Research articleEffects of gibberellin on the activity of anammox bacteria
Introduction
Anammox is a novel biological process of converting NH4+ and NO2− into N2 under anaerobic conditions (Strous et al., 1999). Compared to conventional nitrification and denitrification process, it produces less sludge, CO2 and N2O with no need of oxygen and external carbon source, thus saving operational costs (Zekker et al., 2012a; Tenno et al., 2016, 2017, 2018). Therefore, the research of anammox has received increasing attention (Kimura et al., 2011), and there were more than 100 full-scale anammox plants put into operation worldwide (Lackner et al., 2014). In complete WWTP partial nitrification can be used with respective oxygen consumption, nevertheless deammonification occurs with denitrification in real conditions and can be applied, such as partial nitritation-anammox process, which also reduced consumption and lower organic carbon demand (Rikmann et al., 2018; Zekker et al., 2012a). Although anammox bacteria are widely distributed in natural ecosystems (Zhu et al., 2013), the cultivation of anammox bacteria is still facing challenges in practical applications. The major obstacle is the slow growth of anammox bacteria whose doubling-time reaches nearly 11 days (Strous et al., 1998), which largely extends the start-up time of anammox process (Joss et al., 2009). Besides, anammox bacteria are sensitive to external environment and usually inactivated by many factors, such as substrate, DO, pH, temperature, and illumination (Raudkivi et al., 2017; Zekker et al., 2015; Zhang et al., 2016; Jin et al., 2012).
In order to shorten the start-up time of anammox reactors and enrich anammox bacteria, many researchers have employed various methods to retain anammox biomass in the system or to accelerate the activity of anammox bacteria (Ibrahim et al., 2016). These methods mainly include the design of reactors (Jin et al., 2008), the application of carriers (Tomar and Gupta, 2016), the addition of anaerobic granular sludge (Tomar et al., 2015), and other physical and chemical methods (Yin et al., 2015a). The up-flow anaerobic sludge blanket (UASB) was found to be a suitable anammox reactor, where the maximum NRR reached 74.3–76.7 kg N m−3 day−1 (Tang et al., 2011). Other types of anammox treatment systems could be noted as a promising wastewater treatment technology, For example, moving bed biofilm systems, which used buoyant carrier media as a biofilm growth support material and retained anammox bacteria effectively (Zekker et al., 2012b,c). Also, some municipal water treatment experiments were conducted at low temperature in biofilms as comparison at −5, 10 and 15 °C, which could be used also as a source of nutrient removal tank (Daija et al., 2016). In addition, some kinds of metal ions have been verified to be useful for enhancing anammox bacterial activity, such as Fe(II), Fe(III), Cu(II), and Ni(II) (Liu and Horn, 2012; Chen et al., 2014). Yin et al. (2015a) discovered that electric field could enhance the activity of anammox bacteria and increase the crude enzyme activities and cell quantities. Duan et al. (2011) indicated that the maximum NRR could be increased by 25.5% when the ultrasound intensity was 0.3w cm−2. Wang et al. (2013) and Yin et al. (2015b, 2016) enhanced the activity of anammox bacteria by adding graphene oxide in system, with an increase of the maximum NRR of 27.4%.
GA, a kind of plant hormone, can regulate the growth of plant and impact various developmental stages, including stem elongation, germination, seed dormancy, flowering, sex expression, aging leaves and suppression of fruits. The most prominent role of GA is to accelerate cell elongation and promote cell division and expansion (Cui et al., 2000). However, few studies have addressed the effects of GA on the bacteria. Wu (2012) showed that GA could accelerate the growth and activity of photosynthetic bacteria by promoting cell metabolism, increasing nutrient uptake, and strengthening cell viability. Liu et al. (2013) have shown that GA has a significant role in promoting the growth of rhizobia. Furthermore, no studies have ever considered the effect of GA on the anammox bacteria so far.
Hence, the present study is aimed at investigating the short-term impacts of GA on anammox bacterial activity and evaluating the changes in the properties of anammox granular sludge under the stimulation of GA. Additionally, the effect of different concentrations of GA on the microbial community structure of anammox bacteria was examined. This research may provide a better understanding of the role of GA in the anammox process, which might explore a good way for improving the activity and growth of anammox bacteria, as well as increasing the potential for solving the problem of the lack of anammox biomass.
Section snippets
Seed sludge and synthetic wastewater properties
The anammox sludge was taken out from the stably laboratory-operated SBR reactor whose sludge concentration was 4300 mg L−1, nitrogen loading rate (NLR) was 120 g N·m−3·day−1 and NRR was 39.6 mg N·g−1 VSS·day−1 after 200 days of operation. In addition, the temperature and the rotate speed of the SBR reactor were controlled at 30 ± 1 °C and 140 rpm, respectively. The anammox sludge obtained was washed in phosphate buffer (0.14 g L−1 KH2PO4 and 0.75 g L−1 K2HPO4) to eliminate the influence of
The nitrogen removal performance
As the aim of this present study was to enhance the activity of anammox bacteria in a very short time (within a doubling time) to serve as a reference for further study on the start-up of anammox process, the long-term effect of GA on anammox bacteria was not investigated. Their concentrations of GA were 0 mg L−1 (as control group), 0.1 mg L−1, 0.5 mg L−1, 1.0 mg L−1, and 1.5 mg L−1 respectively. The results were illustrated in Fig. 2 (a–d). Compared with the control group, there was an obvious
Conclusions
The activity and biomass of anammox bacteria were enhanced by adding GA in the short-term experiment. When adding 1.0 mg L−1 GA, the anammox activity was improved, and the NRR increased by 34%. The maximum total EPS increased by 28.6% and the maximum abundance of anammox bacteria increased by 110%. In addition, this study also showed that the most dominant anammox bacteria were identified as the Candidatus Brocadia fulgid, and the Candidatus Brocadia anammoxidans were naturally eliminated in
Acknowledgements
This study was supported by fund for building world-class universities (disciplines) of Renmin University of China (Project No. 2018).
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