Award: OCE-1948163

Volatile organic compounds (VOCs) have become recognized as an important component of the pool of labile dissolved organic compounds in aquatic environments. VOCs released by algae can mediate algal-bacteria interactions and many can be taken up by bacteria as nutrient sources. This project made significant progress towards understanding the ecology of volatile organic compounds in the surface ocean. Our findings show that wide ranges of VOCs are produced during growth of phytoplankton, and many of these compounds are taken up by bacterioplankton that have evolved strategies to take advantage of low concentrations of specific VOCs in complex chemical backgrounds. A core concept this research highlights is that VOCs are actively cycled amongst producers and consumers such that the balance of their activities leads to a VOC pool that can be further acted upon by abiotic processes with important consequences for atmospheric chemistry.

Our findings show the phycosphere is a particularly active zone of VOC cycling with rapid turnover rates ranging from minutes to hours for several of the VOCs we investigated. VOC turnover limits VOC accumulation in the surface ocean, such that low in-water VOC concentrations favor negative air-sea emissions (VOC movement from the atmosphere into the surface ocean). Conditions leading to slow VOC turnover are associated with higher in-water VOC concentrations and positive air-sea emissions. Hydrocarbons were a major component of the VOC pool in the phycosphere of the model diatom, Phaeodactylum tricornutum, and genes involved in oxidation of benzenoids, including benzene, toluene, ethylbenzene/xylenes, were identified in some of the phycosphere bacteria we investigated. Phycosphere bacteria that were motile, chemotactic, and could attach to the diatom showed a larger range of VOC uptake in larger quantities and encode more hydrocarbon oxidation genes. These VOC-specialist bacteria do not utilize polysaccharides, while bacteria previously described as macromolecular specialists do not take up VOCs.

Our research identified many steps in the metabolism of the diatom, P. tricornutum, where VOCs are formed as intermediates or endproducts.  Temporal patterns in VOC production gives new understanding of the fundamental processes leading to VOC accumulation, and these patterns were described by photosynthetic metabolism that shifts depending on the diel cycle and phases of the cell cycle. 

Key highlights of our findings that are likely to be important for understanding the biogeochemistry of dissolved organic matter in the surface ocean include:

• VOCs are an important component of the labile pool of dissolved organic compounds in the oceans. 
• The phycosphere is a hot-spot of VOC-mediated phytoplankton-bacterioplankton interactions.
• VOC production and consumption results in short residence times for many VOCs that are highly reactive.
• VOC specialists are bacterioplankton with traits adapted to accessing VOCs sourced from phytoplankton and transiently present in low concentrations.
• Viral infection can change the amount and types of VOCs released by phytoplankton cells.

We anticipate these findings will encourage new research in areas ranging from basic science of VOC production and metabolism to predictions of large scale VOC air-sea flux. For example,

• Understanding the regulatory mechanisms driving physical interactions between phytoplankton and bacterioplankton.
• Development of models exploring the spatial and temporal impacts of VOC cycling in the phycosphere.
• Contributing to next generation chemical transport models that incorporate biological processes underlying VOC accumulation in the surface ocean.
• Identifying the ecological roles of different VOCs.

We partnered with Oregon State University's SMILE (Science and Math Investigative Learning Experiences) program to develop interactive activities for K-12 students. Two lessons were developed during this project. "Smells of Nature" and “Scents and Spheres” and connected with K-12 learning standards. Lessons were delivered to 40 K-12 teachers representing 33 rural and urban school districts in Oregon. The teachers then used, and continue to use, the lessons and materials in their classrooms and with after school STEM clubs.

• "Smells of Nature: Six mystery boxes containing VOCs embedded in unscented coconut oil were used to help engage students with VOCs. First, the student opens and smells a mystery box and tries to identify the scent (VOC). Clues are then provided in the form of flip cards to help trigger a memory for the VOC. Students learn about the sources of VOCs, the reasons why they are produced, and their chemistry. Mystery boxes contained one of each of the following: dimethylsulfide (the smell of the ocean), alpha pionene (the smell of pine trees), vanillin, d-limonene (the smell of citrus), l-limonene (the smell of dill or terpentine), and geosmin (the smell of dirt). 
• "Scents and Spheres" teaches students about environmental sources of common VOCs and how these VOCs can travel throughout the spheres of our planet. We chose to focus on methane, geosmin, alpha-pinene, dimethylsulfide, and hexanal. Four cards for each VOC were created that described the roles of these VOCs in different environments. When combined through student discussion, networks are created showing the flow of these compounds through the geosphere, hydrosphere, biosphere, and atmosphere.

 

Last Modified: 10/29/2025
Modified by: Kimberly H Halsey