Abstract
Coastal marine sediments, where important biological processes take place, supply essential ecosystem services. By their location, such ecosystems are particularly exposed to human activities as evidenced by the recent Deepwater Horizon disaster. This catastrophe revealed the importance to better understand the microbial processes involved on hydrocarbon degradation in marine sediments raising strong interests of the scientific community. During the last decade, several studies have shown the key role played by microorganisms in determining the fate of hydrocarbons in oil-polluted sediments but only few have taken into consideration the whole sediment’s complexity. Marine coastal sediment ecosystems are characterized by remarkable heterogeneity, owning high biodiversity and are subjected to fluctuations in environmental conditions, especially to important oxygen oscillations due to tides. Thus, for understanding the fate of hydrocarbons in such environments, it is crucial to study microbial activities, taking into account sediment characteristics, physical-chemical factors (electron acceptors, temperature), nutrients, co-metabolites availability as well as sediment’s reworking due to bioturbation activities. Key information could be collected from in situ studies, which provide an overview of microbial processes, but it is difficult to integrate all parameters involved. Microcosm experiments allow to dissect in-depth some mechanisms involved in hydrocarbon degradation but exclude environmental complexity. To overcome these lacks, strategies have been developed, by creating experiments as close as possible to environmental conditions, for studying natural microbial communities subjected to oil pollution. We present here a review of these approaches, their results and limitation, as well as the promising future of applying “omics” approaches to characterize in-depth microbial communities and metabolic networks involved in hydrocarbon degradation. In addition, we present the main conclusions of our studies in this field.
INTRODUCTION
Since Amoco Cadiz oil spill in 1978, many other oil spills such as Exxon Valdez (1989), Erika (1999), Prestige (2002), Deepwater Horizon (DH, 2010) have occurred in marine ecosystems. Such oil spill catastrophes generate a lot of indignation among the human populations, especially in coastal areas where environmental injuries are obvious. Cleanup efforts are urgently implemented in order to mitigate the toxic impact of petroleum compounds on the environment. However, the complete recovery of the ecosystem functioning is difficult to achieve because our knowledge on microbial communities, main actors involved in biodegradation processes, is still limited. The main issues regarding marine oil pollution have been already discussed (for review, see ), and microbial processes involved on hydrocarbon degradation extensively described (; ). However coastal marine sediments constitute particular ecosystems, especially intertidal zones where environmental conditions are daily modified according to tide level that in turn drive microbial degradation processes (Figure 1A). Although the impact of petroleum on microbial communities resulting in ecological succession, modifications of microbial populations following the hydrocarbon degradation, has been largely demonstrated (; , ; ; ; ) several scientific ecological questions remains to be solved. Among these questions, the organization of microbial community structures facing the presence of spilled oil, the mechanisms involved in their adaptation conducting to efficient hydrocarbon degradation, the structure/function relationship and the contribution of functional redundancy to microbial community resilience are some of the current burning questions which responses, at the applied point of view, will help to conduct appropriate bioremediation strategies such as bio-augmentation and bio-stimulation. The presence of a pollutant, such as petroleum, in the environment highlights also the importance to address academic concerns in microbial ecology, contributing more generally to the ecological theory. The importance to apply theory in microbial ecology has been emphasized by , especially for addressing population ecology, micro-organisms interactions, community assembly and the biodiversity-function relationships. Different approaches including in situ studies and laboratory experiments have been developed in order to test ecological hypothesis, decipher the role of microbial communities in the ecosystem functioning and understand the microbial behavior in front of a pollution. Microbial experimental systems have been particularly useful to address ecological questions by simplifying microbial systems and allowing experimental controls (see reviews by , ). Combined with the recent advances in meta-omics’ technologies that provides powerful tools for analyzing microbial communities, their diversity and their functioning as a whole (for review, see ; ), such experimental microbial systems are promising approaches to gain new insights on functional networks involved in hydrocarbon degradation processes in marine coastal sediments. We review here the recent progress on the ecology of microbial communities involved on hydrocarbon degradation in marine coastal sediments, attained by both in situ and experimental approaches. We present the main conclusions on our work in this field indicating the convenience of using experimental ecology to improve our knowledge in hydrocarbon microbial ecology.
