Iron oxide driven methanogenesis and methanotrophy in methanic sediments of Helgoland Mud Area, North Sea

Abstract

Elevated dissolved iron concentrations (Fe2 ), as signpost for on-going iron oxide reduction in the methanic zone, are currently being detected in a wide range of marine environments. The various mechanisms that result in Fe2 release into porewater are a subject of intense debate amongst sediment geo-microbiologists. While abiotic cryptic sulfur cycling is suggested for some sites, biotic mechanisms potentially mediate iron reduction in many other sites, including the Helgoland Mud Area, North Sea. Iron oxide dependent anaerobic oxidation of methane (Fe-AOM) is primarily hypothesised as the biotic mechanism driving iron reduction in the methanic zone but organic matter degradation linked iron reduction could also play a role. Beyond geochemical data however, physiological evidence demonstrating that these processes occur and the microorganisms involved is rather scarce. In chapter two, a short-term radiotracer based experiment revealed that Fe-AOM is indeed feasible in the methanic zone of Helgoland Mud Area, albeit at very low rates under close to in situ conditions (0.095 A /- 0.03 nmol cm 3 d 1). Despite the low rates, these estimates represent the first demonstration of Fe-AOM in a marine environment bearing geochemical preconditions for Fe-AOM to occur in situ. Additionally in long-term incubations, various iron oxides (lepidocrocite, hematite and magnetite) stimulated Fe-AOM in sediments from the methanic zone. Especially with crystalline magnetite, ANME-2a were highly enriched after 250 days showing clearly, and for the first time, that ANME-2a are involved in Fe-AOM. Previous studies from the Helgoland Mud Area revealed that aromatic hydrocarbons are likely the preferred fermentation substrate in the methanic zone. This may have led to the strong correlations between fermentative bacteria, methanogenic archaea (which use fermentation products) and Fe2 concentrations. Chapter three investigated this possibility further, initially in sediment incubations and subsequently in highly enriched cultures. With benzoate as the only carbon substrate, enrichment efforts with crystalline iron oxides (magnetite and hematite) led to concurrent iron reduction and methanogenesis from benzoate degradation. In contrast, with poorly crystalline lepidocrocite, benzoate degradation and methanogenesis was slower. Thus, concurrent reduction of crystalline iron oxides facilitates organic matter degradation while poorly crystalline lepidocrocite inhibits the process. Therefore, a likely scenario might be in play in Helgoland Mud Area, whereby buried crystalline iron oxide phases which make up to 1.6 weight % of sediment volume could be advantageous to the microbial communities. These crystalline iron oxides likely facilitate methanogenic organic matter degradation while being reduced concurrently, thereby contributing to the Fe2 pool detected in porewater. Additionally, we uncovered the clostridial family Halobacteroidaceae as previously unknown benzoate degraders from marine sediments. In chapter four, sediment incubations with an easily fermentable substrate (glucose) revealed that crystalline iron oxides could act as conduits for electron transfer, as electron acceptors for iron reduction or act as both under various temperature regimes. Furthermore, iron reduction was more favorable under lower temperatures than at mesophilic conditions and dissimilatory iron reducers from the order Desulfuromonadales were enriched during iron reduction. These findings substantially advance the current state of the art regarding the biotic mechanisms that drive the apparent concurrent iron reduction in methanic zones of marine sediments. Besides providing direct evidence for Fe-AOM, the body of work presented in this thesis demonstrates the various ways iron oxides could facilitate methanogenic organic matter degradation in ferruginous methanic marine sediments. The exact molecular guides for these various processes should be subject of future studies.