Fixing the Broken Phosphorus Cycle: Wastewater Remediation by Microalgal Polyphosphates

Abstract

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.

Responses of Chlamydomonas reinhardtii during the transition from P‐deficient to P‐sufficient growth (the P‐overplus response): The roles of the vacuolar transport chaperones and polyphosphate synthesis

“…2019b, Slocombe et al. 2020). This phenomenon reflects a P assimilation above a physiological threshold (0.2–1 %P) in microalgae (Powell et al.…”

“…However, there are opportunities to utilize organisms that cause eutrophication in P-polluted waters (Cordell et al 2009, Filippelli 2018, Yuan et al 2018 for recovering P during wastewater treatment Shilton 2014, Solovchenko et al 2019b). Indeed, microalgae have been shown to synthesize and accumulate dense granules of polyphosphate (polyP; Powell et al 2009, Dyhrman et al 2012, Aksoy et al 2014, Ota et al 2016, Mühlroth et al 2017, Sanz-luque et al 2020a), a feature that would make them ideal for P recovery from wastewater (Powell et al 2009, Mukherjee et al 2015, Solovchenko et al 2019a,b, Slocombe et al 2020) and the subsequent production of P-enriched biofertilizers (Mukherjee et al 2015, Solovchenko et al 2019b, Slocombe et al 2020.…”

“…Despite these observations concerning the potential functions of polyP in algae, the regulatory mechanisms that link changes of the concentration of P in the environment to polyP synthesis and breakdown are still poorly understood Aksoy 2015, Dyhrman 2016). This knowledge will be critical to better understand the P cycle in aquatic ecosystems and optimize polyP synthesis in microalgae for removing and recycling P from wastewater (Solovchenko et al 2019b, Slocombe et al 2020.…”

Redefining Water Treatment: Identification of WWTPs as an Earth System Problem and Circular Economic Eco-Bog System to Challenge It

“…To achieve circular economy in wastewater treatment, a new theoretical approach in harmony with nature and with maximum recycling and carbon capture is needed. WWTPs act like artificial biogeochemical cycles themselves and additional cyclic components may also be integrated [1]- [3]. This study identifies WWTPs as a component and a general problem of Earth system (Table I).…”

New developments in biological phosphorus accessibility and recovery approaches from soil and waste streams

“…On lab‐scale, microalgal polycultures in ponds were used to remove 50 to 90% of total P (out of 0.0032 ± 0.0006 g L –1 ) from the secondary effluent [83]. Commonly used algae for these applications are from the genera Chlamydomonas , Chlorella , Euglena , and Scenedesmus [31,84,85]. Microalgae can also be applied in MFC for P recovery [86].…”