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Dimethylsulfide Emission: Climate Control by Marine Algae?
(Released November 2003)

 
  by Katina Bucher Norris  

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Introduction

Ever notice the salty sea smell when you're out on or near the ocean? It's the salt spray tossed from wind-driven white caps and breaking waves, but the smell isn't from the salt alone. Gases diffuse across the air-sea interface, many of which are synthesized and emitted by microalgae. One of these gases is a sulfur based compound that has a strong characteristic odor. It has been suggested that variations in algal production of these natural gases play an important role in moderating our climate through their aerosols' effect on backscattering solar radiation and in cloud formation.1

Trace concentrations of a sulfurous gas were discovered in the Earth's atmosphere about three decades ago. The gas was also found to occur in ocean surface waters. Scientists have identified the sulfurous gas as dimethylsulfide (DMS). While it may sound like a noxious pollutant, dimethylsulfide is a naturally produced biogenic gas essential for the Earth's biogeochemical cycles. Learning more about this crucial gas will enhance our understanding of food chains and global scale climate processes, and allow for more intelligent environmental management.

In the ocean dimethylsulfide is produced through a web of biological interactions. Certain species of phytoplankton, microscopic algae in the upper ocean, synthesize the molecule dimethylsulfoniopropionate (DMSP) which is the precursor to DMS.2 When phytoplankton cells are damaged, for example by grazing zooplankton or viral lysis, they release their contents into the seawater. Bacteria and phytoplankton are involved in degrading the released algal sulfurous compound DMSP to DMS and other products. A portion of the DMS diffuses from saltwater to the atmosphere. Once it is transferred to the atmosphere the gaseous DMS is oxidized to tropospheric sulfate aerosols, and these particulate aerosols act as cloud condensation nuclei (CCN), attracting molecules of water. Water vapor condenses on these CCN particles, forming the water droplets that make up clouds.3 Clouds affect the Earths radiation balance and thereby greatly influence its temperature and climate. DMS represents 95% of the natural marine flux of sulfur gases to the atmosphere, and scientists estimate that the flux of marine DMS supplies about 50% of the global biogenic source of sulfur to the atmosphere.4

The Production of DMS

Marine phytoplankton live in the sunlit waters of the world's upper oceans, an ecosystem covering about two thirds of the planet. Dimethylsulfoniopropionate produced within phytoplankton cells is thought to have a number of important physiological functions. Some microalgal species contain a high percentage of intercellular DMSP. This compound may act as an osmolyte, a neutral solute that reacts minimally with the contents of the cell while protecting it from drying out, or in the cell's response to salinity changes.5 In cold environments DMSP may act as a cryoprotectant, protecting the cell from freezing. It has also been suggested that DMSP acts as an antioxidant, scavenging free radicals and helping the algae to tolerate stressful conditions, such as high solar radiation or iron deficient water.6 Research indicates a direct link between oxidative stressors and the dynamics of DMSP and DMS in marine phytoplankton.7

The synthesis of the volatile organic compound DMS seems to be species specific. While many algae do produce high concentrations of DMS, for example prymesiophytes and dinoflagellates, including many bloom forming taxa e.g. Phaeocystis, Emiliania, and Alexandrium, lesser amounts are found in other phytoplankton.8 There is not a linear relationship between the concentration of DMS in ocean surface waters and the density of phytoplankton, because different phytoplankton have DMSP in varying amounts and not all have the enzyme DMSP-lyase. The DMS concentration is dependent on numerous biotic interactions, most not well understood yet, as investigating the microbial food web is a technical challenge.

The DMS concentration results from the combined effects of DMSP production and removal processes. DMSP is release by damaged phytoplankton cells due to physical stress (e.g. turbulence, zooplankton grazing or viral lysis) and subsequently transformed by phytoplankton and bacterial enzymes to DMS. Many bacteria have DMSP-lyase and are thought to play a significant part in converting the algal DMSP to DMS, while other types of bacteria consume the DMSP.9 Photochemical reactions and ultraviolet radiation can degrade DMS to further break down products, removing DMS. The rate of DMS flux to the atmosphere is primarily dependent on its concentration in sea water.

