Maximum specific growth rate of anammox bacteria revisited

Anammox bacteria have long been considered to be slow-growing bacteria. However, it has recently been reported that they could grow much faster than previously thought when they were cultivated in a membrane bioreactor (MBR) with a step-wise decrease in the solid retention time (SRT). Here, we reevaluated the maximum specific growth rates (μ_max) of three phylogenetically distant anammox bacterial species (i.e. "Ca. Brocadia sinica", "Ca. Jettenia caeni" and "Ca. Scalindua sp.") by directly measuring 16S rRNA gene copy numbers using newly developed quantitative polymerase chain reaction (qPCR) assays. When free-living planktonic "Ca. B. sinica" and "Ca. J. caeni" cells were immobilized in polyvinyl alcohol (PVA) and sodium alginate (SA) gel beads and cultivated in an up-flow column reactor with high substrate loading rates at 37 °C, the μ_max were determined to be 0.33 ± 0.02 d^-1 and 0.18 d^-1 (corresponding doubling time of 2.1 day and 3.9 day) from the exponential increases in 16S rRNA genes copy numbers, respectively. These values were faster than the fastest growth rates reported for these species so far. The cultivation of anammox bacteria in gel beads was achieved less than one month without special cultivation method and selection pressure, and the exponential increase in 16S rRNA gene numbers was directly measured by qPCR with high reproducibility; therefore, the resulting μ_max values were considered accurate. Taken together, the fast growth is, therefore, considered to be an intrinsic kinetic property of anammox bacteria.