What is Phosphate?
Phosphorus (P) is an essential macronutrient which all plants require. As phosphate (PO43-) , it is the backbone of life in DNA, RNA, cell membranes and ATP, therefore essential for energy transfer, protein activation and metabolic regulation [1] . For its pivotal role in photosynthesis and growth, phosphorus is a crucial element for food production [2] .
As a Fertilizer
The phosphate fertilizer that feeds the world is mined from finite, inequitably distributed phosphate rock reserves; Morocco and China alone make up 70% of global demand. However, reserves of the purest phosphate are set to deplete in 50 to 100 years, and sub-optimal, more expensive phosphate 100 to 200 years after that [3] . Supply aside, the crop yields needed to feed a hungry, growing global population will significantly increase phosphate demand [4] . Phosphate fertilizer prices have been historically high and volatile with grave consequences: an 800% price spike in 2008 against a backdrop of market turbulence lead to fertilizer and food riot. Even now, one sixth of the world’s farmers cannot afford inorganic fertilizers, capping crop yields [5].
A graph of the price in dollars per megatonne ($/mt) of phosphate fertilizer products from Janurary 1960 to February 2021.
DAP : Diammonium Phosphate; TSP : Trisodium Phosphate
As a Pollutant
Phosphate does not only fertilise crops, however. Alongside nitrogen , it is one of the main nutrients causing eutrophication of water bodies by promoting the growth of potentially toxic algal blooms that block light and are decomposed by respiring, oxygen-depleting bacteria [6] , reducing fish and invertebrate diversity [7] . As such, removal of phosphate is a crucial step in wastewater treatment [8, 9] . While the importance of reducing nutrient levels is widely recognised [10] , we identified potential in reutilizing the removed phosphorus for downstream agricultural application. Phosphate is commonly removed by chemical precipitation, but the co-precipitation of unwanted metal contaminants makes safe phosphate recycling difficult [11, 12] . A more effective method is needed.
Meet Phoby, our helpful (but less than subtilis) little Bacillus.
Closing the Loop
We recognised the pressing urgency of these dual problems with the understanding that left unchecked, our dwindling supply of phosphate would pose a civilizational challenge and our negligent nutrient pollution would scar aquatic ecosystems permanently. Our solution is PhoBac: an engineered Bacillus subtilis strain that can take up excess phosphate when grown in wastewater. Then, when reinserted into soil, it releases phosphate when it senses a crop under phosphorus stress, allowing phosphorus to flow back into the plant roots. This is an sustainable solution that will close the loop on phosphorus use, solving both our precarious supply and wasteful use of our nutrient wealth.