How are Phytoplankton Involved in Cloud Formation?

This is Jim Fiorendino, your host for On the Ocean

Earth’s ocean and atmosphere form a complex interconnected system. With over 71% of Earth’s surface covered by water, the dynamics of the world’s oceans can have considerable impacts on atmospheric processes such as cloud formation. Studying cloud formation is important because clouds influence Earth’s heat budget and, consequently, Earth’s climate and weather. Clouds form when water condenses or ice crystals grow on aerosol particles in the atmosphere. The breaking of waves on the ocean entrains bubbles in the water, which rise and burst. Bubbles bursting and waves breaking launch water droplets containing organic material into the atmosphere, which dry out and form cloud condensation nuclei or ice nucleating particles. A diagram of this process is shown in Figure 1 below.

Dr. Daniel Thornton of the Texas A&M University Department of Oceanography is currently working with a team of scientists to understand the role of oceanic processes in cloud formation. Specifically, Dr. Thornton is studying the production of organic material in the oceans by phytoplankton.

Phytoplankton are microscopic photosynthetic organisms that form the base of oceanic food webs. They are known to form compounds that promote cloud formation, such as exopolymers. Exopolymers are large, irregularly-shaped molecules comprising roughly 10% of the organic matter in the ocean. This material leaks out of cells, particularly when they are eaten or die (Figure 2). Once in the ocean, this material may be carried to the surface by bubbles, where it is concentrated in a thin layer known as the sea surface microlayer. Wave activity can transfer this material to the atmosphere. Dr. Thornton hopes to link the biology of marine phytoplankton to the composition of organic matter in the ocean and the properties of marine organic aerosols in the atmosphere.

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Figure 1. Ecosystem processes affecting the formation of marine aerosol (a) Phytoplankton fix inorganic carbon into organic matter. Phytoplankton productivity is affected by environmental factors such as the availability of energy (sunlight) and nutrients. A web of interactions (green arrows) between organisms affects the formation and transformation of organic matter. Heavy green arrows emphasize processes that produce large amounts dissolved organic matter (DOM) and particulate organic matter (POM). The metagenome represents the genetic potential of the community, including metabolic pathways that affect the fate of organic matter. Labile organic matter is utilized by heterotrophic bacteria, most of which is ultimately remineralized back to inorganic carbon (CO2). A small proportion of the organic matter fixed by photosynthesis may join the large pool of refractory organic matter in the ocean. Organic matter from the ecosystem fluxes to the atmosphere as volatile organic carbon (VOC) compounds and organic-rich primary marine aerosol (red arrows). (b) Volatile organic compounds (such as dimthyl sulfide (DMS)) flux from ocean to atmosphere, where they undergo oxidation reactions leading to the formation of secondary organic aerosol that may act as cloud condensation nuclei. Breaking waves entrain bubbles that scavenge organic matter from the water and burst through the sea surface microlayer (SML), throwing organic-rich primary aerosol into the atmosphere. A proportion of the primary aerosol may act as cloud condensation nuclei (CCN) or ice nucleating particles (INP).                                                                                                                                                                                                                                                                                                                                Brooks SD & Thornton DCO (accepted) Marine aerosols and clouds. Annual Reviews of Marine Science
Figure 2: Cell permeability in the diatom Thalassiosira weissflogii (CCMP 1051) visualized by epifluorescence microscopy. Cells were stained with SYTOX Green (Invitrogen, Life Technologies, Grand Island, U.S.A.), a membrane-imperme- able nucleic acid stain. Chlorophyll autofluorescence is shown in red and SYTOX Green stained nucleic acids are shown in green. A. Intact cell containing chlorophyll but no SYTOX Green staining and therefore intact cell membranes. B. Intact cell containing chlorophyll with compromised cell membranes revealed by the staining of an intact nucleus with SYTOX Green. C. Dying cell with low chlorophyll autofluorescence, a disrupted nucleus and compromised cell membranes. Image courtesy of Jie Chen. Scale bar = 10 μm.                          Thornton (2014) European Journal of Phycology 49: 20-46

Contributing Professor – Dr. Daniel Thornton