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    Polyphosphate synthesis is an evolutionarily ancient phosphorus storage strategy in microalgae
    (Elsevier B.V., 2023-06-02) Cliff A; Guieysse B; Brown N; Lockhart P; Dubreucq E; Plouviez M
    To assess the ubiquity of the potential for inorganic polyphosphate (polyP) synthesis in microalgae, we searched databases for algal homologues to the polyP polymerase VTC4 of Chlamydomonas reinhardtii. Homologues of this protein were found within >40 species of microalgae known to inhabit marine, freshwater, and terrestrial environments. Phylogenetic analysis demonstrated that these proteins were organized into clades aligning with their taxonomic relationships. These similarities and evolutionary relationships suggest that polyP synthesis represents an ancient ability that has evolved with species as the microalgal lineage has spread out over time. Based on these results and prior knowledge on P metabolism, C. reinhardtii, Chlorella vulgaris, Desmodesmus cf. armatus, Gonium pectorale, and Microcystis aeruginosa were further tested in bioassays known to trigger the synthesis of polyP within dense granules, by addition of P following a period of P depletion. While the cellular P content of C. reinhardtii, G. pectorale, M. aeruginosa, and D. cf. armatus increased to similar maxima, ranging from 2.6 ± 0.5 % to 3.6 ± 1.3 % 24 h after P repletion, P content only reached 1.2 ± 0.2 % in C. vulgaris, suggesting a lesser ability to accumulate polyP than the strains of the other species. Models of predicted VTC4 proteins were generated from the four eukaryotic species tested and showed that the microalgae share the conserved VTC catalytic core and SPX phosphate-sensing domains found in the yeast VTC4 proteins. This confirms the role of microalgal VTC4 as polyP polymerase and suggests a similar regulation of VTC4 proteins to the one described in yeast. Further work is now needed to uncover the assembly of the microalgal VTC complex and its regulation. A deeper study of the microalgal VTC structure could also help to understand whether differences in VTC structures can explain observed differences in P accumulation kinetics.
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    Biological phosphorus removal by microalgae in waste stabilisation ponds : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Environmental Engineering at Massey University, Palmerston North, New Zealand
    (Massey University, 2009) Powell, Nicola
    Waste stabilisation ponds (WSP) are an important wastewater treatment technology used by thousands of communities around the world. Unfortunately, phosphorus removal in WSP is generally low and inconsistent. The aim of this work was to investigate biological phosphorus removal by microalgae in WSP. Luxury uptake of phosphorus, which is the accumulation of polyphosphate, is known to occur in microalgae in natural systems such as lakes; however, this mechanism has not previously been studied under WSP conditions. Three methods were used in the laboratory to investigate luxury uptake and it was shown for the first time that luxury uptake of phosphorus can occur in microalgae under typical WSP conditions. Acid-insoluble polyphosphate (AISP) is a form of phosphorus storage and acid soluble polyphosphate (ASP) is used for synthesis of cellular constituents. However, the findings of this thesis indicate that ASP may also act as a form of short term storage. The environmental factors influencing luxury uptake were investigated using laboratory experiments conducted under controlled conditions. The key environmental factors were the phosphate concentration in the wastewater, light intensity and temperature. A higher phosphate concentration increased the amount of ASP accumulation and also resulted in AISP being stored within the cells instead of being consumed for growth. Higher light intensity increased ASP accumulation, but as a consequence of elevated growth, the ASP was rapidly consumed. Temperature influenced the rate of AISP accumulation and little if any was accumulated at low temperatures. The fate of polyphosphate in the sludge layer was also studied and it was shown that polyphosphate was degraded resulting in phosphate release. Therefore, to maximise phosphorus removal the microalgae needs to be harvested. Field work showed that at times the biomass contained almost four times the amount of phosphorus required for growth which confirms that luxury uptake does indeed occur in full-scale WSP. To improve phosphorus removal in WSP both luxury uptake and the biomass concentration need to be maximised simultaneously. With this new understanding of biological phosphorus removal in WSP and the key environmental factors required it may be possible to develop a new phosphorus removal process utilising luxury uptake by microalgae.