobased and biodegradable materials via a carbon neutral bioprocess, could help us in accomplishing some of the Sustainable Development Goals (SDGs) adopted by the United Nations such as Goal #12 ‘Responsible Production and Consumption,’ and Goal #13 ‘Climate Action.’ Our results will help provide feasible solutions for energy, water, and food crises, solid waste problems, and global warming.”

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