DMS Effects on Grazing

DMSP degradation products (i.e. DMS and acrylate) may also act as deterrents to grazing by herbivores. These compounds are released from phytoplankton when they are damaged by grazing zooplankton (e.g. ciliates, tintinnids, copepods, fish and invertebrate larvae).10 Certain species of phytoplankton contain DMSP-lyase that quickly converts the benign DMSP to the unpalatable DMS and acrylate when the cell is injured. These sulfurous compounds that have been proposed to act as chemical defenses against consumption by zooplankton. In feeding experiments, Wolfe et al. found that zooplankton grazers, including protozoan herbivores, prefer to consume algae without DMSP-lyase.11

Some species of the larger ocean plants, the seaweeds, also contain DMSP, and ecologists are determining whether the compound or its breakdown products are a deterrent to herbivore consumption as well. Van Alstyne et al. surveyed Pacific seaweeds growing along the shores of Oregon and Washington, and detected DMSP in numerous green algal species and in one red alga.12 DMSP-lyase activity was found in Ulva fenestra and Polysiphonia hendryi, so these species have the enzyme to convert DMSP to DMS and depending upon the pH, either acrylate or acrylic acid. In laboratory feeding preference trials, sea urchins (Strongylocentrotus) were attracted to diets containing DMSP and avoided diets containing acrylic acid, except at the lowest test concentration. Acrylic acid appears to be a successful deterrent against the two sea urchin species tested, but it did not discourage the herbivorous crustacean Idotea from feeding.13

The Transfer of DMS to the Air and the Marine Food Web

Along Antarctic polar fronts, upwelling brings nutrient rich water to the surface, supporting dense algal blooms. Zooplankton, including krill (Euphausia superba), exploit this enriched environment by swarming to feed on these phytoplankton blooms. As the phytoplankton are consumed, DMSP is released and converted to DMS, and a portion of it is volatized to the atmospheric boundary layer. In fact, elevated concentrations of DMS above the ocean surface can be an indication of dense aggregations of zooplankton feeding beneath the surface. Other animals within the Antarctic marine food web (e.g. fish and squid) are also abundant in these more productive environs.

Large populations of Antarctic procellariiform seabirds are also associated with sites of high DMS concentrations.14 Field tests indicate that these birds are sensitive to a variety of odors associated with their prey, including the odors of phytoplankton (DMS) and krill. Nevitt hypothesized that these seabirds may have an excellent sense of smell and use the sulfurous gas to locate the most productive feeding grounds in the visually monotonous open sea environment.15 Using DMS as a foraging cue would improve the birds' success, as the odor guides them to areas of higher productivity and greater densities of prey (krill and other crustaceans). Procellariiform seabirds have evolved large, well-developed olfactory organs.

Nevitt et al. set up experiments in sub-Antarctic waters, near South Georgia, deploying DMS-scented surface slicks in one area and controls of slicks without DMS scent in a second area to observe the birds' behavioral responses.16 Results from paired slick experiments showed that some species of petrels and other birds can detect and are attracted to the DMS-scented surface slicks, while other birds which are probably visual predators, were not. In another experiment, DMS was wafted in air plumes to observe the birds' flight behavior. The same species that were attracted to scented slicks turned in response toward the air-borne DMS. DMS is part of the olfactory landscape over the Antarctic Ocean. Planktivorous seabirds seem to use DMS as an olfactory signal to navigate to zooplankton-rich sites.17

DMS and the Global Sulfur Cycle and Climate

A key process in the sulfur cycle is the transfer of volatile sulfur compounds from the sea to the land via the atmosphere. DMS is the dominant biogenic sulfur compound in the marine atmosphere and essential to the global sulfur cycle. The gaseous DMS is photo-oxidized to sulfated aerosols in the atmosphere. Berresheim et al. established a relationship between DMS, sulfate aerosols, and cloud condensation nuclei.18 Because the sulfate aerosols function as cloud condensation nuclei, DMS has a significant impact on the Earth's climate. Plankton production of DMS and its escape to the atmosphere is believed to be one of the mechanisms by which the biota can regulate the climate.

