Cell Aggregation and Aerobic Respiration Are Important for Zymomonas mobilis ZM4 Survival in an Aerobic Minimal Medium

Abstract

Zymomonas mobilis produces ethanol from glucose near the theoretical maximum yield, making it a potential alternative to the yeast Saccharomyces cerevisiae for industrial ethanol production. A potentially useful industrial feature is the ability to form multicellular aggregates called flocs, which can settle quickly and exhibit higher resistance to harmful chemicals than single cells. While spontaneous floc-forming Z. mobilis mutants have been described, little is known about the natural conditions that induce Z. mobilis floc formation or about the genetic factors involved. Here we found that wild-type Z. mobilis forms flocs in response to aerobic growth conditions but only in a minimal medium. We identified a cellulose synthase gene cluster and a single diguanylate cyclase that are essential for both floc formation and survival in a minimal aerobic medium. We also found that NADH dehydrogenase 2, a key component of the aerobic respiratory chain, is important for survival in a minimal aerobic medium, providing a physiological role for this enzyme, which has previously been found to be disadvantageous in a rich aerobic medium. Supplementation of the minimal medium with vitamins also promoted survival but did not inhibit floc formation. IMPORTANCE The bacterium Zymomonas mobilis is best known for its anaerobic fermentative lifestyle, in which it converts glucose into ethanol at a yield surpassing that of yeast. However, Z. mobilis also has an aerobic lifestyle, which has confounded researchers with its attributes of poor growth, accumulation of toxic acetic acid and acetaldehyde, and respiratory enzymes that are detrimental for aerobic growth. Here we show that a major Z. mobilis respiratory enzyme and the ability to form multicellular aggregates, called flocs, are important for survival, but only during aerobic growth in a medium containing a minimum set of nutrients required for growth. Supplements, such as vitamins or yeast extract, promote aerobic growth and, in some cases, inhibit floc formation. We propose that Z. mobilis likely requires aerobic respiration and floc formation in order to survive in natural environments that lack protective factors found in supplements such as yeast extract.

Increased salt tolerance in Zymomonas mobilis strain generated by adaptative evolution

“…These cells constituted a loosened pellet, found as sediment in the fully-grown culture (Fig. 1 ), however, we did not observe any floc formation which was shown to be beneficial for stress resistance in Z. mobilis [ 25 ]. Based on the cell shapes and the growth profile (Figs.…”

Genome Wide Phosphoproteome Analysis of Zymomonas mobilis Under Anaerobic, Aerobic, and N2-Fixing Conditions

“…Unlike most anaerobic biofuel producers, Z. mobilis is highly tolerant to oxygen, although its growth rate and ethanol yield decrease significantly under aerobic conditions (Tanaka et al, 1990; Yang et al, 2009; Martien et al, 2019). Recent studies have indicated that during aerobic growth, oxidative damage to iron-sulfur (FeS) clusters constitutes a major factor influencing Z. mobilis metabolism and that respiratory enzymes and the ability to form multicellular aggregates are important for its survival (Jones-Burrage et al, 2019; Martien et al, 2019). Despite these and other recent advances (Yang et al, 2009; Rutkis et al, 2016; Strazdina et al, 2018), much remains to be learned about the regulation of Z. mobilis physiology during aerobic growth.…”

Increased salt tolerance in Zymomonas mobilis strain generated by adaptative evolution

“…These cells constituted a loosened pellet, found as sediment in the fully-grown culture ( Fig. 1), however, we did not observe any oc formation which was shown to be bene cial for stress resistance in Z. mobilis [25]. Based on the cell shapes and the growth pro le (Fig.…”