Researchers at the Lawrence Berkeley National Laboratory (Berkeley Lab), UC Irvine and the University of Illinois Urbana-Champaign (UIUC) have used microbes to turn human urine into hydroxyapatite. The team published their work in Nature Communications.
The scientists genetically modified yeast to take the phosphorus and calcium from urine and create hydroxyapatite, a mineral naturally produced by humans and animals to build bones. Manufactured hydroxyapatite is used in surgery and dentistry. It is also strong and tough enough to be used as a building material or a plastic replacement.
The osteoyeast cell where the outline of the vacuole, the cellular organelle where calcium is accumulated forming hydroxyapatite, is fluorescing red. The calcium inside the vacuole is fluorescing green. Credit: Berkeley Lab
Creating “osteoyeast”
The yeast created by the research team is a strain of Saccharomyces boulardii and is closely related to the species used to brew beer and make bread, and the best-known yeast-based probiotic. This type of yeast takes minerals from its environment and stores them in a membrane compartment. This activity is similar to that of osteoblasts, specialized animal cells that make hydroxyapatite and form bone.
Due to this similarity, “just mild tweaking was sufficient to convert the yeast into a cell factory for hydroxyapatite,” said Yasuo Yoshikuni, head of the DNA Synthesis Science Program at the Joint Genome Institute (JGI), who co-authored the study.
The modified yeast, which they call “osteoyeast”, mimics osteoblasts, which are difficult and expensive to culture. Osteoyeast also maintains the low-maintenance lifestyle of yeast, making it a cost-efficient alternative.
Reduce, reuse, “pee-cycle”
Using urine as a mineral source for ammonia and phosphate, an emerging trend, is sometimes called “pee-cycling.” Inspired by this trend, the research team saw an opportunity to use urine as a mineral source for their osteoyeast.
According to Yoshikuni, ammonia and phosphate are cheap enough that pee-cycling has not become a widespread practice. However, the osteoyeast can take the phosphorus and calcium in urine and create hydroxyapatite, which is more expensive. In the process, the yeast also collects ammonia.
“Today, we use about 1% of the world’s energy to make fertilizers from nitrogen gas,” Yoshikuni said. “If we’re able to produce both hydroxyapatite and make nitrogen fertilizer from the ammonia, we could potentially replace a significant portion of total demand of nitrogen, saving energy while also dramatically reducing the costs at wastewater facilities.”
In 2017, the U.S. Department of Energy estimated that municipal wastewater treatment plants consumed more than 30 terawatt hours per year of electricity, costing about $2 billion per year. Electricity costs accounted for 25-40% of a wastewater treatment plant’s annual budget.
Jeremy Guest, Sustainable Design Lead for the DOE Center for Advanced Bioenergy and Bioproducts Innovation at UIUC, and Xinyi (Joy) Zhang, a research scientist at UIUC, performed a technoeconomic analysis that simulated hydroxyapatite production systems serving a city.
They estimated that it would cost approximately $19 to make one kilogram of hydroxyapatite, which could be sold for $50-200 in the U.S. The analysis indicated that the system could generate a profit of about $1.4 million per year.
