The biodegradation of dispersed crude oil in the ocean is relatively rapid (a half-life of a few weeks). However, it is often much slower on shorelines, usually attributed to low moisture content, nutrient limitation, and higher oil concentrations in beaches than in dispersed plumes. Another factor may be the increased salinity of the upper intertidal and supratidal zones since these parts of the beach are potentially subject to prolonged evaporation and only intermittent inundation. Therefore, two laboratory experiments are conducted to investigate whether such an increase in porewater salinity results in additional inhibitory effects on oil biodegradation in seashores. In the first experiment, oil biodegradation is investigated in seawater at different salinities by evaporating sampled seawater to a concentrated brine and then adding it to fresh seawater to generate high salinity microcosms. Artificially weathered Hibernia crude oil is added, and biodegradation is followed for 76 days. Results show that the biodegradation of hydrocarbons is substantially inhibited at high salinities, whereby the half-life slows down by 20-fold when increasing the salinity from 30 to 160 g/L. Genera of well-known aerobic hydrocarbonoclastic bacteria are only identified at 30 g/L salt in the presence of oil, and only a few halophilic hydrocarbon-degrading microorganisms show a slight enrichment at higher salt concentrations. In the second experiment, oil biodegradation is investigated in coastal sediments. Lightly weathered Hibernia crude oil is added to beach sand at 1 or 10 mL/kg, and fresh seawater, at salinities of 30, 90, and 160 g/L, is added to 20% saturation. The sand mixtures are placed in glass cylinders, with and without the addition of nutrients. All microcosms are analyzed every 30 days for a total incubation period of 180 days. Results show that the biodegradation of oil is slower at higher salinities, where the half-life increases from 40 days at 30 g/L salt to 58 and 76 days at 90 and 160 g/L salt, respectively, and adding fertilizers somewhat enhances oil biodegradation. Increasing oil concentration in the sand, from 1 to 10 mL/kg, slows the half-life by about 10-fold. Interestingly, the biodegradation of aromatic hydrocarbons is much slower at higher salinities, while that of alkanes is not considerably affected. The relative abundance and diversity of genera vary significantly with the increase in porewater salinity, whereby halophilic hydrocarbon-degrading microorganisms, particularly the ones of the Marinobacter genus, are the most abundant only under hypersaline conditions. Enzymes pertaining to hydrocarbon biodegradation pathways are noticeably less abundant at high salinities, specifically for those pertaining to the degradation pathways of aromatics. Both experiments indicated that high porewater salinity in the upper parts of sandy beaches could significantly slow down the microbial degradation of crude oil, specifically that of the aromatic fraction. Consequently, occasional irrigation with seawater (i.e., to decrease the salinity) with fertilization could be a suitable bioremediation strategy for the upper parts of contaminated beaches. However, given that the high oil concentration in sandy beaches also plays a major role in the persistence of petroleum hydrocarbons on contaminated shorelines, chemically dispersing the spilled oil at sea and preventing it from reaching the coast is probably the most suitable option for its optimal removal from marine and coastal ecosystems.
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