Biological degradation of ethanol in Southern California coastal seawater
Introduction
Increased use of ethanol in gasoline, either as an oxygen additive to increase octane or lower carbon monoxide (CO) emissions and surface ozone (O3) levels or as a renewable fuel to decrease dependence on gasoline, has increased interest in understanding ethanol's cycling in the environment (e.g. Naik et al. 2010; Kirstine and Galbally, 2012, Avery Jr. et al., 2016). In the United States of America (USA), the Environmental Protection Agency (EPA) introduced the Oxyfuel Program and the Reformulated Gasoline Program in the early 1990s to reduce wintertime CO levels in areas with high wintertime CO levels and to reduce urban O3 levels in cities which exceeded surface ozone guidelines. The Oxyfuel Program mandated that gasoline had to have 2.7% oxygen by weight during wintertime and the Reformulated Gasoline Program required that gasoline have 2% oxygen by weight year round. The Renewable Fuel Standard Program, started as part of the Energy Policy Act of 2005, and expanded under the Energy Independence and Security Act of 2007, increased the use of ethanol in gasoline across the USA (www.epa.gov). The Energy Independence and Security Act calls for the use of 36 billion gallons of renewable fuel by 2022. Due to these policies and legislation, almost all gasoline in the USA today contains ethanol. Most gasoline used in the USA today has less than 10% ethanol; however, Flex-Fuel vehicles are capable of using gasoline with up to 85% ethanol (E85; www.eia.gov). Globally many countries have pursued similar or even more aggressive ethanol programs. For example, in Brazil, ethanol use has been mandatory since 1977 and today ethanol blends vary from a minimum of 18% to a maximum of 27.5% (Barros, 2016).
Increased use of ethanol in the USA and globally as a fossil fuel substitute and additive is expected to have an impact on ethanol levels in the atmosphere. Significantly higher levels of ethanol have been measured in urban air in cities that use ethanol additives relative to cities that do not (Nguyen et al., 2001). Based on the work of Jacobson (2007), Millet et al. (2010) estimated that a shift to an E85 automobile fleet in the US would increase ethanol emissions in the USA from 1.3 to 3.4 Tg yr−1 (an increase of 2.1 Tg yr−1). In the atmosphere, gas phase ethanol contributes to the production of acetaldehyde, peroxyacetyl nitrate (PAN), and ozone in urban areas (Tanner et al., 1988) and is a potential secondary aerosol precursor (Blando and Turpin, 2000; Naik et al. 2010). The primary sink for ethanol in the troposphere is reaction with OH (Naik et al. 2010; Kirstine and Galbally, 2012). As a source of ozone and a sink for HOx, increased levels of ethanol can potentially have a significant impact on the oxidizing power of the troposphere (Tanner et al., 1998; Naik et al. 2010; Singh et al., 2004; de Gouw et al., 2005). Ethanol's atmospheric importance and the potential for significant shifts in global ethanol emissions have generated a number of recent attempts to understand the atmospheric ethanol budget (Naik et al. 2010; Kirstine and Galbally, 2012).
Budget calculations agree that the primary source of ethanol to the atmosphere is plant emissions and that the primary atmospheric sink is reaction with OH. Biofuels, industrial processes and biomass burning are smaller sources and in-situ production is a relatively minor source (Naik et al. 2010; Kirstine and Galbally, 2012). Dry deposition to land and wet deposition are minor sinks. However, there are significant discrepancies in the sizes of sources and sinks. Naik et al. (2010) estimated the plant emissions source to be 9.2 Tg/yr with a total source of 14.7 Tg/yr in a baseline simulation. In a more recent budget analysis, Kirstine and Galbally (2012) added a small ocean source (4 Tg/yr) and suggest that the emissions from living plants are significantly higher than suggested in earlier budgets. They report a plant emission source of 26 Tg/yr and a total ethanol source of 42 Tg/yr. They also report a significantly larger OH ethanol sink than previous budget estimates. Overall budget calculations are hindered by the limited database of atmospheric ethanol concentration measurements (Kirstine and Galbally, 2012).
