CHEMISTRY AND THE UNIVERSE
Cloudy, with a Chance of Microbes
by Nancy K. McGuire
October 5, 2015
Marine microorganisms may affect Earth’s climate by driving ice crystal formation in clouds.
Clouds account for roughly two-thirds of the sunlight that the Earth reflects back into space. They are, however, the least understood component of the climate system and the largest source of uncertainty in climate change models. Knowing the amount of ice a cloud contains under a given set of conditions would provide valuable insight into the cloud’s longevity, how much rain or snow it produces, and how much sunlight it reflects.
Figure 1 shows “sun dogs”: bright spots on either side of the sun that are produced when visible light interacts with ice in the atmosphere.
Aerosol particulates drive cloud formation by acting as nuclei around which water droplets or ice crystals condense. These particulates include mineral dust, sea salt, and fossil-fuel residues; but they also come from biological sources. Worldwide, marine sources emit an estimated 5–15 million tonnes of organic submicrometer aerosol every year.
Icy clouds from plankton
How does organic material from deep in the ocean end up high in the clouds? Particles ride to the surface on rising bubbles. At the surface, bursting bubbles create an organic-rich surface microlayer; and they spray organic matter into the air. Air currents carry the smallest particles aloft.
The first reported study of potential ice-nucleating material from the surface microlayer was recently published in Nature. Theodore W. Wilson at the University of Leeds (UK) and 28 coauthors studied the freezing behavior of seawater samples by using lab analysis, modeling, and a laboratory cold stage under conditions that simulate cloud formation.
Crystalline salt particles did not contribute markedly to ice nucleation at low relative humidity, but aerosol particles derived from the surface microlayer did. The Nature study determined that the ice-active particles probably come from ultramicrobacteria, viruses, or extracellular material from bacteria or phytoplankton, including diatoms, a type of alga. Figure 2 shows diatoms that live between crystals of annual sea ice in McMurdo Sound, Antarctica.
The authors found amino acids, polysaccharides, lipids, and proteins from diatom cell walls and exudates in Arctic seawater samples. They showed that the Thalassiosira pseudonana diatom exudates can nucleate ice.
Marine organic material could be an important source of ice-nucleating particles in remote, high-latitude marine environments, according to the modeling studies. Desert mineral dust and industrial pollution particles are less common in these environments. (Nature DOI: 10.1038/nature14986)
Several coauthors of the Nature paper previously demonstrated that diatoms placed in supercooled water vapor increase the ice formation temperature by as much as 13 ºC at relative humidity levels as low as 65%. When sodium chloride was present, the diatoms increased the ice formation temperature as much as 30 ºC. (Knopf, D. A., et al. Nat. Geosci. DOI: 10.1038/ngeo1037)
Related research by some of these scientists showed that organic coatings on sea salt particles contain decomposition products from phytoplankton and other organisms. Two components of these coatings, nonadecanol and nonadecanoic acid, can raise the temperature at which ice crystals form. (School of Marine and Atmospheric Sciences, Stony Brook University [NY])
Two blooms, two outcomes
A recent study published in ACS Central Science examined the effects of phytoplankton blooms on sea spray aerosol composition. In this study, breaking waves from 3400 gal (12,870 L) of natural seawater in a laboratory wave channel (see Figure 3) produced an aerosol overhead. Nutrients added to the seawater spurred two phytoplankton blooms over a 29-day period.
As the first bloom developed, the concentration of ice-nucleating species in the sea spray aerosol, including amphiphilic long-chain alcohols, increased concurrently. The second phytoplankton bloom did not enrich organic species in the submicrometer droplets.
Heterotrophic bacteria concentrations, however, were much higher during the second bloom, possibly because they were fed by the lipids from the first bloom. Bacterial enzymes may have degraded the lipids to less active, more soluble species such as fatty acid salts that were transferred less efficiently into the submicrometer droplets. (ACS Cent. Sci. DOI: 10.1021/acscentsci.5b00148)
Marine aerosols go ashore
Marine organic aerosols could influence inland cloud formation, according to a modeling study reported by B. Gantt at North Carolina State University (Raleigh) and coauthors in Geoscientific Model Development. The authors found that these aerosols travel farther inland than sea salt aerosols. Thus, >10% of the surface submicrometer organic aerosol mass concentration over several inland European cities can be traced to marine sources.
The authors added marine organic aerosol tracers to the GEOS–Chem (Global Earth Observing System Chemistry) model. This new capability made it possible to identify regions with large contributions from aerosols with specific physicochemical properties. (Geosci. Model Dev. DOI: 10.5194/gmd-8-619-2015)
The effects of agricultural and industrial effluents on marine microbial blooms have been studied extensively. We still have much to learn, however, about how human activities affect the interplay between microbes and the atmosphere. We know still less about how this interplay drives long-term climate phenomena.