Project Overview

Summary:

Dr. Aixin Hou at Louisiana State University’s Department of Environmental Sciences, was awarded an RFP-VI grant at $705,732 to conduct the RFP-VI project titled, “A Decade-Long Study on Impact, Recovery, and Resilience in Louisiana Salt Marshes: The Evolution of the Oil Transformation Compounds and Plant-Soil-Microbial Responses to the Deepwater HorizonOil Spill”. The project consisted of 1 other institution (Florida State University), 1 principal investigator (Hou), 3 co-PIs (Drs. Huan Chen, Qiaxin Lin, Amy McKenna); 3 PhD students (Brian Matherne, Mansar Abbas, Grace Cagle); and 1 undergraduate student (Cameron Davis).

The Deepwater Horizon (DWH) oil spill exposed the nation’s largest and most productive wetland-estuary, the Mississippi River Delta coastal wetland ecosystem, to an unprecedented level of oil contamination and potential damage. The coastal marshes support a host of environmental and economic services that depend on a healthy, well-functioning plant-soil-microbial complex to drive the food web base. For ~7 years, the PI’s team has monitored the effects of the DWH oil spill in Louisiana salt marshes through 16 field-based data collections that quantify the impacts and recovery of a broad array of flora, fauna and microbes. Continuation of this research in heavily-oiled shorelines where marsh plants that serve as foundation species suffered severe mortality is critical to assessment of coastal marsh recovery, which to-date is incomplete, and to a better understanding of marsh resiliency to oil contamination. Also, the PI and team have identified highly polar, persistent oil transformation products derived from marsh sediments with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) within the first 4 years after the spill. Thus, the current proposal will (1) document and catalogue the decade-long impact of DWH oil on the coastal marsh plant-soil-microbial complex; (2) quantify rates and controls of long-term recovery; (3) catalogue oil transformation compounds detected in oiled salt marsh sediments up to 10 years after the spill and determine their toxicity; and (4) identify potential correlation between vegetation, microbes and transformed oil compounds in oiled salt marshes. It supports GoMRI theme 2: chemical evolution and biological degradation of oil and subsequent interaction with coastal marshes, and 3: knowledge of environmental effects of oil on marshes and the science of ecosystem recovery.

The combined research expertise of the PI and team in oil impacts on wetlands, the impact of the DWH spill specifically, and oil chemistry highly qualify the team to conduct the proposed research. Collectively, this group has published more than 50 refereed scientific papers on oil spill science; with 14 publications and 8 manuscripts in review on the DWH spill. The ~7-year database on DWH impacts and recovery in coastal salt marshes provides a catalogue to assess long-term resiliency and sustainability. Based on the preliminary characterization of oil transformation, oxidation in the sediments remains incomplete after 4 years and requires a comprehensive study on the symbiotic relationship between hydrocarbon transformation and microbial community. The current proposal emphasizes the ecological assessment of plant structure and function, marsh integrity and interactions in the plant-soil-microbe complex, a combination of cultivation-based and molecular-based biological analysis of microbial communities, and molecular-level characterization of oil chemical functionalities by ultrahigh resolution FT-ICR MS that cannot be identified by any other analytical technique. This proposed research will create a decade-long assessment record, and provide comprehensive integration of petroleum compound evolution and long-term recovery of oil impacted coastal wetlands.

Research Highlights

Dr. Hou’s research to date, which included 5 outreach products and activities, resulted in 1 peer-reviewed publication, 23 presentations at national/international conferences, and 7 datasets submitted to the GoMRI Information and Data Cooperative (GRIIDC), which are available to the public.  Significant outcomes of their research (all related to GoMRI Research Themes 2 and 3) are highlighted below.

