Abstract
Anammox bacteria enable efficient removal of nitrogen from sewage in processes involving partial nitritation and anammox (PN/A) or nitrification, partial denitrification, and anammox (N-PdN/A). In mild climates, anammox bacteria must be adapted to ≤15 °C, typically by gradual temperature decrease; however, this takes months or years. To reduce the time necessary for the adaptation, an unconventional method of ‘cold shocks’ is promising, involving hours-long exposure of anammox biomass to extremely low temperatures. We compared the efficacies of gradual temperature decrease and cold shocks to increase the metabolic activity of anammox (fed batch reactor, planktonic “Ca. Kuenenia”). We assessed the cold shock mechanism on the level of protein expression (quantitative shot-gun proteomics, LCHRMS/MS) and the structure of membrane lipids (UPLCHRMS/MS). The shocked culture was more active (0.66±0.06 vs 0.48±0.06 kg-N/kg-VSS/d) and maintained the relative content of N-respiration proteins at levels consistent levels with the initial state, whereas the content of these proteins decreased in gradually acclimated culture. Cold shocks also induced a more efficient expression of potential cold shock proteins (e.g. ppiD, UspA, pqqC), while putative cold shock proteins CspB and TypA were upregulated in both cultures. Ladderane lipids characteristic for anammox evolved to a similar end-point in both cultures; this confirms their role in anammox bacteria adaptation to cold and indicates a three-pronged adaptation mechanism (ladderane alkyl length, introduction of shorter non-ladderane alkyls, polar headgroup). Overall, we show the outstanding potential of cold shocks for low-temperature adaptation of anammox bacteria and provide yet unreported detailed mechanisms of anammox adaptation to low temperatures.
Highlights
Anammox bacteria can cost-effectively remove nitrogen from the mainstream of wastewater treatment plants (WWTP) via processes such as partial nitritation and anammox (PN/A) as well as nitrification, partial denitrification, and anammox (N-PdN/A)
Initial batch activity tests at 30 ◦C showed that the metabolic ac tivities of the three anammox cultures were 2.60–2.88 kg-N/kg-VSS/d (Fig. 1) which is higher than 1.94±0.09 kg-N/kg-VSS/d in our previous study with biomass from the identical membrane bioreactor (MBR) (Kouba et al, 2019)
It is reasonable to assume that these Csps, especially ppiD, UspA and pqqC, re-started protein synthesis, including the synthesis of N-respi ration proteins (e.g. nitrate oxidoreductase (NXR), hy drazine synthase (HZS),...) responsible for anammox activity; we suggest that more efficient induction of Csps by cold shocks compared to gradual acclimation made anammox cells more active
Summary
Anammox bacteria can cost-effectively remove nitrogen from the mainstream of wastewater treatment plants (WWTP) via processes such as partial nitritation and anammox (PN/A) as well as nitrification, partial denitrification, and anammox (N-PdN/A). In these processes, anammox eliminates the costs associated with the input of organic carbon necessary for conventional heterotrophic denitrification. The mainstream of WWTP is much colder (10–20 ◦C) than most current anammox reactors used to treat mesophilic centrates (30–37 ◦C), making the adaptation of anammox bacteria to low temperatures a crucial issue (Cao et al, 2017; Kouba et al, 2019). Favorable activities were obtained by the enrichment of cold-adapted populations (Hendrickx et al, 2014; Park et al, 2017), by the gradual acclimation of mesophilic cultures to a psychrophilic regime (De Cocker et al, 2018), or by a short-term exposure to extremely low temperature called the cold shock (Kouba et al, 2018)
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