Anaerobic free-living protozoa: growth efficiencies and the structure of anaerobic communities

Temporal variation of eukaryotic community structures in UASB reactor treating domestic sewage as revealed by 18S rRNA gene sequencing

Eukaryotes are important components of ecosystems in wastewater treatment processes. However, little is known about eukaryotic community in anaerobic wastewater treatment systems. In this study, eukaryotic communities in an up flow anaerobic sludge blanket (UASB) reactor treating domestic sewage during two years of operation were investigated using V4 and V9 regions of 18S rRNA gene for amplicon sequencing. In addition, activated sludge and influent sewage samples were also analyzed and used as the references for aerobic eukaryotic community to characterize anaerobic eukaryotes. The amplicon sequence V4 and V9 libraries detected different taxonomic groups, especially from the UASB samples, suggesting that commonly used V4 and V9 primer pairs could produce a bias for eukaryotic communities analysis. Eukaryotic community structures in the UASB reactor were influenced by the immigration of eukaryotes via influent sewage but were clearly different from the influent sewage and activated sludge. Multivariate statistics indicated that protist genera Cyclidium, Platyophrya and Subulatomonas correlated with chemical oxygen demand and suspended solid concentration, and could be used as bioindicators of treatment performance. Uncultured eukaryotes groups were dominant in the UASB reactor, and their physiological roles need to be examined to understand their contributions to anaerobic processes in future studies.

Anoxic ecosystems and early eukaryotes

Through much of the Proterozoic Eon (2.5–0.54 billion years ago, Ga), oceans were dominantly anoxic. It is often assumed that this put a brake on early eukaryote diversification because eukaryotes lived only in oxygenated habitats, which were restricted to surface waters and benthic environments near cyanobacterial mats. Studies of extant microbial eukaryotes show, however, that they are diverse and abundant in anoxic (including sulfidic) environments, often through partnerships with endo- and ectosymbiotic bacteria and archaea. Though the last common ancestor of extant eukaryotes was capable of aerobic respiration, we propose that at least some, and perhaps many, early eukaryotes were adapted to anoxic settings, and outline a way to test this with the microfossil and redox-proxy record in Proterozoic shales. This hypothesis might explain the mismatch between the record of eukaryotic body fossils, which extends back to >1.6 Ga, and the record of sterane biomarkers, which become diverse and abundant only after 659 Ma, as modern eukaryotes adapted to anoxic habitats do not make sterols (sterane precursors). In addition, an anoxic habitat might make sense for several long-ranging (>800 million years) and globally widespread eukaryotic taxa, which disappear in the late Neoproterozoic around the time oxic environments are thought to have become more widespread.