Biodegradation of polycyclic aromatic hydrocarbons in the surface ocean

Increasing quantities of organic pollutants (OP) are being released to the environment, posing a threat to Earth’s life system. In the marine environment, OP pollution caused by oil spill accidents receives a lot of academic and societal attention. However, the magnitude of semi-volatiles OP introduced by atmospheric deposition and by maritime currents is orders of magnitude larger. Little is known, however, about the cycling of these background OP in the oceans and their effects to marine ecosystems. It is believed that an important sink of OP in the marine environment must be microbial biodegradation, since it is widely recognized their capacity to consume many OPs. However, neither the magnitude of biodegradation, the identity of the main degraders nor OP effects to microorganisms, is known. Polycyclic aromatic hydrocarbons (PAH) and other semi-volatile hydrocarbons are among the most abundant OP in the marine environment. Their wide spectrum of physicochemical properties make PAH family an ideal surrogate to study the biogeochemistry of OP in seawater. This thesis focuses on the biodegradation activities under background concentrations of PAH in the upper ocean, and the co-occurring microbial responses to this exposure. The goals of this work are: 1) to get insights into microbial PAH biodegradation under realistic conditions in different upper ocean environments by means of biogeochemical, molecular and genomic approaches, 2) identify the main players and describe the main metabolisms co-occurring along with biodegradation in the interaction between PAH and microorganisms by means of physiological measurements and metatranscriptomic approaches, and 3) localize marine hot spots of PAH biodegradation and describe some of the main physicochemical influencing factors of biodegradation. For these purposes we did short term incubations with background concentrations of PAH, mimicking those conditions found in the surface ocean in several oceanic provinces with contrasted physicochemical conditions. We used molecular biology and analytical chemistry techniques to calculate biodegradation taxes and evaluate the changes in the composition and genetic expression profiles of microbial communities exposed to PAH ambient levels. We found that PAH microbial metabolic capacity is a widespread trait in the global upper ocean although PAH biodegradation rates span in a wide range of values. These findings strongly suggest a fundamental role of microbial degradation as a relevant driver of PAH fate in the upper ocean. We observed that PAH at background concentrations after short term exposure (24-48 hrs) had an impact on microbial communities compositions and functionality. The main taxa responsible for PAH biodegradation at background concentrations varied depending on the site and the habitat, and included many different groups. The battery of genes expressed during PAH exposure included PAH degrading genes plus genes involved in stress response and detoxifying strategies among others. Comparison of results from the different sites identified new environmental drivers affecting the efficiency of PAH biodegradation in the upper ocean beyond those already described and those that are commonly overlooked: bacterial life-style (PA vs free-living), microbial community pre-exposure history to organic pollutants and the habitat (specially the SML). Overall, this thesis adds new information and field-based evidence regarding the microbial controls over the fate and transport of the background PAH concentrations in the surface ocean. In a context of increasing emissions and global change, it becomes crucial to be able to forecast the fate of OP once in the ocean. This requires understanding at a mechanistic level how do microbial metabolic processes and the pool of OP in the ocean intertwine, in order to elucidate a more updated, and always evolving, global carbon cycle.

Bibliographic Details

Main Author:	Martínez Varela, Alícia
Other Authors:	Vila Costa, Maria
Format:	tesis biblioteca
Language:	English
Published:	2022
Subjects:	Organic pollutants (OP), Hydrocarbons, Ensure availability and sustainable management of water and sanitation for all,
Online Access:	http://hdl.handle.net/10261/339688
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