Cyanophycin Granule Polypeptide: a Neglected High Value-Added Biopolymer, Synthesized in Activated Sludge on a Large Scale

Abstract

Recovery of microbial synthetic polymers with high economic value and market demand in activated sludge has attracted extensive attention. This work analyzed the synthesis of cyanophycin granule peptide (CGP) in activated sludge and its adsorption capacity for heavy metals and dyes. The distribution and expression of synthetic genes for eight biopolymers in two wastewater treatment plants (WWTPs) were analyzed by metagenomics and metatranscriptomics. The results indicate that the abundance and expression level of CGP synthase (cphA) are similar to those of polyhydroxyalkanoate polymerase, implying high synthesis of CGP in activated sludges. CGP in activated sludge is mainly polymerized from aspartic acid and arginine, and its secondary structure is mainly β-sheet. The crude yields of CGP are as high as 104 ± 26 and 76 ± 13 mg/g dry sludge in winter and in summer, respectively, comparable to those of polyhydroxyalkanoate and alginate. CGP has a stronger adsorption capacity for anionic pollutants (Cr (VI) and methyl orange) than for cationic pollutants because it is rich in guanidine groups. This study highlights prospects for recovery and application of CGP from WWTPs. IMPORTANCE The conversion of organic pollutants into bioresources by activated sludge can reduce the carbon dioxide emission of wastewater treatment plants. Identification of new high value-added biopolymers produced by activated sludge is beneficial to recover bioresources. Cyanophycin granule polypeptide (CGP), first discovered in cyanobacteria, has unique chemical and material properties suitable for industrial food, medicine, cosmetics, water treatment, and agriculture applications. Here, we revealed for the first time that activated sludge has a remarkable ability to produce CGP. These findings could further facilitate the conversion of wastewater treatment plants into resource recycling plants.

Keywords: activated sludge; biopolymer; bioresource recovery; cyanophycin; polyhydroxyalkanoate.

FIG 1

Abundances and expression of synthetic genes and proposed synthesis pathway of eight biopolymers in activated sludges based on metagenomics and metatranscriptomics: CGP (a), γ-PGA (b), PHA (c), alginate (d), xanthan (e), HA (f), cellulose (g), and succinoglycan (h).

FIG 2

(a) Co-occurrence network between cphA and potential hosts. (b) Relative abundance of cphA potential hosts at genus levels. (Node size represents the degree of connection, and edge thickness represents the size of the correlation; P < 0.05).

FIG 3

Identification and characterization of CGP from four activated sludge samples. (a) Image of CGP suspension at pH 7.5. (b) Image of CGP gels. (c) Image of Sakaguchi reaction of CGP. (d) HPLC analysis of Asp and Arg. (e) HPLC analysis of CGP hydrolysate. (f) Positive Ion ESI-MS analysis of Arg and Asp from CGP hydrolysate. (g) UV–vis absorption spectra of CGP. (h) FTIR of CGP. (i) Curve-fitted amide I region (1,700 to 1,600 cm⁻¹) of CGP FTIR.

FIG 4

Crude yields of the five biopolymers extracted from activated sludges at winter (a) and summer (b).

FIG 5

Adsorption dynamics of Cr₂O₇²⁻ and methyl orange by CGP from different activated sludge samples. (a) The adsorption of Cr₂O₇²⁻ by CGP at pH 5. (b) The optimized binding configuration of Cr₂O₇²⁻ and amidogen group with Cr₂O₇²⁻. (c) The adsorption of methyl orange by CGP at pH 1 and pH 5.