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Atmospheric Chemistry

Marine microbial activity poses restrictions on cloud formation

Article: Wang, X., Sultana, C.M., Trueblood, J., Hill, T.C.J., Malfatti, F., Lee, C., Laskina, O., Moorse, K.A., Beall, C.M., McCluskey, C.S., Cornwell, G.C., Zhou, Y., Cox, J., Pendergraft, M.A., Santander, M.V., Bertram, T.H., Cappa, C.D., Azam, F., DeMott, P.J., Grassian, V.H., Prather, K. 2015. Microbial Control of Sea Spray Aerosol Composition: A Tale of Two Blooms. ACS Cent. Sci. DOI: 10.1021/acscentsci.5b00148

Marine microbial activity poses restrictions on cloud formation

While it’s commonly accepted within the scientific community that Earth’s climate is changing, there are many aspects of climate change that are still not fully understood. For the most part, Earth absorbs energy from the Sun, which generates heat (that’s what ultimately creates weather patterns in the troposphere). But, it turns out there a lots of factors that can influence the amount of solar radiation absorbed and converted into heat, such as the angle of the Earth relative to the sun, ozone in the atmosphere, cloud cover, etc.

Aerosols are a major component of the atmosphere that can influence this radiation budget. Some aerosols may reflect incoming solar radiation, slightly decreasing the amount of radiation the Earth absorbs. On the other hand, some aerosols might absorb solar radiation and cause a warming of the atmosphere. Additionally, some aerosols serve as scaffolding that helps ice accumulate around them, seeding and creating clouds, which also have a large influence on Earth’s radiation budget. This complex nature of aerosols makes it very difficult to understand the true impact they have on regulating climate, making them a hot area of study.

A recent study published by an international collaborative group conducted at Scripps Institute of Oceanography shows how complex biological parameters influence the chemical composition of marine aerosols (also known as sea spray aerosols, or SSA), which can impact climate processes.

How they did it

The researchers, led by Dr. Xiaofei Wang, used a 3,400 gallon wave channel to mimic natural ocean conditions to study SSA generation. They used filtered sea water in their wave chamber and supplied extra nutrients to facilitate phytoplankton growth. They generated breaking waves to naturally produce SSA.

SSA was collected and analyzed by a suite of mass spectrometry methods to determine the organic matter content, the types of organic matter, and elemental ratios. Chlorophyll A was measured throughout as a proxy measurement of phytoplankton biomass. Cell counts were made to assess heterotrophic bacteria abundance and enzyme activity was measured to gauge degradation rates.

What they found

Dr. Wang and his colleagues found that SSA could, very generally, be separated by size classes. The submicron fraction consisted of mainly aliphatic organic compounds (mainly carbons and hydrogens, with little to no oxygen leading to a low oxygen to carbon ratio, such as lipids) while larger organic particles were much more dominated by oxygen-rich molecules with much higher oxygen to carbon ratios (Fig 1).

Figure 1. SSA can be decomposed into various categories that seem to overlap nicely with each other. Small SSA particles tend to be aliphatic rich (AR) compounds with low oxygen to carbon ratios while larger SSA particles tend to be more oxygen rich (OR) compounds with higher oxygen to carbon ratios.

Figure 1. SSA can be decomposed into various categories that seem to overlap nicely with each other. Small SSA particles tend to be aliphatic rich (AR) compounds with low oxygen to carbon ratios while larger SSA particles tend to be more oxygen rich (OR) compounds with higher oxygen to carbon ratios.

The researchers observed two phytoplankton blooms (Fig 2A) that gave very different SSAs in terms of their chemical composition. During the first bloom, the SSA had a much higher organic matter fraction (fOM) than the second bloom. Additionally, the first bloom contained much more aliphatic rich fraction than the second bloom while the oxygen rich fraction remained relatively high in both blooms (Fig 2B).

Figure 2. A) Chlorophyll A (Chl-a – green shaded regions) concentrations show two distinct bloom events during the 30 day experiment (on at roughly day 10-15, and another from day 20-25). Heterotrophic bacteria (HB) cell counts (blue boxes) show a general increase throughout the experiment. The organic fraction (FOM) of the SSA shows large differences in the composition of SSA between the two blooms. B) The AR fraction (FAR) and organic content of SSA show similar patterns as in 3A. Organic content (OC) remains relatively stable throughout. C) OR fraction (FOR) shows a contrasting pattern, where FAR is much higher in the first bloom, FOR increases during the first bloom and remains relatively consistent throughout the rest of the experiment. Sea Salt Organic Compound (SSOC) also shows a general increase throughout the experiment. D) Ice nucleating particles (INP) follow similar trends as the FAR, showing that AR compounds are likely better at seeding clouds in the atmosphere.

