Microbial life associated with low-temperature hydrothermal venting and formation of barite chimneys at Loki's Castle vent field

A low-temperature diffuse venting area with numbers of small barite chimneys is located on the flank of the large sulphide mound of the Loki’s Castle black smoker vent field at the Arctic Mid-Ocean Ridge (AMOR). White cotton-like microbial mats on top of the barite chimneys and associated siboglinid tubeworms were observed. The temperature was determined to 20°C for the surface sediment and 0°C for the white microbial mats, just above the ambient bottom seawater temperature of -0.8°C. The microbial mats were sampled using a remote operating vehicle (ROV) equipped with a hydraulic sampling cylinder (biosyringe) and the chimneys using an aluminum scuffle box. Black colored interior flow channels surrounded by white outer sections of nearly pure barite, were observed. Scanning electron microscopy (SEM) of mats showed numerous microbial cells and large amounts of extracellular thread-like material with attached barite crystals. Inside the chimneys microbial cells are partially embedded in barite, and individual crystals are also frequently covered by extracellular material. The microbial activity could thus have an important influence on the nucleation and growth of the barite crystals and thus on the formation of the chimneys. To reveal the microbial community structure, 16S rRNA gene sequence tag-encoded pyrosequencing (1.1 x 104 - 3.5 x 104 amplicons per library) followed by taxonomic classification of the reads using the MEGAN software, were performed. Organisms assigned to a genus of sulfide oxidizers (Sulfurimonas) within the e-Proteobacteria were abundant in each chimney structure; the white microbial mats (86-96% of the reads), the white barite (36% of total reads); the black flow channel (9.9%). The second most dominating taxon in the white chimney barite, including 26% of the reads, was anaerobic methanotrophs (ANME) of the ANME-1 clade, indicating anaerobic methane oxidation (AOM) as a major microbial process. Furthermore, the novel AOM associated clade, GOM-arc1 was apparently highly abundant (14.3% of total reads). These latter taxa were identified but clearly less abundant in the mats (ANME-1, 0.7%; GOM-arc1, 2.7%) as well as in black flow channel (ANME-1, 8%; GOM-arc1, 0.49%). Other dominating taxa in the flow channel were; Planctomycetales, 13.5%; Thiotrichales, Leucotrix, 8.8%; Thaumarchaeota, Marine Group 1, 9.35%; Pseudomonadales, Psychrobacter, 7.2%; Rhodobacterales, Rhodobacteraceae, 6.1%; Candidate division TM7, 5.9%; Flavobacteriales, 4.8% and Methylococcales, 3.5% altogether indicating a more diverse microbial community, performing methane, sulfur and ammonia oxidation as well as heterotrophic processes. To further clarify the relationship between crystallization, chimney growth and microbial activity, and the potential for preservation of biosignatures in barite formations, these data will be supplemented by geochemical characteristics, more detailed SEM observations and knowledge of In situ activities determine by analysis of community transcriptome and proteome.