Phosphorus (P), in the form of phosphate derived from either inorganic (P i) or organic (P o) forms is an essential macronutrient for all life. P undergoes a biogeochemical cycle within the environment, but anthropogenic redistribution through inefficient agricultural practice and inadequate nutrient recovery at wastewater treatment works have resulted in a sustained transfer of P from rock deposits to land and aquatic environments. Our present and near future supply of P is primarily mined from rock P reserves in a limited number of geographical regions. To help ensure that this resource is adequate for humanity's food security, an energy-efficient means of recovering P from waste and recycling it for agriculture is required. This will also help to address excess discharge to water bodies and the resulting eutrophication. Microalgae possess the advantage of polymeric inorganic polyphosphate (PolyP) storage which can potentially operate simultaneously with remediation of waste nitrogen and phosphorus streams and flue gases (CO 2 , SO x , and NO x). Having high productivity in photoautotrophic, mixotrophic or heterotrophic growth modes, they can be harnessed in wastewater remediation strategies for biofuel production either directly (biodiesel) or in conjunction with anaerobic digestion (biogas) or dark fermentation (biohydrogen). Regulation of algal P uptake, storage, and mobilization is intertwined with the cellular status of other macronutrients (e.g., nitrogen and sulphur) in addition to the manufacture of other storage products (e.g., carbohydrate and lipids) or macromolecules (e.g., cell wall). A greater understanding of controlling factors in this complex interaction is required to facilitate and improve P control, recovery, and reuse from waste streams. The best understood algal genetic model is Chlamydomonas reinhardtii in terms of utility and shared resources. It also displays mixotrophic growth and advantageously, species of this genus are often found growing in wastewater treatment plants. In this review, we focus primarily on the molecular and genetic aspects of PolyP production or turnover and place this knowledge in the context of wastewater remediation and highlight developments and challenges in this field.

Remediation using micro-algae offers an attractive solution to environmental phosphate (Pi) pollution. However, for maximum efficiency, pre-conditioning of algae to induce luxury phosphorus (P) uptake is needed. Here we show that natural pre-conditioning can be mimicked through over-expression of a single gene, the global regulator PSR1 (Myb transcription factor: Phosphate Starvation Response 1), raising P levels to 8% dry cell weight from 2% in control. Complete removal of Pi occurred in log phase, unlike the control. This was associated with increases in PolyP granule size and uptake of Mg2+, the principal counterion. Hyper-accumulation of P depended on a feed-forward mechanism, where a small set of Class I genes were activated despite abundant external Pi levels. This drove a reduction in external Pi levels, permitting more genes to be expressed (Class II), leading to more Pi -uptake. These discoveries enable a bio-circular approach of recycling nutrients from wastewater back to agriculture.

Remediation using micro-algae offers an attractive solution to environmental phosphate (PO43-) pollution. However, for maximum efficiency, pre-conditioning of algae to induce ‘luxury phosphorus (P) uptake’ is needed. To replicate this process, we targeted the global regulator PSR1 (Myb transcription factor: Phosphate Starvation Response 1) for over-expression in algae. Manipulating a single gene (PSR1) drove uptake of both PO43- and a Mg2+ counter-ion leading to increased PolyP granule size, raising P levels 4-fold to 8% dry cell weight, and accelerated removal of PO43- from the medium. Examination of the gene expression profile showed that the P-starvation response was mimicked under P-replete conditions, switching on luxury uptake. Hyper-accumulation of P depended on a feed-forward mechanism, where a small set of ‘Class I’ P-transporter genes were activated despite abundant external PO43- levels. The transporters drove a reduction in external PO43- levels, permitting more genes to be expressed (Class II), leading to more P-uptake. Our data pointed toward a PSR1-independent mechanism for detection of external PO43- which suppressed Class II genes. This model provided a plausible mechanism for P-overplus where prior P-starvation elevates PSR1 and on P-resupply causes luxury P-uptake. This is because the Class I genes, which include P-transporter genes, are not suppressed by the excess PO43-. Taken together, these discoveries facilitate a bio-circular approach of recycling nutrients from wastewater back to agriculture.