Frances E. Hopkins

Ocean acidification and marine trace gas emissions

The oceanic uptake of man-made CO2 emissions is resulting in a measureable decrease in the pH of the surface oceans, a process which is predicted to have severe consequences for marine biological and biogeochemical processes [Caldeira K, Wickett ME (2003) Nature 425:365; The Royal Society (2005) Policy Document 12/05 (Royal Society, London)]. Here, we describe results showing how a doubling of current atmospheric CO2 affects the production of a suite of atmospherically important marine trace gases. Two CO2 treatments were used during a mesocosm CO2 perturbation experiment in a Norwegian fjord (present day: ∼380 ppmv and year 2100: ∼750 ppmv), and phytoplankton blooms were stimulated by the addition of nutrients. Seawater trace gas concentrations were monitored over the growth and decline of the blooms, revealing that concentrations of methyl iodide and dimethylsulfide were significantly reduced under high CO2. Additionally, large reductions in concentrations of other iodocarbons were observed. The response of bromocarbons to high CO2 was less clear cut. Further research is now required to understand how ocean acidification might impact on global marine trace gas fluxes and how these impacts might feed through to changes in the earths future climate and atmospheric chemistry.

Decreased marine dimethyl sulfide production under elevated CO2 levels in mesocosm and in vitro studies

Environmental context As atmospheric CO2 levels rise due to human activities, more of the gas dissolves in the oceans, increasing their acidity. The effect of these seawater changes on marine organisms is largely unknown. We examine the consequences of higher CO2 levels on the production by plankton of dimethyl sulfide, a climatically active gas. We find that higher CO2 levels leads to lower concentrations of dimethyl sulfide in the seawater, which has potentially important implications for the future climate. Abstract The oceans have absorbed approximately half of the CO2 produced by human activities and it is inevitable that surface seawaters will become increasingly acidified. The effect of lower pH on marine organisms and ocean–atmosphere exchanges is largely unknown but organisms with CaCO3 structural components are likely to be particularly affected. Because calcifying phytoplankton are significant producers of dimethyl sulfide (DMS), it is vital to understand how lower seawater pH may affect DMS production and emission to the atmosphere. Here we show, by mesocosm (Raunefjorden, Norway, April–May 2003) and in vitro studies, that the net production of DMS and its cellular precursor dimethylsulfoniopropionate (DMSP) is approximately halved in microbial communities subjected to doubled CO2 levels. Our findings provide evidence that the amount of DMS entering the atmosphere could decrease in the future. Because atmospheric oxidation of DMS can lead to climate cooling by increasing cloud albedo, a consequence of reduced DMS emissions from a lower pH ocean would be an enhancement in global warming.

Ocean acidification has different effects on the production of dimethylsulfide and dimethylsulfoniopropionate measured in cultures of Emiliania huxleyi and a mesocosm study: a comparison of laboratory monocultures and community interactions

Environmental context. Approximately 25 % of CO2 released to the atmosphere by human activities has been absorbed by the oceans, resulting in ocean acidification. We investigate the acidification effects on marine phytoplankton and subsequent production of the trace gas dimethylsulfide, a major route for sulfur transfer from the oceans to the atmosphere. Increasing surface water CO2 partial pressure (pCO2) affects the growth of phytoplankton groups to different degrees, resulting in varying responses in community production of dimethylsulfide. Abstract. The human-induced rise in atmospheric carbon dioxide since the industrial revolution has led to increasing oceanic carbon uptake and changes in seawater carbonate chemistry, resulting in lowering of surface water pH. In this study we investigated the effect of increasing CO2 partial pressure (pCO2) on concentrations of volatile biogenic dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP), through monoculture studies and community pCO2 perturbation. DMS is a climatically important gas produced by many marine algae: it transfers sulfur into the atmosphere and is a major influence on biogeochemical climate regulation through breakdown to sulfate and formation of subsequent cloud condensation nuclei (CCN). Overall, production of DMS and DMSP by the coccolithophore Emiliania huxleyi strain RCC1229 was unaffected by growth at 900 μatm pCO2, but DMSP production normalised to cell volume was 12 % lower at the higher pCO2 treatment. These cultures were compared with community DMS and DMSP production during an elevated pCO2 mesocosm experiment with the aim of studying E. huxleyi in the natural environment. Results contrasted with the culture experiments and showed reductions in community DMS and DMSP concentrations of up to 60 and 32 % respectively at pCO2 up to 3000 μatm, with changes attributed to poorer growth of DMSP-producing nanophytoplankton species, including E. huxleyi, and potentially increased microbial consumption of DMS and dissolved DMSP at higher pCO2. DMS and DMSP production differences between culture and community likely arise from pH affecting the inter-species responses between microbial producers and consumers.