In the salt marshes of northern Barataria Bay, one of the most heavily oiled areas, impacts of the Deepwater Horizon oil spill were severe in some areas and moderate in others. Nine and a half years after the spill, the average concentration of total petroleum hydrocarbons (TPH) in the surface soil in heavily oiled marshes was approximately 50 mg/g dry soil, significantly higher than that of reference marshes. High oil concentration adversely affected the coastal salt marsh vegetation. Although there has been variable recovery in oiled shoreline marshes after the spill, total live plant aboveground biomass was significantly lower (p<0.0001) than that of reference marshes 9.5 years after the spill. In addition, impact and recovery were oiling intensity-specific and plant species-specific. In moderately oiled marshes, Spartina alterniflora recovered 9 months after the spill; however, Juncus roemerianus did not recover until 2-3 years. In heavily oiled marshes, there was little recovery of Juncus 9.5 years after the spill although Spartina was able to recover in 2-3 years. As a result, the DWH oil spill has changed the vegetation structure from a Spartina-Juncus mixed community to predominantly Spartina in heavily oiled marsh. The results suggest that even almost a decade after the DWH oil spill, heavy oiling still severely affected coastal salt marsh structure and function and plants of heavily oiled marshes have not fully recovered.

Total cultivable heterotrophic bacterial abundance was similar across the reference, moderately and heavily oiled categories at most times after the spill, but was found to be significantly lower on several occasions where sediments had extremely high total petroleum hydrocarbon (TPH) concentrations, suggesting that heavy oil contamination hinders overall microbial activity. The populations of oil-degrading bacteria increased with increased oiling following the spill, but the extent to which the bacterial populations increased declined with time. In the 2019 samplings, no significant differences were found across treatments. The temporal pattern indicates the microbial community succession with time when the environment changes.

 Bacterial OTUs in Louisiana salt marsh sediments were predominant with Proteobacteria, followed by Bacteroidetes, Chlorofleix and Acidobacteria. Bacterial communities were significantly altered on both heavily and moderately oiled sites at the early stages of oiling. The communities experienced a recovery with time, with a faster speed of recovery occurring in moderately oiled sites in comparison to heavily oiled marshes. Nevertheless, the bacterial communities at heavily oiled sites did not return to the primitive community structure of the reference sites after almost a decade since the oil spill but have evolved into a new state. Potential hydrocarbon-degrading Alpha-Proteobacteria, Firmicutes and Bacteroidetes were replaced with Chloroflexi, Chlorobi, and Acidobacteria. Sequences of PAH-RHDα GP gene from heavily oiled sites were grouped into two distinct gene clusters. The genes became more diverse in 2018 than in 2011, indicating the oil-degrading bacteria have evolved into a more diversified functional community with time.

The concentration of oil contamination from heavily and moderately oiled sites exceeded non-oiled reference sites for nine years after the spill. Combining chromatographic fractionation, conventional GC MS, and ultrahigh-resolution FT-ICR Mass Spectrometry analyses, we identified tens-of-thousands of biotic and abiotic crude oil transformation products that remained persistent and kept evolving in the environment. Heavily oxygenated transformation products, which are mostly only accessible by FT-ICR MS, spanned a wide range of chemical functionalities.

Isolation of interfacially active compounds across various field samples revealed highly oxidized, polar compounds that form the interfacial layer. Fractionation of interfacial material combined with FT-ICR MS expanded the characterization of surface-active compounds and yielded a 10-fold increase in the number of assigned formulas. We have also identified a continuum of acidic species, including biosurfactants generated in situ in highly bioactive salt marsh samples.

Toxicity of the non-GC amenable fraction of oil contaminants was measured to determine the health risk of weathered oil residues left in the salt marsh environment and was linked to known molecular compositional changes identified by FT-ICR MS. Since chemical functionalities of these petrogenic contaminants dictate their toxicity, water-solubility, stable emulsion formation, and bioavailability, molecular level characterizations are necessary for understanding the fate and long-term impact of these emerging environmental contaminants.

Our data synthesis has focused on addressing the significant (p ≤ 0.05) differences in the salt marsh soil microbial communities among reference, moderately oiled, and heavily oiled sites. The results indicate that the variation in the soil microbial communities among oiling levels was best explained by differences in live aboveground vegetation, bulk density, and TPH. However, a spatial trend was present in the microbial data that was also significantly correlated to bulk density and total vegetation. Oiling level explained a significant portion of variation in the microbial communities after removing the variation shared by the environmental variables and the spatial trend.

 Proposal Abstract - RFP-VI PI Aixin Hou