In-Situ Metatranscriptomic Analyses Reveal the Metabolic Flexibility of the Thermophilic Anoxygenic Photosynthetic Bacterium Chloroflexus aggregans in a Hot Spring Cyanobacteria-Dominated Microbial Mat.

Shigeru Kawai,Erik Trampe,Arisa Nishihara,Vera Thiel,Shin Haruta,Satoshi Hanada,Michael Kühl,Joval N Martinez,Mads Lichtenberg,Marcus Tank

doi:10.3390/microorganisms9030652

Abstract

Chloroflexus aggregans is a metabolically versatile, thermophilic, anoxygenic phototrophic member of the phylum Chloroflexota (formerly Chloroflexi), which can grow photoheterotrophically, photoautotrophically, chemoheterotrophically, and chemoautotrophically. In hot spring-associated microbial mats, C. aggregans co-exists with oxygenic cyanobacteria under dynamic micro-environmental conditions. To elucidate the predominant growth modes of C. aggregans, relative transcription levels of energy metabolism- and CO2 fixation-related genes were studied in Nakabusa Hot Springs microbial mats over a diel cycle and correlated with microscale in situ measurements of O2 and light. Metatranscriptomic analyses indicated two periods with different modes of energy metabolism of C. aggregans: (1) phototrophy around midday and (2) chemotrophy in the early morning hours. During midday, C. aggregans mainly employed photoheterotrophy when the microbial mats were hyperoxic (400–800 µmol L−1 O2). In the early morning hours, relative transcription peaks of genes encoding uptake hydrogenase, key enzymes for carbon fixation, respiratory complexes as well as enzymes for TCA cycle and acetate uptake suggest an aerobic chemomixotrophic lifestyle. This is the first in situ study of the versatile energy metabolism of C. aggregans based on gene transcription patterns. The results provide novel insights into the metabolic flexibility of these filamentous anoxygenic phototrophs that thrive under dynamic environmental conditions.

Highlights

Members of the genus Chloroflexus are thermophilic, filamentous anoxygenic phototrophs (FAPs) in the phylum Chloroflexota
Two sets of nuo operons have been described for other, non-phototrophic bacteria such as Ignavibacterium album [83] and “Candidatus Thermonerobacter thiotrophicus” [18], which might indicate specification to different O2 levels rather than phototrophic versus respiratory electron transport. This might indicate a potential higher O2 tolerance for the 14-gene set and a higher oxygen affinity for the 12-gene set. Because these results indicate that chemoheterotrophic, respiratory metabolism does not take place during highlight and superoxic conditions at midday, when the 14-gene set is highly transcribed, it is hypothesized that this NADH:menaquinone oxidoreductase plays a role in phototrophic electron transport, perhaps by donating electrons from NADH similar to as it has been proposed to be the case in the cyclic electron transport chain in heliobacteria [84]
This study suggests that C. aggregans uses its metabolic flexibility and capability for both phototrophic and chemotrophic growth to optimize its performance under the varying environmental conditions in its natural habitat, the microbial mat community at Nakabusa Hot Springs

Summary

Introduction

Members of the genus Chloroflexus are thermophilic, filamentous anoxygenic phototrophs (FAPs) in the phylum Chloroflexota (formerly Chloroflexi). They are well known to have the ability to grow photoheterotrophically under anaerobic conditions and chemoheterotrophically under aerobic conditions in the laboratory [1,2,3]. Microorganisms 2021, 9, 652 growth in the laboratory has been observed only in a small number of isolated strains C. aggregans strains NA9-6 and ACA-12, which were isolated from Nakabusa Hot Springs (Nagano Prefecture, Japan), can grow photoautotrophically with hydrogen gas (H2 ) [8] and sulfide [10] as the electron donors in pure culture, respectively. Fermentative growth has been shown in two isolates of C. aurantiacus, strains B3 and UZ [12]

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