These budget estimates also highlight our current limited understanding of the role of the oceans in cycling ethanol into or out of the troposphere. Naik et al. (2010) suggested that there might be a large missing remote oceanographic source of ethanol. Beale et al. (2010) reported the first measurements of ethanol concentrations in seawater with levels ranging from 2 to 33 nM in the Atlantic Ocean. Based on their measurements and estimated atmospheric levels, Beale et al. (2010) calculated ethanol air-sea fluxes and showed that fluxes were sometimes out of and sometimes into the ocean. When ethanol levels were at a maximum in seawater (night time in Mauritanian upwelling and daytime in oligotrophic water), the oceans were a source of ethanol to the atmosphere. Conversely, when ethanol levels were a minimum, the oceans tended to be a sink. Based on the complete dataset, they report an average flux of ethanol out of the ocean and suggested that the oceans could be an important source of ethanol to the atmosphere. In their budget calculations, Kirstine and Galbally (2012) assumed a small global oceanographic source based on the work of Beale et al. (2010). More recently, Avery Jr. et al. (2016) measured concentrations of ethanol up to 598 nM ethanol in fresh, estuary, and coastal waters. Even with these relatively high aqueous phase concentrations, they reported under-saturated freshwater and estuary surface waters, and concluded that these waters are likely a significant sink for atmospheric ethanol. They suggested that aqueous ethanol is rapidly converted to acetaldehyde by microorganisms in these ecosystems. Their coastal water measurements, like the Beale et al. (2010) open ocean measurements, suggested fluxes that are at times into and at times out of the ocean. Expanding the database of seawater ethanol measurements and improving our understanding of the processes that produce and destroy ethanol in seawater would improve our understanding of the role of the oceans in cycling ethanol into or out of the troposphere.
Ethanol can be lost in seawater via physical, chemical, photochemical, and/or biological pathways. Little is known about potential ethanol chemical and photochemical loss processes in seawater. While there is some indirect evidence for a photochemical ethanol sink in seawater (Beale et al., 2010), limited absorption by ethanol in the UV-VIS region suggests direct photolysis is unlikely. Ethanol is expected to be rapidly biodegraded in aquatic ecosystems. There have been a number of studies that have looked at ethanol biodegrading in the context of gasoline spills and underground storage tank leaks with ethanol as an additive (Corseuil et al., 1998; Schaefer et al., 2010). Much of this work has focused on the potential impact of ethanol biodegradation on BTEX (benzene, toluene, ethylbenzene, xylenes) compounds in gasoline during gasoline spills and leaks. Reported ethanol biodegradation rates are relatively fast and ethanol biodegradation tends to slow the degradation of other species, which allows BTEX compounds to travel longer distances (Corseuil et al., 1998; Powers et al., 2001; Schaefer et al., 2010). However, for the most part, ethanol biodegradation rates reported in these gasoline spill studies have been made at high ethanol concentrations representative of gasoline spills, and most have been aquifer microcosm studies (Corseuil et al., 1998; Ruiz-Aguilar et al., 2002; Chen et al., 2008; Schaefer et al., 2010) or field aquifer studies (Pirnie, 1998; Zhang et al., 2006). There is some evidence in these studies that the ethanol biological degradation rate is concentration dependent, increasing at lower concentrations (Chen et al., 2008). The microorganism and nutrient environment is also likely to be very different in aquifers and various surface waters. There is one estimate of the biological degradation rate in surface waters based on a single river sample in the literature (Apoteker and Thevenot, 1983; Howard et al., 1991; Pirnie, 1998). To the best of our knowledge, there are no measurements of biodegradation of ethanol for ambient ethanol non-spill levels in seawater. To add to our understanding of ethanol cycling in the environment and to help constrain the role of the oceans in cycling ethanol into or out of the atmosphere, the rates of chemical and biological consumption of ethanol in seawater were measured in southern California coastal waters over a six-month period at near ambient ethanol levels. Preliminary photochemical degradation experiments were also carried out to test the extent of an ethanol photochemical loss process.
Section snippets
Site and sample preparation
Pacific Ocean water samples (1 L) were collected between September 2015 and April 2016 in coastal waters at the Santa Ana River mouth (SAR) at Huntington State Beach (HSB) (33o37′32″ N; 117o57′01″ W) in Orange County, Southern California, U.S.A. The site is a relatively well studied site which has been described in some detail by others (e.g. Grant et al., 2001; Boehm et al., 2002; Izbicki et al., 2004; Ahn et al., 2005; Ahn and Grant, 2007; de Bruyn et al., 2017). The SAR is typical of
Ancillary measurements
During the winter months in southern California, regional storm activity increases and with it rainfall and river stream flow. Santa Ana River stream gauge flow and regional rainfall totals (station 121) are plotted as a function of time in Fig. 1 (top panel). Rainfall totals were obtained from the Orange County Public Works (OCPW, 2019) and stream gauge data was obtained from the United States Geological Survey (USGS, 2019). A total of 17.6 cm of rain were recorded over the study period. There
Conclusions
An understanding of the processes that produce and destroy ethanol in seawater would help constrain our understanding of the cycling of ethanol into or out of the oceans. Theoretically, ethanol can be produced and lost in seawater via photochemical, biological, and chemical mechanisms and physical processes like air-sea exchange. Overall, the relative strengths of these potential sources and sinks are not well known. While conclusions can be drawn from observed trends in ethanol concentrations
Acknowledgements
The authors thank the National Science Foundation (OCE # 1233091; CHE #1337396) for funding this work. AH was supported by a fellowship from the Grand Challenges Initiative at Chapman University.
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