Comment: Closing phosphorus cycle from natural waters: re-capturing phosphorus through an integrated water-energy-food strategy.

Phosphorus (P) reserve, largely derived from phosphate rock, microalgae utilization technologies may greatly accelerate is essential for crop growth to support the growing world population. However, a significant proportion of phosphorus used as a fertilizer runs into natural waters, causing eutrophication and ecological damage. Moreover, most P in the food is eventually discharged as waste after being digested by human and animals. Thus, industrial activities have created a one-way flow of non-renewable P from rocks to farms to lakes, rivers and oceans. It has been suggested that P reserve can only support global food security for up to 125 years (Gilbert, 2009). Due to technological and economic reasons, existing wastewater treatment and environmental stewardship laws permit a relatively high level of P (~0.3 mg/L) to be discharged into natural environment, which will cause eutrophication. Phosphorus recovery has become an important issue for agricultural sustainability and aquatic ecology. Existing technologies for re-capturing P from concentrated wastes still remain unsustainable due to high chemical and energy costs (Zhou et al., 2017; Zhang et al., 2017). We predict that in the near future we have no option but to find cost-effective and sustainable methods to re-capture P from the natural waters where it currently ends up. Harmful algal bloom has become an important issue in eutrophic natural waters, which represent a great threat to public health and cause ecological degradation (Conley et al. 2009). However, microalgae are exceptionally effective in turning low levels of dissolved P into concentrated particulate P, and at very much faster rates than the geological mineralisation processes that created our P reserves. Recent developments have made it possible to remove large scale harmful algal blooms through flocculation using ecologically friendly geo-engineeredmaterials (Pan et al. 2006, 2011; Li and Pan 2013; Li et al. 2015; Shi et al. 2016; Yuan et al. 2016). This principlemakes it possible to harvest algal biomass and take P out of natural waters cost-effectively by combining with conventional air floating technologies. We predict that there will be a tipping point to make it feasible to re-capture from natural waters when the supplying of food, water and energy to the growing global population becomes unsustainable, and