Microbial diversity and respiratory processes in hydrothermal sediment

In marine hydrothermal sediments metal-rich, reduced fluids and oxidised seawater form steep geochemical gradients supporting abundant, but as yet poorly defined, chemosynthetic life and biogeochemical cycling. This project elucidates the functional diversity of chemosynthetic micro-organisms, and metabolic processes on the geochemistry of ferruginous (-50% Fe), arsenic-rich (-400 ppm As) hydrothermal sediment at Santorini, Greece (20-40°C, pH 6.0-6.3, Eh 0 to 155 mV). Culture and molecular-phylogenetic techniques revealed abundant chemosynthetic prokaryotes capable of transforming a range of chemical species common to marine and hydrothermal sediments via: anaerobic Fe(III), sulfate, nitrate and As(V) reduction, and nitrate-dependant Fe(lI) oxidation; micro-aerobic Fe(lI) monosulfide (FeS) oxidation; and aerobic As(lII) and free-sulfide oxidation. Geochemical analyses showed that oxidised species [Fe(III), As(V), sulfate and presumably Mn(IV)] dominated in the suboxic surface sediment layer (0-5 cm deep), but that reduced species [namely Fe(II), As(III), Mn(II)] increased in the lower suboxic-anoxic transition zone (5-20 em depth). From a biological perspective sulfate is the energetically least favourable of the oxidised species to be reduced, and sulfate concentrations were consistent throughout the portion of the sediment column analysed (0-30 em depth). The presence of a black precipitate (probably FeS) at the lower limit of the transition zone, however, did suggest at least a small amount of sulfate reduction was occurring. In contrast, no nitrate was detected indicating rapid reduction of this . energetically favourable compound at· the water-sediment interface. Microbiological and geochemical data, in combination, indicated that redox cycling of Fe, Mn and As were most likely key biogenic processes in biogeochemically stratifying the sediment, whereas S-cycling bacteria were comparatively minor contributors. Other principal findings included the cultivation of a novel neutrophilic Fe(II)-oxidiser related to marine microaerophilic Fe(II)oxidising bacterium Mariprofundus ferroxydans. This bacterium dominated the Fe(lIl) (oxy-)hydroxide-rich surface sediment, and most probably plays a significant role in catalysing suboxic Fe(lI) oxidation. Fe(II)-oxidising prokaryotes comprise a functional group not well understood in marine settings. Additionally, bacteria belonging to a ubiquitous, heterotrophic marine genus, Marinobacter, were shown to oxidise Fe(II). Of these, Marinobacter santoriniensis, was isolated and characterised, and also shown to both oxidise As(llI) and respire As(V), providing a rare example of a marine prokaryote capable of these functions. Futher to this, incubation experiments, conducted to establish the effect of microbial respiration on arsenic mobilisation, demonstrated that microbial Fe(lll) reduction largely controlled shifts in arsenic partitioning in the sediment. Specifically, results support other studies (mainly concerned with the toxic effects of arsenic) showing arsenic sequestration by Fe(II)-bearing minerals. This is contrary to the more common supposition of arsenic release to porewaters upon reductive dissolution of Fe(lIl) minerals. Overall, these results highlight the importance of microbial enzymatic processes in sediment geochemistry.EThOS - Electronic Theses Online ServiceGBUnited Kingdo