Atmosphere science Biological oceanography Physical oceanography

Ice ice diatom: how microscopic algae govern ice formation in the clouds

Wilson, TW and Ladino, LA et al. A marine biogenic source of atmospheric ice-nucleating particles. Nature. 10 September 2015. Doi:10.1038/nature14986

Conjuring diatoms … author out of ideas.

Clouds are masses of water, particulates, and chemicals that hover over the earth, varying in their composition depending on altitude. The relationship of clouds to climate is a complicated one, where the ability of clouds to scatter and reflect the intense rays of the sun is counteracted by the insulating properties of water. Scientists are interested in understanding the formation of clouds to improve climate models, while geoengineers might like to know a thing or two about how clouds work toward potentially manipulating climate itself.

Ice particles floating about in the clouds play an important role in the longevity and reflective properties of clouds. Ice formation is accelerated by airborne particulates that bring water molecules together in their nanoscale cracks and crevices seeding ice crystals in a process known as nucleation. A major question in ocean science is the marine origin of particulates that nucleate ice crystals in clouds.

A multi-national team of scientists lead by Theodore Wilson and Luis Ladino think they have found a plausible biological source for nucleating ice particles in the atmosphere that originates in the surface of the ocean. Results of their study appear in last week’s issue of Nature.

You don't Hokusai?! Model showing transport of organic aerosol particles by bubbles that burst at the sea surface.
You don’t Hokusai? Model showing ejection of organic particles into the atmosphere by bubbles that burst at the sea surface. (Image courtesy Wilson et al., Nature, 2015)

Sea-spray is thought to be a major source of particulate matter entering the atmosphere. Put simply, bubbles are formed as a result of mixing of water in air by the action of waves break, and are temporarily trapped below the surface. As the bubbles rush upward they entrap particulate matter, which is released into circulation as an enriched particulate load when they burst at the surface.  Hence, Wilson and co-workers first set out to explore whether the thin layer of surface seawater is enriched in matter that accelerates formation of ice (i.e., nucleating particles). They examined properties of seawater droplets collected from the surface and subsurface at conditions relevant for formation of low altitude mixed-phase clouds (composed of a mixture of liquid water and ice) as well as high altitude ice clouds. In both cases they found that ice formation was more favorable for surface water samples than for subsurface samples, consistent with enrichment of nucleating particles at the water-air interface. For mixed-phase clouds this meant formation of ice at higher temperatures for droplets sampled at the surface as compared to a few meters below the surface, while in the case of ice clouds nucleation was observed at lower humidities for surface samples as compared to subsurface samples. Taken together, these results are consistent with enrichment of ice nucleating matter in surface waters.

Fraction frozen. Panel ‘a’ depicts temperature data for droplets collected at various sites either from the surface microlayer (SML) or from sub-surface seawater (SSW). Droplets from the SML freeze a higher temperatures than SSW controls. Panel ‘b’ shows the relationship between ice nucleating particles (INPs) to dissolved organic carbon. This trend is later applied to a model of dissolved organic carbon in the ocean surface to generate a prediction of ice nucleating particulates in the atmosphere. (Image courtes Wilson et al., Nature, 2015)

Wilson and co-workers next explored the nature of the mystery material driving nucleation. They first wanted to distinguish whether they should be looking for material of animal (organic) or mineral (inorganic) origin. Organic particles such as proteins tend to be thermally unstable as compared to their inorganic counterparts. Upon boiling, the researchers found a decrease in nucleating activity, indicating that a significant organic component may be at play in nucleation. Moreover, looking to terrestrial environments for inspiration they noted that particles emitted by plants (like pollen) and fungi are demonstrated sources of nucleating agents.

Visualizing diatom exudates in sea surface samples. Panels ‘a’ and ‘b’ show X-ray images of two arctic samples analyzed for signature of diatom exudate. Panel ‘c’ show a spectroscopic comparison of the pictured samples to a laboratory prepared sample of diatom exudate (in red); the trace generated is signature of the elemental composition of diatom exudate. (Wilson et al., Nature, 2015)

Next, Wilson and co-workers performed an elemental composition analysis to identify specific biological signatures in their surface samples. They identified a chemical signature consistent with single-cell algae known as a diatom that is found abundantly in surface of waters. Whole and partial diatom cells have been previously shown to possess ice-nucleating behaviors. However, filtering out larger particles using a 0.2-micron filter (reflective of the average diameter of a bacterial cell) indicated that sub-cellular sized particles play a significant role in nucleating activity. To validate the nucleating role of diatom exudate, the researchers reared diatoms in the lab and filtered away the cells to test for the nucleating potential of cellular exudate. They found that that a 0.1-micron-filtered cultures exhibited significant nucleation activity. Hence, Wilson and co-workers showed that diatom poop could plausibly be a significant component in sea-spray that contributes to ice nucleation.

Explaining nucleating particles through surface dissolved carbon. Using measurements from this study, Wilson et al. extrapolated the concentration of ice nucleating particles at the surface (INPs) as depicted in the map in panel ‘a’. Panel ‘b’ shows how observed particulate concentrations line up with those predicted by the model for the Southern Oceans. (Image courtesy Wilson et al., Nature, 2015)

To conclude their study Wilson and co-workers sought global-scale validation for their findings. Based on their measurements of dissolved carbon in their sea surface samples, they extrapolated the global distribution of sea surface layer nucleating particulates from global predictions of dissolved organic carbon. Their model based only on marine inputs was able to explain measured nucleating particulate content over the Southern Oceans.  They further explored the ability of particulates to actually reach altitudes where they would be relevant for ice formation, and found that marine organic inputs rivaled terrestrial mineral inputs in regions of high biological activity (hight production of organic matter) and strong winds (effective transport) such as the Southern Oceans and North Atlantic.

The work of Wilson and co-workers demonstrates for the first time that organic material of likely biological origin in the surface layer of the ocean is capable of nucleation of ice at conditions relevant to two routes of cloud formation. Their study highlights a new and potentially significant role for biology in regulating the climate over the world’s oceans. Intrigued? Get a PhD!


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