FIGURE 1
IN SITU MICROBIAL ECOLOGY IN COASTAL MARINE POLLUTED SEDIMENTS
Opening the microbial black box involved in hydrocarbon degradation needs to take into consideration the whole ecosystem. In situ studies allow to reach the entire microbial communities in their actual context, considering (biotic and abiotic) ecological interactions. Over the past few years, field studies have been performed in marine sediments addressing the impact of oil on microbial communities, by following their organization and/or by characterizing their composition. In most cases this has been approached by spatial comparisons of contrasting contaminated and uncontaminated sites (
Different molecular approaches, mainly involving 16S rRNA gene analyses such as DGGE (
These studies allowed to assess the complexity of autochthonous microbial communities related to the oil pollution, revealing in situ changes in microbial diversity and their selective response to the presence of oil. Nevertheless, in situ approaches have many drawbacks since information access on ecophysiology of oil-degrading microorganisms, their activity and degradation pathways remain still limited.
EXPERIMENTAL ECOLOGY APPROACHES IN MARINE COASTAL SEDIMENTS STUDIES
Experimental approaches have been developed to address the ecological role of microbial communities on hydrocarbon mitigation in marine sediments. These approaches have progressed as scientific questions arise with the concomitant advances of microbial molecular ecology techniques. Enrichment cultures (or slurries), similar approaches to those used for microbial strains isolation, have been used to tackle the impact of crude oil and petroleum hydrocarbon compounds on bacterial communities. Usually, sediments are mixed with a minimal medium (more often artificial sea water) containing crude oil or selected hydrocarbon compounds, and maintained at laboratory under agitation. Following the dynamic modifications of microbial communities in the slurries, fingerprinting techniques and 16S rRNA gene libraries analyses have shown the ecological succession of microbial populations in response to hydrocarbon compounds in pristine deep marine sediments (
MIMICKING ENVIRONMENTAL CONDITIONS IN EXPERIMENTAL ECOLOGY
In intertidal zones, microbial degradation processes are driven by environmental conditions that are daily modified according to tide level (Figure 1A). An original microcosm (Figure 1B) system maintaining the structure of muddy sediments under tidal cycles was set up ensuring conditions close to those prevailing in the natural environment of coastal marine sediments (
CONCLUDING REMARKS
We reviewed here recent approaches implemented in order to assess microbial processes involved on hydrocarbon degradation in marine coastal sediments. Microbiologists have developed several approaches, including more or less sophisticated experimental systems and in situ studies to answer the scientific questions regarding the microbial mechanisms that take place in response to oil and hydrocarbon contaminations. Figure 2 summarize the main advantage and limitations for the approach considered in addressing the scientific questions. Experimental ecology using experimental systems mimicking as close as possible the environmental conditions combine the advantages of lab controlled systems with the possibility of extrapolation to the real situation found in complex ecosystems. Such approaches offer the opportunity to conduct experiments in replicates, crucial advantage for robust statistical analyses as highlighted by
FIGURE 2

Understanding microbial processes involved on hydrocarbon degradation in marine coastal sediments. The general scheme from scientific questions to microbial processes is presented together with the comparison of experimental approaches developed at different environmental complexity.
Statements
Acknowledgments
This research was funded by the French National Agency (ANR) under the DHYVA (ANR 2006 SEST 09) and DECAPAGE (ANR 2011 CESA 006 01) projects. We would like to thank the CEDRE for the setup of the microcosms and all partners of the DHYVA and DECAPAGE project for their useful discussions. We acknowledge the Regional Platform for Environmental Microbiology PREMICE supported by the Aquitaine Regional Government Council (France).
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
microcosm, mesocosm, molecular ecology, intertidal sediments, experimental ecology, omic’s approaches
Citation
Cravo-Laureau C and Duran R (2014) Marine coastal sediments microbial hydrocarbon degradation processes: contribution of experimental ecology in the omics’era. Front. Microbiol. 5:39. doi: 10.3389/fmicb.2014.00039
Received
31 October 2013
Accepted
21 January 2014
Published
12 February 2014
Volume
5 - 2014
Edited by
Ian M. Head, Newcastle University, UK
Reviewed by
Kathleen Scott, University of South Florida, USA; Guang Gao, Nanjing Institute of Geography and Limnology, China
Copyright
© 2014 Cravo-Laureau and Duran.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Robert Duran, Equipe Environnement et Microbiologie, Université de Pau et des Pays de l’Adour, IPREM UMR CNRS 5254, BP 1155, 64013 Pau Cedex, France e-mail: robert.duran@univ-pau.fr
This article was submitted to Aquatic Microbiology, a section of the journal Frontiers in Microbiology.
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