Missing links in the sulfur cycle of marine sediments

The sediments underlying our oceans are inhabited by massive numbers of uncharacterised microorganisms, which are present in similar quantities to those in the oceans. Because oceans cover more than 70% of Earth and sediments can be kilometres thick, the tremendous size of this living subsurface biosphere means that it contributes substantially to global cycles of elements such as carbon, to and from the oceans and atmosphere. Within marine environments, sulfur-containing chemicals are abundant and play essential roles in the growth of all organisms. In sediments, microorganisms can use these sulfur chemicals to breath when oxygen is not available. Because most sediments lack oxygen, growth via breathing with sulfur occurs in the great majority of sediments,. These particular microorganisms are therefore key players in sediment ecosystems. Contemporary research has shown that the chemistry of sulfur in marine environments is more complicated than previously understood. We now know that various sulfur chemicals exist and most of these are rapidly used by microorganisms to breath and drive growth. This means these newly understood aspects of the marine sulfur cycle must also play key roles in the cycling of other elements, such as carbon. Although the chemistry of these processes is starting to be better understood, there is a complete lack of knowledge about which microorganisms control these processes, and how these processes influence the diversity of microorganisms and their interactions with each other and the environment. This project therefore aims to identify and understand the microorganisms controlling these recently understood dimensions of the marine sulfur cycle. Because most microorganisms (>99%) cannot be grown in the laboratory, this project will use cutting-edge tools to by-pass these limitations and study these organisms directly in the environment. These tools include the latest DNA sequencing technologies and innovative environmental genomics techniques, which will be combined with integrated experimental and environmental studies in order reveal and understand these missing links in the marine sulfur cycle. This work will therefore contribute to our fundamental understanding of: i) what influences the existence of different types of microorganisms that live in the marine subsurface; ii) the functions of some of the numerous unknown groups of microorganisms living beneath our oceans, i.e., microbial dark matter; iii) how microorganisms influence and interact with elemental cycles such as those of carbon, sulfur and metals, which are highly interconnected; and iv), additional broader aspects such as the processes and microorganisms involved in the generation of sulfide chemicals, which can have detrimental effects to processes in areas such as bioremediation or in environments such as oil reservoirs and pipelines.

The sediments underlying our oceans are actually living and breathing systems, inhabited by immense numbers of uncharacterised microorganisms. This extensive 'subsurface biosphere' plays important roles in global cycles of elements such as carbon in the oceans. In marine environments, sulfur chemicals are especially abundant and play essential roles for the growth of all organisms. In sediments, microorganisms can use these sulfur chemicals to 'breath' when oxygen is not available. Because most sediments lack oxygen, growth via breathing with sulfur occurs is one of the most important processes. These particular microorganisms are therefore key players in sediment ecosystems. Previous chemistry-based research has shown sulfur cycling in the seafloor is more complicated than previously understood. We now know that various sulfur chemicals exist and most of these are rapidly used by microorganisms to breath and/or support growth. Nevertheless, there is a lack of knowledge about which microorganisms control certain aspects of these processes. This project therefore aimed to identify and understand the microorganisms controlling these recently understood dimensions of the marine sulfur cycle. The results from this project identified a new major group of bacteria in marine sediments as important sulfur cycling microbes. These Acidobacteria were shown to represent one of the main sulfur cycling groups that were previously unknown, and suggest they are involved in multiple sulfur chemical transformations - making them somewhat sulfur-cycling specialists. Studying this group was also important, because Acidobacteria are the dominant bacterial group in terrestrial soils, but relatives from marine environments have barely been studied in detail. We therefore provided new insights into the functions of this major lineage of bacteria. We also showed that potential for sulfur cycling of the unusual sulfur chemicals is actually widespread among bacteria in marine sediment, and that most of the major populations of microbes are probably involved in sulfur cycling in different ways. This shows how the sulfur cycle had a persuasive influence on the evolution and functions of many microbes that live in sediments.