Figure 2. A) Chlorophyll A (Chl-a – green shaded regions) concentrations show two distinct bloom events during the 30 day experiment (on at roughly day 10-15, and another from day 20-25). Heterotrophic bacteria (HB) cell counts (blue boxes) show a general increase throughout the experiment. The organic fraction (FOM) of the SSA shows large differences in the composition of SSA between the two blooms. B) The AR fraction (FAR) and organic content of SSA show similar patterns as in 3A. Organic content (OC) remains relatively stable throughout. C) OR fraction (FOR) shows a contrasting pattern, where FAR is much higher in the first bloom, FOR increases during the first bloom and remains relatively consistent throughout the rest of the experiment. Sea Salt Organic Compound (SSOC) also shows a general increase throughout the experiment. D) Ice nucleating particles (INP) follow similar trends as the FAR, showing that AR compounds are likely better at seeding clouds in the atmosphere.

 

The researchers propose that the heterotrophic bacteria are the main cause for this difference in the SSA. As the first phytoplankton bloom declines, the heterotrophic bacteria, which degrade the dead/dying phytoplankton, increase in abundance (blue squares in Fig 2A). These heterotrophs often consume aliphatic compounds (such as lipids) since they give the most amount of energy. As bacteria increase in abundance, the compounds they like to consume the most (aliphatic compounds) will decrease while the compounds they do not consume (oxygen rich compounds) stay relatively constant.

To test this hypothesis, the researchers created a simple model to simulate the relationship between the amount of aliphatic rich fraction in SSA and heterotrophic bacteria activity. Their model assumed that phytoplankton death and decay was the source of aliphatic compounds and that the change in the aliphatic rich fraction of SSA was dependent on the activity and abundance of heterotrophic bacteria that consume aliphatic compounds. While their model is simplified and not very precise, they were able to simulate very similar trends to what they observed in their tank (Fig 3). As heterotrophic bacteria activity (represented as lipase enzyme activity) increases, the aliphatic rich fraction of SSA decreases, mimicking their observations.

 

Figure 3. Box model of aliphatic rich labile (ARL) compounds found in SSA can be determined based on heterotrophic bacteria activity (represented by the enzyme ligase). Lines a, b, and c represent model outputs based on different starting parameters. Overall, the general trend matches what was observed in the tank – as heterotrophic bacteria increase in abundance and activity, the FAR decreases, supporting their hypothesis that microbial degradation of organic compounds influences the composition of SSA, and ultimately its ability to seed clouds.

Figure 3. Box model of aliphatic rich labile (ARL) compounds found in SSA can be determined based on heterotrophic bacteria activity (represented by the enzyme ligase). Lines a, b, and c represent model outputs based on different starting parameters. Overall, the general trend matches what was observed in the tank – as heterotrophic bacteria increase in abundance and activity, the FAR decreases, supporting their hypothesis that microbial degradation of organic compounds influences the composition of SSA, and ultimately its ability to seed clouds.

 

Why it matters

This is important work that links biological activity (on a microscopic scale) to large-scale climate processes. Heterotrophic bacteria, through their eating habits, can significantly alter the chemical composition of sea spray aerosols. These differences in SSA chemistry directly impacts atmospheric ice nucleation (Fig 2D). More ice nucleating particles (INPs) were present during the first, aliphatic rich, phytoplankton blooms. Presumably, this means that SSA from that first bloom would have likely created more clouds than the second bloom.

With this new understanding, climate scientists can use biological data (ex. Chlorophyll A data, bacteria cell counts, enzyme activities) to inform and improve their models to predict cloud and aerosol formation. Additionally, any alterations in the marine microbial community, whether they are natural alterations (like seasonal variations in community structure) or anthropogenically driven (climate change, pollution, ocean acidification, etc.) might have greater impacts than previously thought – expanding beyond just the marine ecosystem to potentially influence global climate. This would make protecting the biological diversity and stability of marine ecosystems even more important than previously realized.

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