Understanding variability in algal solid-liquid separation process outcomes by manipulating extracellular protein-carbohydrate interactions.

Coagulation-flocculation followed by sedimentation or dissolved air flotation (DAF) are processes routinely used for separating microalgae from water; however, during algae separation then can exhibit inconsistent separation, high coagulant demand, and high operating cost. To circumvent these problems, previous studies reported the development of a novel DAF process in which bubbles were modified instead of particles. While this process was shown to be sustainable and inexpensive, the problem of inconsistent algal separation across species remained. Recent research has suggested that this could be due to the varying concentration and character of algal-derived proteins and carbohydrates within the extracellular organic matter (EOM) and their associated interactions. This hypothesis is tested in the current study using the novel modified-bubble DAF process, which has been highly susceptible to EOM protein and carbohydrate concentrations and character. Biomolecular additives (commercially available proteins and carbohydrates, and algal-extracted proteins) of widely differing molecular weight (MW) and charge were dosed in varying proportions into samples containing either Chlorella vulgaris CS-42/7, Microcystis aeruginosa CS-564/01, or Microcystis aeruginosa CS-555/1 after removing the intrinsic EOM. These cell-rich suspensions were then subject to flotation using cationic bubbles modified with poly(diallyldimethylammonium chloride) (PDADMAC). When additives were dosed independently, separation increased from <5% to up to 62%. The maximum separation was obtained when the dose was double the respective biopolymer concentration measured in the intrinsic EOM for the equivalent species, and, in the case of protein additives, when MW and charge were >50 kDa, and >0.5 meq·g-1, respectively, irrespective of the species tested. When evaluating steric- and charge-based protein-carbohydrate interactions on cell separation by simultaneously dosing high MW and high charge protein- and carbohydrate-additives, enhanced separation of up to 79% was achieved. It is suggested that enhanced cell separation is achieved due to proteins and carbohydrates bridging with cells and forming protein-carbohydrate-cell suprastructures in the presence of a flocculant, e.g. PDADMAC, and this only occurs when the intrinsic EOM comprises proteins and carbohydrates that have high MW (>25 kDa) and charge (>0.2 meq·g-1), and interactions with each other and with the cell surface.

Detailed algal extracellular carbohydrate-protein characterisation lends insight into algal solid-liquid separation process outcomes.

The effectiveness of algal solid-liquid separation processes has been impacted by the strong influence of algal extracellular organic matter (EOM), where the composition of proteins and carbohydrates and their associated interactions have been implicated. However, despite this, no studies have analysed the detailed protein and carbohydrate composition in EOM in relation to their impacts on separation. Hence, the aim of this study was to explore the relationship between the variety of carbohydrates and proteins present in the EOM of select algal and cyanobacterial samples and the associated separation performance to better understand the influence of specific biopolymers. The protein and carbohydrate composition of the EOM of three species - Microcystis aeruginosa CS-555/1, Chlorella vulgaris CS-42/7 and Microcystis aeruginosa CS-564/01, previously observed to result in variable treatment performance were investigated. The carbohydrates were analysed via high-performance anion-exchange chromatography (HPAEC) with pulsed amperometric detection (PAD) while the proteins were analysed using sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) combined with liquid chromatography-mass spectrometry (LC-MS). Ten unique monosaccharides were identified; of these, the greatest proportion of charged uronic acid carbohydrates were present in the EOM of M. aeruginosa CS-564/01. The protein profiling revealed that M. aeruginosa CS-564/01 had a greater proportion and concentration of proteins >75 kDa when compared to M. aeruginosa CS-555/1 or C. vulgaris CS-42/7. It was determined that three serine- and two threonine-based proteins, detected in greater concentrations in M. aeruginosa CS-564/01 than CS-555/1, could covalently interact with carbohydrates (OHenderson et al., 2010a, 2010b-linked glycosylation). These proteins have the ability to form numerous localised networks with carbohydrates and cells in the presence of coagulant molecules, thereby providing a good hypothesis to explain the excellent treatment performance observed for M. aeruginosa CS-564/01 previously. It is proposed that the uronic acids in M. aeruginosa CS-564/01 could interact with proteins via glycosylation, explaining why the coagulant demand for this strain remained low despite the high charged carbohydrate concentration. Overall, it is proposed that process performance could be impacted by: (a) physicochemical characteristics and (b) carbohydrate-protein interactions.