Award: OCE-2022184

Ethanol is used as a fossil fuel substitute and additive.  It enters the atmosphere where it reacts to form acetaldehyde, a hazardous air pollutant. There are large uncertainties in the role of the oceans in cycling these species into and out of the atmosphere. Ethanol and acetaldehyde concentrations in water depend on biological sources and sinks from bacteria, photochemical sources and sinks from sunlight and exchange between the air and water. Research goals were to: 1) measure biological degradation rates of ethanol and acetaldehyde; 2) measure biological production of acetaldehyde from ethanol; 3) measure/estimate ethanol and acetaldehyde levels in air and surface waters; 4) measure/estimate ethanol and acetaldehyde photoproduction rates; and 5) study which types of bacteria process ethanol and acetaldehyde.

The study site was Newport Bay, an estuary in Orange County, Southern California. Water in the bay is close to freshwater at the inlet during the rainy season and close to seawater at the outflow. Water samples were collected up to three times a week over one year from site 1 at the inlet, site 2 halfway down near a salt marsh, and site 3 near the outflow. This study was carried out during a severe drought.

Results showed that:

1) Chemical and physical properties varied between sites and by season. Variability was controlled by: 1) freshwater flowing into the bay at site 1; 2) summer phytoplankton blooms; and 3) tidal flushing of seawater into the bay. Freshwater at site 1 brought increased bacteria, dissolved organic matter and chlorophyll and decreased salinity, resulted in gradients from sites 1 to 3 which were strongest in the wet winter months. Summer-time phytoplankton blooms increased chlorophyll, bacteria and dissolved organic carbon (DOC) levels. These maxima were stronger at sites 2 and 3 because of nutrient differences and the decreased influence of freshwater.  This was an unusually dry year; in wetter years, gradients from sites 1 to 3 would be stronger and bloom driven changes less noticeable.

2) Microbial community structure varied between sites and with season, driven by the same processes as above and nutrient availability. Season and site had a significant impact on microbial community composition, while tidal flow did not.

2) Rate constants for acetaldehyde and ethanol uptake and the biological conversion of ethanol to acetaldehyde decreased from sites 1 to 3 and increased in summer. Changes were driven by: 1) increases in bacteria associated with freshwater inflow and summer-time phytoplankton blooms; 2) increases in bacterial metabolism with higher temperatures in summer; and 3) shifts in microbial community structure from estuary flows, nutrient levels, temperature, and seasonal sources and sinks.  Correlations with temperature were strong. Correlations with bacteria levels were weak. Correlations with chlorophyll and DOC were strong at sites 2 and 3.  Some bacteria types were related to ethanol and acetaldehyde degradation rates (Rhonoluna limnophila, Luminiphilus syltensis, Roseobacter denitrificans, Acidothermus cellulolyticus, Roseovarius).

3) Acetaldehyde concentrations were estimated from biodegradation rates and photoproduction rates.  Differences between sites were not significant. However, driven by the seasonal cycle in biological uptake, there was a well-defined seasonal cycle with a maximum in late Winter/early Spring (February/March) and a minimum in Fall (Sept/Oct). Comparison with atmospheric levels from the literature suggested that acetaldehyde would usually move from the atmosphere into the water and occasionally, during the early spring, out of the water.  Ethanol biodegradation was always higher than photochemical production, suggesting these waters are a sink for ethanol from the atmosphere.

3) For most of the year, the biological acetaldehyde production rate was low relative to the photochemical production rate (<20%). However, this increased into the summer, reaching ~80%, suggesting that biological production of acetaldehyde can be significant compared to photoproduction. This could be different in wetter years when the amount of dissolved organic matter and photochemical production rates would be higher.

This research is important because: 1) it improves our understanding of the processes that control the concentrations of acetaldehyde and ethanol in coastal waters; 2) increases our understanding of carbon flow and microbial communities in coastal waters; and 3) improves estimates of the flow of these species between the air and sea and their impact on the chemistry of the atmosphere.

Five undergraduate students (chemistry, biochemistry, environmental science and biology majors) worked on this project, receiving training in research techniques including bacterial and optical measurements, data analysis and scientific writing skills. Two students presented their research at a national American Chemical Society meeting. Two students have gone on to graduate programs, one to medical school and two to careers as lab technicians. One paper has been published in an environmental science journal and an additional three papers are in preparation for publication in aquatic science and biogeosciences journals.

Last Modified: 12/13/2024
Modified by: Catherine D Clark