Microbial life and metabolism in Mediterranean Sea extreme environments

Abstract

The Mediterranean Sea is one of the most intensively investigated areas of the world. Despite this, it lags other regions of the world in studies of its deep sea and extreme environments. The contribution of Mediterranean extreme environments to global and regional diversity and biogeochemical cycles is poorly understood. Prokaryotes are the major component of Earth biodiversity, they outnumber any other living form and posses the greatest metabolic plasticity. This is especially true in extreme environments, where prokaryotes often constitute the only life form and play a key role in the functioning of those systems. Aim of this thesis is to investigate the diversity and role of prokaryotes in Mediterranean Sea extreme environment. The samples analyzed in this work were obtained in different oceanographic cruises and field trip around the Mediterranean Sea and are representatives of low- and high-energy environments. The used approach is a mix of traditional microbiology and ecology techniques combined with cutting-the-edge molecular tools, integrating laboratory and field survey. I started investigating the community structure in shallow-water sub-surface sediments evidencing a shift in the prokaryotic community composition. Despite the high biopolymeric organic C amount, results showed the major importance of the environmental factors influencing the availability of inorganic electron donors/acceptors, suggesting that lithotrophy could be an important metabolic pathway in shallow sub-surface sediments. In deep sea surface sediments across the Mediterranean Sea, community structure was instead dominated by Bacteria, and major driving force were trophic variables and geographic position within the basin, suggesting that overlaying water masses play a critical role in shaping deep-sea benthic prokaryotic communities. Different factors controlled prokaryotes distribution in shallow-water hydrothermal vents and seeps. In shallow-water vents prokaryotes distribution was controlled by temperature gradients rather than trophic resources. In cold-seeps environments the major drivers were both the presence of hydrocarbons and organic matter. In both areas the diversity was high, with a community evenly dominated by chemoautotrophic and heterotrophic metabolic strategies. Finally, the results obtained in different environments are brought together and emerging trends are analyzed. In the future, to gain a deeper understand of the diversity and functioning of extreme environments, we will need to integrate ecology, microbiology, geochemistry, oceanography and molecular biology and shift our scale of measuring, reasoning and interpreting the microbial world from “human” to “microbial”.