Dimethylsulfide (DMS) Cycle, Ocean / Atmosphere Exchange
Recreated From: http://me-www.jrc.it/dms/dms.html

The radiation balance has a fundamental effect on Earth's climate. About one third of the solar radiation that reaches the Earth is reflected back into space by clouds and from earth surfaces, such as ice and snow. The atmosphere absorbs some solar energy, but most of the other two thirds is absorbed by the land and oceans, which are warmed by the sunlight. The sun's energy is converted into heat, and the land and oceans then radiate a portion of this energy back as outgoing long-wave radiation (infrared), also known as terrestrial radiation. As this energy is radiated back out, it warms the atmosphere and continues on into space. The amount of solar energy received by the Earth, the planetary albedo (the amount reflected back) and the emitted terrestrial radiation, makes up the Earth's radiation balance. If the Earth receives more energy than it loses, the result is global warming, and if it loses more energy than it receives, the result is global cooling.

Albedo is an important factor in the radiation balance, and clouds have the major effect on albedo. The optical properties of a cloud are a key issue to understanding and therefore predicting global climate change. A cloud's optical properties are related to the size distribution and number of its droplets. The more cloud condensation nuclei, the smaller the size of its water droplets and the higher the density of water droplets since the same amount of water vapor is distributed among a greater number of CCN.19 This affects the radiative properties (reflectance, transmittance and absorbance) of the cloud.

Clouds affect both incoming solar and outgoing thermal infrared fluxes; low thick clouds act as shields, blocking and reflecting solar radiation back into space which cools the planet, but high clouds can also trap outgoing heat (longwave radiation), warming the planet. Data indicate that clouds have an overall net cooling effect.20 The smaller droplet size will likely decrease precipitation, resulting in a longer lifetime for a cloud.21 Climate change scientists realized their models had a poor ability to reproduce the effects of clouds, so they set a priority to observe, measure and learn about clouds' physical properties and radiative fluxes. Several international programs (CERES and other projects) are observing clouds from space using multiple satellites to more accurately quantify cloud properties and their impact on albedo. The results of these programs will improve the modeling of cloud physics, so climate models will provide a more credible simulation of climate change projections. DMS may influence both the hydrologic cycle and the global heat budget through its part in cloud formation, and may alter rainfall patterns and temperatures.

The Earth as an Organism

Although progress has been made in understanding the biological, physical and chemical reactions in the DMS cycle, much remains to be investigated. In order to discover the factors controlling the atmospheric concentration of DMS, we need to know the following about what affects its production in the ocean and escape to the atmosphere: 1) which phytoplankton species are high in DMSP and which have DMSP-lyase, 2) the species composition of the phytoplankton community and its succession in an area, 3) their global distribution and population density, and 4) other biotic interactions that effect DMS concentrations (e.g. zooplankton and bacteria populations). Abiotic factors, such as sea surface temperature and mixed layer depth, also have a direct impact on DMS production.

The Gaia Hypothesis suggests that the Earth (biosphere and more) functions as an organism to maintain homeostasis, to keep the planet fit for life.22 NASA used the fact that organisms actively change the atmosphere' s chemistry through their living processes to search for extraterrestrial life. DMS is part of the Earth's ocean-atmosphere feedback loop, a climate stabilizing mechanism, moderating temperatures on Earth. Sciare et al. found a direct link between sea surface temperature and atmospheric DMS over a large area in the southern Indian Ocean. They estimate that an increase in temperature would increase the atmosphere's DMS concentration and have a negative feedback on the original warming.23

The photic zone is a complex habitat where phytoplankton, although tiny in size, through their number and volume are critical to Earth's natural cycles. Phytoplankton activities have global consequences on atmospheric chemistry through their production of oxygen, are the base of most ocean food chains, interact in the global carbon cycle by using the greenhouse gas carbon dioxide, and are a carbon sink through their sedimentation of carbonates to the sea floor. Microalgae also play an important role in mediating global climate through variations in DMS production and its influence on cloud formation.

It is imperative to understand the chemical reactions involving dimethylsulfide on a global scale, from polar to tropical waters, so that models can be developed to accurately estimate the natural and human impacts on atmospheric chemistry and climate. To be better stewards of our planet we need to understand its natural cycles, and the environmental consequences of human actions. As a practical measure we need to sustain our life support system, as we are all dependent on natural systems for breathable air, clean water, dependable rainfall, and temperature and climate stability.

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