Microbial responses to increased salinity in oiled upper tidal shorelines

The microbial degradation of crude oil stranded on seashores is usually much slower than that of dispersed oil at sea. This is usually attributed to nutrient limitation and higher oil concentrations on beaches than in dispersed plumes, in addition to the low moisture content and transient hypersaline conditions of the upper intertidal and supratidal zones. Here, we investigated the effect of hypersaline conditions, oil concentration, and nutrient limitation on the hydrocarbonoclastic activity. Beach sand, with oil concentrations of 1 and 10 mL/kg, was infiltrated with seawater at salinities of 30, 90, and 160 g/L to 20% saturation, with and without the addition of nutrients. Oil chemistry, 16 S rRNA gene amplicon, and metagenomic sequencing analyses were conducted every 30 days for 180 days. The biodegradation of aromatic hydrocarbons was much slower at higher salinities, while that of alkanes was not noticeably affected, and the addition of nutrients only marginally enhanced the biodegradation of hydrocarbons. The relative abundance and diversity of genera varied substantially with the increase in pore water salinity, whereby halophilic hydrocarbon-degrading microorganisms, particularly in the Marinobacter genus, were significantly abundant only under hypersaline conditions. The enzymes were considerably less abundant at high salinities, mainly for those associated to aromatic hydrocarbon degradation pathways. Consequently, occasional inundation of the upper parts of oiled sandy beaches with seawater to decrease the pore water salinity and reseed the contaminated region with indigenous hydrocarbon degraders, along with the concomitant addition of fertilizers, could be a suitable, non-destructive, and inexpensive response tool for enhancing the biodegradation of beached crude oil, particularly that of the aromatic fraction.