Air exposure of coral is a significant source of dimethylsulfide (DMS) to the atmosphere

Corals are prolific producers of dimethylsulfoniopropionate (DMSP). High atmospheric concentrations of the DMSP breakdown product dimethylsulfide (DMS) have been linked to coral reefs during low tides. DMS is a potentially key sulfur source to the tropical atmosphere, but DMS emission from corals during tidal exposure is not well quantified. Here we show that gas phase DMS concentrations (DMSgas) increased by an order of magnitude when three Indo-Pacific corals were exposed to air in laboratory experiments. Upon re-submersion, an additional rapid rise in DMSgas was observed, reflecting increased production by the coral and/or dissolution of DMS-rich mucus formed by the coral during air exposure. Depletion in DMS following re-submersion was likely due to biologically-driven conversion of DMS to dimethylsulfoxide (DMSO). Fast Repetition Rate fluorometry showed downregulated photosynthesis during air exposure but rapid recovery upon re-submersion, suggesting that DMS enhances coral tolerance to oxidative stress during a process that can induce photoinhibition. We estimate that DMS emission from exposed coral reefs may be comparable in magnitude to emissions from other marine DMS hotspots. Coral DMS emission likely comprises a regular and significant source of sulfur to the tropical marine atmosphere, which is currently unrecognised in global DMS emission estimates and Earth System Models.

Attribution of Atmospheric Sulfur Dioxide over the English Channel to Dimethylsulfide and Changing Ship Emissions

Atmospheric sulfur dioxide (SO2) was measured continuously from the Penlee Point Atmospheric Observatory (PPAO) near Plymouth, United Kingdom, between May 2014 and November 2015. This coastal site is exposed to marine air across a wide wind sector. The predominant southwesterly winds carry relatively clean background Atlantic air. In contrast, air from the southeast is heavily influenced by exhaust plumes from ships in the English Channel as well as near Plymouth Sound. A new International Maritime Organization (IMO) regulation came into force in January 2015 to reduce the maximum allowed sulfur content in ships’ fuel 10-fold in sulfur emission control areas such as the English Channel. Our observations suggest a 3-fold reduction in shipemitted SO2 from 2014 to 2015. Apparent fuel sulfur content calculated from coincidental SO2 and carbon dioxide (CO2) peaks from local ship plumes show a high level of compliance to the IMO regulation (> 95 %) in both years (∼ 70 % of ships in 2014 were already emitting at levels below the 2015 cap). Dimethyl sulfide (DMS) is an important source of atmospheric SO2 even in this semi-polluted region. The relative contribution of DMS oxidation to the SO2 burden over the English Channel increased from about one-third in 2014 to about one-half in 2015 due to the reduction in ship sulfur emissions. Our diel analysis suggests that SO2 is removed from the marine atmospheric boundary layer in about half a day, with dry deposition to the ocean accounting for a quarter of the total loss.

A New Perspective at the Ship-Air-Sea-Interface: The Environmental Impacts of Exhaust Gas Scrubber Discharge

Shipping emissions are likely to increase significantly in the coming decades, alongside increasing emphasis on the sustainability and environmental impacts of the maritime transport sector. Exhaust gas cleaning systems (“scrubbers”), using seawater or fresh water as cleaning media for sulfur dioxide, are progressively used by shipping companies to comply with emissions regulations. Little is known about the chemical composition of the scrubber effluent and its ecological consequences for marine life and biogeochemical processes. If scrubbers become a central tool for atmospheric pollution reduction from shipping, modeling, and experimental studies will be necessary to determine the ecological and biogeochemical effects of scrubber wash water discharge on the marine environment. Furthermore, attention must be paid to the regulation and enforcement of environmental protection standards concerning scrubber use. Close collaboration between natural scientists and social scientists is crucial for progress toward sustainable shipping and protection of the marine environment.