Biogeochemical interactions in plumes

The massive output of microbial biomass that characterises the early stages of event plumes potentially affects, and is affected by, hydrothermal constituents like H2S, Mn2+, Fe2+, H2, CH4. These reducing substances represent important energy sources as in other hydrothermal environments. 

Mn oxidation and scavenging in plumes appear to be dominated by microbial activity (see review in Winn etal., 1995). Plume bacteria are often characterised by capsules of abundant extracellular polymer matrices that are believed to scavenge Mn from solution (e.g. Cowen etal., 1986, 1999). Radiotracer experiments (54Mn uptake) and elevated microbial biomass at plume depths suggest that microbial metabolic activity enhances Mn scavenging (Cowen etal., 1986). However, it is not clear if this activity includes active Mn2+ oxidation or is simply due to non-enzymatic processes. 

In comparison, Fe oxidation and deposition appear to be much less common in plumes. About half the Fe in the hydrothermal fluids combines with H2S and is rapidly transformed into Fe sulphides within a few seconds of release (e.g. Rudnicki Elderfield, 1993 James etal., 1995), and much of the Fe2+ that escapes sulphide precipitation is rapidly and spontaneously oxidized in well-oxygenated seawater making it difficult to evaluate the bacterial contribution to the redox transformations of hydrothermal Fe in plumes (Lilley etal., 1995 Winn etal., 1995). Nevertheless, high Fe/Mn particles and Fe-encrusted capsule forms have been observed in plumes at Axial Volcano (JDFR) the physicochemical characteristics of the capsules may be responsible for the passive or surface-enhanced deposition of iron (Cowen etal., 1999). Thiosulphate, the primary product of sulphide autooxidation, may also serve as a useful energy source, but this is yet to be documented in hydrothermal plumes (Winn etal., 1995 Cowen German, 2002). 

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