Amino acids in surface sediments of the Zannone Island shelf (Western Mediterranean Sea): possible bioindicators of submarine hydrothermal activity

Abstract This study shows results from amino acid analysis of seafloor sediment sampled between ∼80 and ∼130 m depth, within an active hydrothermal field located off Zannone Island (Tyrrhenian Sea). The total hydrolysable amino acid (THAA) content was determined at eight stations, three located within the main hydrothermal depression (i.e. the Zannone Giant Pockmark, ZGP) and five located in the surrounding seafloor. THAA concentration ranged between ∼2 and ∼11 nmol mg-1 dw and the lowest THAA concentrations were found at stations located within the ZGP. Amino acids resistant to degradation or originating from bacteria (β-alanine, histidine, leucine, arginine and lysine) had higher relative abundances inside the ZGP, whereas amino acids susceptible to degradation or originating from cellular plasma (proline, phenylalanine and tyrosine) showed higher relative abundances outside the ZGP. Degradation and reactivity indices were calculated, and the sedimentary organic matter in the three outside ZGP stations B07, B08 and B09 was found to be the most labile and reactive. Together with THAA concentrations, sediment grain size and carbon percentages were measured at 12 stations, five located within ZGP and seven located outside the ZGP. At all stations, the percentage of sand was ∼75% or higher. Carbon percentages were lower in samples located inside the ZGP than in the surrounding stations. The southern outside ZGP station B12 was found to be similar in terms of amino acids composition to the three inside ZGP stations, suggesting the influence of the hydrothermal fluids at this station.