Understanding the biochemical characteristics of struvite bio-mineralising microorganisms and their future in nutrient recovery
Introduction
Biological struvite (bio-struvite) has been identified as a route to recover phosphorus (P) from municipal wastewater streams (Soares et al., 2014). Microorganisms play an important role in struvite bio-mineralisation through different metabolic activities (Sinha et al., 2014) and by precipitation of specific structures or substances for microbial processes (Arias et al., 2017). Five microbial strains, Halobacterium salinarum, Bacillus pumilus, Brevibacterium antiquum, Myxococcus xanthus, and Idiomarina loihiensis, have been reported to be involved in biologically driven struvite formation in liquid streams (Table 1, González-Muñoz et al., 2008; Soares et al., 2014). M. xanthus, I. loihiensis and H. salinarum were reported to produce extracellular polymeric substances (EPS), which may fix cations and contribute to mineral heterogeneous nucleation and precipitation (González-Muñoz et al., 2010, 2008; Merroun et al., 2003). Most of the selected microorganisms can use O2 as an electron acceptor (Table 1). H. salinarum has been reported to be able to use dimethyl sulfoxide (DMSO) et al.,as an electron acceptor under anaerobic conditions, and use photophosphorylation in the presence of light (Table 1).
B. pumilus and M. xanthus can use carbohydrates as a carbon source but this does not apply to B. antiquum and H. salinarum According to the literature, all the selected microorganisms can use protein/amino acids as a carbon source (Robinson, 2014; Trujillo and Goodfellow, 2015). The utilisation of organic carbon sources depended on enzyme production, and the rates of enzyme-catalysed reactions optimally performed under appropriate temperature, pH and salinity ranges (Silva et al., 2016). The selected microbial strains have been reported to grow in pHs from 5.5 to 9, and temperatures ranging from 20 to 45 °C (Table 1). The halotolerant microorganisms B. antiquum, H. salinarum and I. loihiensis can live in environments containing high NaCl (Gavrish et al., 2004; González-Muñoz et al., 2008; Mesbah and Wiegel, 2005), particularly H. salinarum, which can survive at extremely high NaCl concentrations (17.4–30.16%, Table 1).
Although some of the biochemical properties and growth conditions of selected microorganism have been reported in the literature, some of the values are controversial and further verification and characterisation is required. Statistical experimental design is recognised as an approach widely used for parameter screening in optimisation studies (Massey et al., 2009). By using such design, Simoes et al. (2017) investigated the significant factors required for B. antiquum growth, and maximised the growth rates in wastewater streams by screening and optimising a number of factors.
This study aims to investigate the biochemical properties of the selected microorganisms owing to their capability to produce struvite through bio-mineralisation. For industrial exploitation of microorganisms, the investigation of biochemical characterisation is critical for appropriate processes design and meeting microbial requirements by optimising reactor operational conditions. The temperature, pH, electron acceptor, and organic carbon source are among the most important environmental parameters affecting microbial growth and organic substance synthesis (Silva et al., 2016). Knowledge of such parameters will allow the design of reactors/processes and operational conditions to ensure proliferation of the selected microorganisms, and even out-compete other microorganisms in mixed cultures, for eventual enhanced P recovery by struvite from waste streams.
Section snippets
Microbial strains and culture solution
Five microbial strains were used in this study: H. salinarum & B. antiquum (DSM 671 & DSM 21545, German Resource Centre for Biological Material, Germany), B. pumilus (GB43, LGC Standards, Middlesex, UK), M. xanthus & I. loihiensis (CECT 422 & MAH1/CECT 5996, Spanish Type Culture Collection, University of Valencia, Paterna, Spain). The microorganism were grown in synthetic B41 solution comprising 4 g/L yeast extract, 2 g/L MgSO4·7H2O and 2 g/L K2HPO4 (Da Silva et al., 2000). The solution was
Microbial properties and enzyme production
B. pumilus and B. antiquum were identified as Gram-positive and M. xanthus, H. salinarum and I. loihiensis as Gram-negative, which agrees with previously published information (Table 1). In particular, B. pumilus formed crusted two-cell clusters or tetrads in B41 media, which were not observed in the other four microbial cultures. Such cell structures did not grow in size but had the potential to aggregate together or onto the crystal surface. Similar cell structures were observed as
Conclusion
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Proteins/amino acids were the preferred organic carbon sources for the five microorganisms investigated.
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B. pumilus, M. xanthus, H. salinarum and B. antiquum were able to produce urease.
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I. loihiensis was found to be a facultative anaerobe able to use O2 and NO3–N as an electron acceptor.
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The preferred temperature for all selected microorganisms was within the mesophilic range (22–34 °C); most microorganisms preferred a neutral pH and NaCl concentrations less than 1% w/v, whereas I. loihiensis
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