Description: This study examines the representativeness of low-temperature hydrothermal fluid samples with respect to their chemical and microbiological characteristics. Within this scope, we investigated short-term temporal chemical and microbial variability of the hydrothermal fluids. For this purpose we collected three fluid samples consecutively from the same spot at the Clueless field near 5 degrees S on the southern Mid-Atlantic Ridge over a period of 50 min. During sampling, the temperature was monitored online. We measured fluid chemical parameters, characterized microbial community compositions and used statistical analyses to determine significant differences between the samples. Overall, the three fluid samples are more closely related to each other than to any other tested habitat. Therefore, on a broad scale, the three collected fluid samples can be regarded as habitat representatives. However, small differences are apparent between all samples. One of the Clueless samples even displayed significant differences (P-value 〈 0.01) to the other two Clueless samples. Our data suggest that the observed variations in fluid chemical and microbial compositions are not reflecting sampling artefacts but are related to short-term fluid variability due to dynamic subseafloor fluid mixing. Recorded temporal changes in fact reflect spatial heterogeneity found in the subsurface as the fluid flows through distinctive pathways. While conservative elements (Cl, Si, Na and K) indicate variable degrees of fluid-seawater mixing, reactive components, including Fe(II), O(2) and H(2)S, show that chemical and microbial reactions within the mixing zone further modify the emanating fluids on short-time scales. Fluids entrain microorganisms, which modify the chemical microenvironment within the subsurface biotopes. This is the first study focusing on short-term microbial variability linked to chemical changes in hydrothermal fluids.

Description: Lilliput was discovered in 2005 as the southernmost known hydrothermal field along the Mid-Atlantic Ridge. It is exceptional in that it lacks high-temperature venting probably because of a thickened crust. The absence of thermophilic and hyperthermophilic prokaryotes in emissions supports the argument against the presence of a hot subsurface at Lilliput, as is typically suggested for diffuse emissions from areas of high-temperature venting. The high phylogenetic diversity and novelty of bacteria observed could be because of the low-temperature influence, the distinct location of the hydrothermal field or the Bathymodiolus assemblages covering the sites of discharge. The low-temperature fluids at the Lilliput are characterized by lowered pH and slightly elevated hydrogen (16 nM) and methane (∼2.6 μM) contents compared with ambient seawater. No typical hydrogen and methane oxidizing prokaryotes were detected. The higher diversity of reverse tricarboxylic acid genes and the form II RubisCO genes of the Calvin Benson-Bassham (CBB) cycle compared with the form I RubisCO genes of the CBB cycle suggests that the chemoautotrophic community is better adapted to low oxygen concentrations. Thiomicrospira spp. and Epsilonproteobacteria dominated the autotrophic community. Sulfide is the most abundant inorganic energy source (0.5 mM). Diverse bacteria were associated with sulfur cycling, including Gamma-, Delta- and Epsilonproteobacteria, with the latter being the most abundant bacteria according to fluorescence in situ hybridization. With members of various Candidate Divisions constituting for 25% of clone library sequences we suggest that their role in vent ecosystems might be more important than previously assumed and propose potential mechanisms they might be involved in at the Lilliput hydrothermal field.