The ocean is teeming with microscopic life that despite their minuscule size, greatly impact our world’s ecosystem and climate. A large majority of these organisms are considered planktonic, those that are suspended in the ocean waters and rely on the current for movement. Phytoplankton are a type of plankton that are autotrophic and use just photosynthesis as a carbon source. Not only do they serve as a food source for larger organisms, phytoplankton also produce the majority of the world’s oxygen and also play roles in cloud formation.
The seeding of a cloud
Clouds begin as cloud condensation nuclei (CCNs) or cloud seeds. CCNs are small particles approximately 0.2 microns in size and can be in the form of dust, soot, sea salt, or sulfur-containing compounds. CCN particles attract and hold water molecules from the surrounding environment. This leads to the formation of cloud droplets. Individual cloud droplets collide and coalescence with one another and rapidly grow in size. Once the droplets reach approximately 1 mililiter in size, they can fall from the sky as rain.
How do marine microbes contribute to cloud formation?
The link between cloud formation and marine microbes may seem tenuous at first glance. However, life is all chemistry right? Phytoplankton produce dimethysulfoniopropionate (DMSP) as an osmoprotectant, some of which leak out of the cell into the ocean. This excess DMSP is degraded by marine bacteria to dimethysulfide (DMS) which is subsequently released into the atmosphere. Once in the atmosphere, DMS is rapidly oxidized to aerosol sulfates that serve as CCNs.
One such bacteria-phytoplankton model of DMSP metabolism involves the phytoplankton Alexandrium tamarense and the bacterium Ruegeria pomeroyi. A. tamerense produces DMSP which is degraded by R. pomeroyi. DMSP has two metabolic fates in R. pomeroyi, one of which results in DMS production. Researchers in the Moran lab at the University of Georgia are trying to understand what triggers expression of genes in each DMSP degradation pathways. It is currently unknown what controls the fate of DMSP but it is possible that changes in the physical environment or microbial community structure influences how much DMS is produced.
Role of DMS in modulating climate
We often hear about the climate gasses carbon dioxide and methane, but other gasses such as DMS may also modulate changes in climate. There are two opposing hypothesis that link DMS production by microbes to the climate. The first of these hypotheses is the CLAW hypothesis (named after the scientists behind this hypothesis). The CLAW hypothesis proposes a negative feedback loop in which increased energy from the sun drives phytoplankton growth (potentially due to increased photosynthesis). The increased phytoplankton biomass leads to more DMSP and DMS production that in turn, increases cloud cover. Once cloud cover has increased, more sunlight is reflected away from the earth, bringing the ocean temperature back to where it started.
However, other factors are involved in modulating ocean temperatures, likely causing an increase in ocean temperature that may inhibit growth of phytoplankton (the anti-CLAW hypothesis). This increase in temperatures may lead to ocean stratification, where waters with different properties (ex: temperature) form layers that cannot mix. As an outcome, phytoplankton growth may decline since nutrients from the ocean’s depth are unable to reach the surface waters where phytoplankton thrive. This decrease in phytoplankton growth results in reduced DMSP and DMS production and further reduce cloud coverage to increase ocean temperatures even further.
Solar radiation management projects designed to reflect sunlight have been proposed in attempts to reduce global warming. However any climate manipulations will have an effect on marine and terrestrial ecosystems. It is clear that microbes have the potential to mitigate global warming and understanding how microbes affect cloud formation may better inform strategies being developed to impede climate change.
