Ocean Algae Reveal Surprising VOC Secrets Under Changing Light

Marine phytoplankton, the microscopic photosynthetic organisms drifting at the base of the ocean food web, are far more chemically active than their size suggests. A new study published in Frontiers in Photobiology has revealed that two common marine algae produce strikingly different suites of volatile organic compounds (VOCs) depending on the light conditions they experience during growth, with implications for how scientists model ocean-atmosphere chemistry and climate feedbacks. The research, conducted by Hawra H. Almubarak, Holger H. Buchholz, and Kimberly H. Halsey at Oregon State University, challenges the assumption that faster-growing phytoplankton simply emit more VOCs, pointing instead to deeply rooted evolutionary strategies shaped by each species’ preferred light environment.

VOCs are low-molecular-weight carbon compounds that easily evaporate into the gas phase. In the ocean, they are estimated to represent 30 to 40 percent of the dissolved organic carbon pool and serve as both metabolic substrates and chemical signals for marine bacteria. Once produced, they can be consumed by heterotrophic microbes, react with other dissolved chemicals, or escape across the sea surface into the atmosphere. There, they participate in the formation of secondary organic aerosols, which influence cloud formation and climate, and modulate the availability of the hydroxyl radical, a key oxidant that controls the atmospheric lifetime of methane and helps maintain stratospheric ozone. Despite their importance, the biological processes governing which VOCs are produced, and in what quantities, remain poorly resolved.

To isolate the effect of growth rate on VOC production, the team turned to a controlled laboratory approach. They grew two model marine phytoplankton species, the green alga Dunaliella tertiolecta and the diatom Thalassiosira weissflogii, in semi-continuous cultures under constant low light (10 micromol photons per square meter per second) or high light (180 micromol photons per square meter per second). By passaging cultures repeatedly through fresh medium, the researchers ensured that cells had undergone at least ten divisions under identical conditions, achieving what biologists call steady-state growth: a physiological state where light absorption is perfectly balanced with downstream carbon metabolism and cell division proceeds at a constant, predictable rate. This design eliminated the confounding effects of short-term light fluctuations, nutrient limitation, or stress responses that often complicate VOC measurements in the field.

The results were unexpected. Under low light, D. tertiolecta grew at 0.6 per day and produced a diverse array of VOCs, including acetone, isoprene, and methanethiol, with oxygen-containing compounds making up 72 percent of the total VOC pool. But when the same species was grown under high light at a doubled growth rate of 1.2 per day, the researchers detected no VOCs at all. This was the opposite of what the team had hypothesized. Meanwhile, T. weissflogii, the diatom, produced VOCs under both light conditions, with its total VOC pool reaching 5.5 times that of low-light D. tertiolecta under high light. Even more strikingly, the types of VOCs produced by the diatom remained nearly identical whether cells were growing slowly in dim light or quickly in bright light.

Only two VOCs were shared between the two species across all treatments, underscoring how species-specific VOC production can be. Seven VOCs were unique to D. tertiolecta and seven were unique to T. weissflogii. In the green alga, acetone dominated the low-light VOC profile, likely arising from fatty acid beta-oxidation as an auxiliary energy pathway when photosynthetic electron transport is limited. Isoprene, which the researchers interpret as linked to chlorophyll biosynthesis rather than high-light stress protection, appeared only in low-light cells where chlorophyll content was 7.1-fold higher than in high-light cells. Methanethiol, derived from the metabolism of dimethylsulfoniopropionate (DMSP), an abundant cellular osmolyte, was also detected only under low light, consistent with field observations showing elevated methanethiol during light-limited periods in the subarctic North Atlantic.

The diatom told a different story. T. weissflogii produced ethanol as its dominant VOC under both light conditions, with ethanol and one other oxygenated compound comprising 88 to 90 percent of the total VOC pool. Flux-based modeling of a closely related diatom, Thalassiosira pseudonana, has shown that ethanol release serves as a fermentation product to balance cellular redox state, specifically the ratio of NADH to ATP, across a wide range of growth rates. This generalized redox regulatory strategy appears to be a hallmark of diatom metabolism that D. tertiolecta lacks. Instead, the green alga channels excess photosynthetic electrons into glycerol synthesis, a pathway that keeps volatile metabolites from accumulating and diffusing out of the cell.

The researchers propose that these contrasting VOC production patterns reflect fundamental differences in how each species has adapted to its ecological niche. D. tertiolecta, a chlorophyte, is adapted to relatively stable, well-lit waters with moderate nutrient availability. Under these conditions, tight metabolic control minimizes metabolic overflow, and VOC production is essentially shut down when growth is fast and energy is abundant. Diatoms, by contrast, thrive in dynamic environments such as coastal waters and deep mixed layers where light availability shifts dramatically over hours and days. Their success in these variable conditions may depend on a more relaxed control of metabolite pools, allowing constitutive VOC release as a byproduct of the flexible redox balancing that enables rapid responses to changing light.

The findings carry significant implications for atmospheric and climate science. Current models of ocean-atmosphere VOC flux rely on limited knowledge of which phytoplankton taxa produce which compounds under what conditions. If VOC production is governed more by evolutionary life-history strategy than by instantaneous growth rate, then predicting how ocean VOC emissions will respond to climate-driven changes in light availability, mixed-layer depth, and species composition becomes far more complex. The results also suggest that light-limited growth, which occurs in deep chlorophyll maxima, during bloom senescence, and in seasonally dark polar waters, may be an important and previously unrecognized source of isoprene and other climate-active VOCs.

The study does leave open questions. The researchers did not attempt to separate VOCs produced directly by algal metabolism from those generated through photochemical degradation of dissolved organic carbon released by the cells, a sequential process that can sometimes account for the majority of acetaldehyde and acetone production in seawater. Future experiments exposing cell-free spent medium to light could disentangle these two production pathways. Additionally, the team notes that exposing D. tertiolecta to unpredictable light pulses, mimicking the variable conditions diatoms naturally encounter, might reverse the observed patterns, causing the green alga to produce a broader VOC array while the diatom remains unaffected. Testing this prediction across a wider range of phytoplankton taxa and nutrient conditions will be essential for building a comprehensive picture of how the ocean’s microscopic forests shape the air we breathe.

Subject of Research: Light-dependent volatile organic compound production in marine phytoplankton

Article Title: Light-driven growth rate-dependent volatile organic compound production reflects adaptative strategies in Dunaliella tertiolecta and Thalassiosira weissflogii

Article References: Almubarak, H. H., Buchholz, H. H., & Halsey, K. H. (2026). Light-driven growth rate-dependent volatile organic compound production reflects adaptative strategies in Dunaliella tertiolecta and Thalassiosira weissflogii. Frontiers in Photobiology, 4, Article 1810167. https://doi.org/10.3389/fphbi.2026.1810167

Image Credits: AI Generated

DOI: 10.3389/fphbi.2026.1810167

Keywords: phytoplankton, volatile organic compounds, Dunaliella tertiolecta, Thalassiosira weissflogii, diatom, green alga, photosynthesis, light intensity, isoprene, acetone, ethanol, ocean-atmosphere exchange