Syntrophy in anaerobic global carbon cycles

Syntrophy is an essential intermediary step in the anaerobic conversion of organic matter to methane where metabolically distinct microorganisms are tightly linked by the need to maintain the exchanged metabolites at very low concentrations. Anaerobic syntrophy is thermodynamically constrained, and is probably a prime reason why it is difficult to culture microbes as these approaches disrupt consortia. Reconstruction of artificial syntrophic consortia has allowed uncultured syntrophic metabolizers and methanogens to be optimally grown and studied biochemically. The pathways for syntrophic acetate, propionate and longer chain fatty acid metabolism are mostly understood, but key steps involved in benzoate breakdown and cyclohexane carboxylate formation are unclear. Syntrophic metabolism requires reverse electron transfer, close physical contact, and metabolic synchronization of the syntrophic partners. Genomic analyses reveal that multiple mechanisms exist for reverse electron transfer. Surprisingly, the flagellum functions were implicated in ensuring close physical proximity and synchronization of the syntrophic partners.

Introduction

Syntrophy can mean any type of crossfeeding of molecules between microbial species whereby a more restricted definition is applied when discussing anaerobic syntrophic metabolism. Here, anaerobic syntrophy is defined as a thermodynamically interdependent lifestyle where the degradation of a compound such as a fatty acid occurs only when degradation end products, usually hydrogen, formate, and acetate, are maintained at very low concentrations. This typically occurs in cooperation with a second microorganism, usually a methanogen, that consumes the product with high affinity (Table 1). For example, the degradation of butyrate with hydrogen and acetate production is thermodynamically unfavorable unless these metabolites are maintained at very low levels by methanogens. This anaerobic metabolism, especially when methanogenesis is the driver of the terminal electron accepting reactions, often involves consortia with tightly coupled syntrophic partnerships [1, 2•, 3]. Syntrophic interactions also occur in sulfate-reducing environments as evidenced by sulfate-reducing consortia involved in anaerobic methane oxidation [4].

Anaerobic syntrophy differs from other types of microbial metabolism like aerobic fatty acid metabolism or denitrification in that a consortium of interacting microbial species rather than a single microbial species is needed to mineralize organic compounds [5]. A wide range of compounds including alcohols, fatty and aromatic acids, amino acid, sugars, and hydrocarbons are syntrophically degraded [2•, 3]. Syntrophy is essential for the complete conversion of natural polymers such as polysaccharides, proteins, nucleic acids, and lipids to CO₂ and CH₄ (Figure 1) [6]. Initially, fermentative bacteria hydrolyze the polymeric substrates such as polysaccharides, proteins, and lipids, and ferment the hydrolysis products to acetate and longer-chain fatty acids, propionate, alcohols, CO₂, formate, and H₂. These products plus some amino acids and aromatic compounds are then syntrophically metabolized to the methanogenic substrates, H₂, formate, and acetate [2•, 3]. Lastly, two different groups of methanogens, the hydrogenotrophic methanogens and the acetotrophic methanogens, complete the process by converting acetate, formate, and hydrogen produced by other microorganisms to methane and carbon dioxide.

Syntrophic fatty and aromatic acid metabolism accounts for much of the carbon flux in methanogenic environments [2•, 3]. Many aromatic compounds are converted to benzoyl-CoA, which is further metabolized by syntrophic consortia [3]. Drake et al. [7] coined the term “intermediary ecosystem metabolism” analogous to intermediary cellular metabolism to emphasize the importance of the intermediate steps that occur after polymer hydrolysis as the main drivers of methanogenesis. Our knowledge of intermediary ecosystem metabolism is incomplete because we have only limited information of the in situ occurrence and activity of key players. This is, in part, due to the difficulty in culturing and studying microorganisms involved in syntrophic metabolism.

From a thermodynamic point of view, anaerobic syntrophy represents an extreme lifestyle [8^••]. Even when hydrogen, formate, and acetate are low, the Gibbs free energy change for syntrophic metabolism is very close to the minimum increment of energy required for ATP synthesis, which is predicted to be about −15 to −20 kJ mol⁻¹ [2^•]. In some cases, syntrophic consortia grow at free energy changes of −10 kJ mol⁻¹ or less [9•, 10•]. Low energy yields mean that growth rates (<0.005 h⁻¹) and growth yields (2.6 g dry weight mole⁻¹ propionate) are low [9•, 10•]. Maintenance energy values for syntrophic metabolizers (0.1 to 7.5 kJ h⁻¹ mol C⁻¹) are an order of magnitude below that predicted from the empirical relationship derived from maintenance energy values of diverse microorganisms grown at different temperatures [9•, 10•]. The low maintenance energy requirements indicate that syntrophic bacteria are well adapted to an energetically stressed lifestyle. Mechanisms by which syntrophic consortia conserve energy when their thermodynamic driving force is very low are not well understood, but whole genome sequencing approaches are providing us with more insight into the metabolic capability of these organisms.