Abstract
BackgroundCaldicellulosiruptor species have gained a reputation as being among the best microorganisms to produce hydrogen (H2) due to possession of a combination of appropriate features. However, due to their low volumetric H2 productivities (QH2), Caldicellulosiruptor species cannot be considered for any viable biohydrogen production process yet. In this study, we evaluate biofilm forming potential of pure and co-cultures of Caldicellulosiruptor saccharolyticus and Caldicellulosiruptor owensensis in continuously stirred tank reactors (CSTR) and up-flow anaerobic (UA) reactors. We also evaluate biofilms as a means to retain biomass in the reactor and its influence on QH2. Moreover, we explore the factors influencing the formation of biofilm.ResultsCo-cultures of C. saccharolyticus and C. owensensis form substantially more biofilm than formed by C. owensensis alone. Biofilms improved substrate conversion in both of the reactor systems, but improved the QH2 only in the UA reactor. When grown in the presence of each other’s culture supernatant, both C. saccharolyticus and C. owensensis were positively influenced on their individual growth and H2 production. Unlike the CSTR, UA reactors allowed retention of C. saccharolyticus and C. owensensis when subjected to very high substrate loading rates. In the UA reactor, maximum QH2 (approximately 20 mmol · L−1 · h−1) was obtained only with granular sludge as the carrier material. In the CSTR, stirring negatively affected biofilm formation. Whereas, a clear correlation was observed between elevated (>40 μM) intracellular levels of the secondary messenger bis-(3′-5′)-cyclic dimeric guanosine monophosphate (c-di-GMP) and biofilm formation.ConclusionsIn co-cultures C. saccharolyticus fortified the trade of biofilm formation by C. owensensis, which was mediated by elevated levels of c-di-GMP in C. owensensis. These biofilms were effective in retaining biomass of both species in the reactor and improving QH2 in a UA reactor using granular sludge as the carrier material. This concept forms a basis for further optimizing the QH2 at laboratory scale and beyond.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0201-7) contains supplementary material, which is available to authorized users.
Highlights
Amid the findings of vast reserves of shale oil and convenient negligence towards its side-effects on the environment, the utopian world of ‘hydrogen economy’ still looks distant
Fermentation setup and culture medium To study the effect of any excretion of C. saccharolyticus on the growth of C. owensensis and vice versa, batch cultures of each were performed in biological duplicates and previously collected cell-free culture supernatant of one organism was added into the batch medium of another prior to inoculation
When grown together in co-culture do, C. saccharolyticus and C. owensensis form substantial amounts of biofilm, improving substrate conversion and Volumetric H2 productivity (QH2). Such a constructed co-culture is an effective means to be exploited in any bioreactor designed for biomass retention, such as up-flow anaerobic (UA) reactors
Summary
Amid the findings of vast reserves of shale oil and convenient negligence towards its (alleged) side-effects on the environment, the utopian world of ‘hydrogen economy’ still looks distant. One of the best, yet not ideal, groups of bacteria with the natural ability to produce H2 [7] Within this genus, Caldicellulosiruptor saccharolyticus and Caldicellulosiruptor owensensis are two of the best-studied species, both known to produce H2 near the theoretical maximum of 4 mol · mol−1 [8,9]. Caldicellulosiruptor species have gained a reputation as being among the best microorganisms to produce hydrogen (H2) due to possession of a combination of appropriate features. Due to their low volumetric H2 productivities (QH2), Caldicellulosiruptor species cannot be considered for any viable biohydrogen production process yet.
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