Concentrations, distributions, and energy yields of dissolved neutral aldoses in a shallow hydrothermal vent system of Vulcano, Italy

Abstract

Large numbers of heterotrophic microorganisms inhabit hydrothermal systems, but few organic geochemistry studies have been carried out on these environments. This study presents the first data on total dissolved neutral aldose (TDNA) composition in a shallow hydrothermal system on the Island of Vulcano, Italy. Samples were collected from geothermal wells, sediment seeps, and shallow submarine vents. Temperatures and geochemical conditions varied between sample locations. Concentrations of TDNA were 56–55,128 nM, a larger range than those reported for non-hydrothermal systems. TDNA yields ranged from 0.47% to 16.54%. The highest yields were observed in sediment seeps and these yields are higher than any reported for non-hydrothermal systems. Similar to other marine environments, glucose and galactose had the highest mole fractions in most samples. Mole fractions of the deoxysugars fucose and rhamnose were low, indicating that algal material is not a likely source for TDNA. High TDNA concentrations correlated with high amino acid (AA) concentrations, indicating that the source and sink processes for AA and TDNA may be similar. High concentrations of both TDNA and AA occurred in samples with low pH and high temperature, but only pH showed a significant correlation with organic parameters. At the in situ geochemical conditions and per mole of aldose reacted, fermentation yields 220–420 kJ, anaerobic respiration yields 500–2400 kJ, and aerobic respiration yields ∼2750 kJ. The generally high aldose yields indicate a large fraction of biologically labile material, and energy calculations show that aldose respiration is more exergonic than chemolithotrophic reactions within the system.