Sheng Yangjie, a doctoral student at the School of Oceanography, Shanghai Jiao Tong University, published a research paper titled “Temperature fluctuation alleviates the negative effects of warming on marine diatoms: comparison between Thalassiosira sp. and Nitzschia closterium f. minutissima” in the journal Biogeosciences as co-first author. Associate Professor Feng Yuanyuan from the School of Oceanography, Shanghai Jiao Tong University, is the corresponding author. Other collaborators include Researcher Wang Yuntao from the Second Institute of Oceanography, Ministry of Natural Resources, and Dr. Sana Ben Ismail from the Marine Environment Laboratory of the National Institute of Marine Science and Technology of Tunisia. School of Oceanography in Shanghai Jiao Tong University is the first affiliation of the paper.

Against the backdrop of global mean climate change, marine phytoplankton are also influenced by environmental heterogeneity in the ocean. A typical manifestation is the oscillatory fluctuations of environmental conditions around long-term climate means, across different temporal and spatial scales. In almost all marine environments, surface seawater temperature displays obvious periodic fluctuations due to diurnal and seasonal solar radiation patterns, air–sea interactions, tides, upwelling, and other ocean dynamic processes. The frequency and magnitude of these temperature fluctuations vary with region and season. As global mean temperature continues to rise, the frequency of extreme weather events will also increase, altering the amplitude and frequency of temperature oscillations.

This study selected two ecologically important centrate diatom Thalassiosira sp. and pennate diatom Nitzschia closterium f. minutissima and conducted laboratory semi-continuous culture experiments under four temperature scenarios: constant 20 °C, fluctuating 20 ± 4 °C, constant 25 °C, and fluctuating 25 ± 4 °C. Key physiological indicators including growth rate, cellular elemental composition, chlorophyll a, protein and biogenic silica content, and sinking rate were measured to reveal the physiological effects of warming and temperature fluctuations, as well as interspecific differences in response (Figure 1).

The results showed that temperature fluctuations significantly alleviated the inhibitory effects of high temperature on diatom growth. At an average temperature of 25 °C, compared with 20 °C, the growth rates of both diatoms decreased significantly under constant temperature conditions, while growth increased again under fluctuating temperature conditions, indicating stronger thermal tolerance (Figure 2). Meanwhile, the physiological responses of the two diatoms to warming and temperature fluctuations exhibited clear interspecific differences. For Thalassiosira sp., warming under constant temperature significantly reduced growth rate but increased cellular elemental content and sinking rate. Under temperature fluctuation, however, warming significantly reduced particulate organic carbon and biogenic silica contents as well as sinking rate, while increasing protein content to cope with heat stress. Nitzschia closterium f. minutissima exhibited greater environmental resilience, with warming increasing intracellular particulate organic carbon, nitrogen, and phosphorus contents under both constant and fluctuating temperatures. The interaction between warming and temperature fluctuation produced antagonistic effects on most physiological parameters of Thalassiosira sp., but synergistic effects on those of Nitzschia closterium f. minutissima.

Figure 3. Schematic diagram of the physiological response patterns of Thalassiosira sp. and Nitzschia closterium f. minutissima to warming under constant and fluctuating temperature scenarios

The ecological effects of temperature fluctuations also depend on background mean temperature. At the lower mean temperature (20 °C), temperature fluctuations increased cellular carbon and silicon contents and sinking rate, favoring carbon export to the deep ocean. However, at higher temperature (25 °C), they suppressed the carbon export potential of Thalassiosira sp., suggesting that continued ocean warming may alter the relative contributions of different diatoms to the biological carbon pump (Figure 3). This study highlights that environmental heterogeneity such as temperature fluctuation is an underestimated key factor under the context of ocean warming. The interaction of temperature fluctuation with warming will be species-specific and may further influence future phytoplankton community structure and related marine carbon cycling processes.

This work was financially supported by the National Natural Science Foundation of China (grant no. 42276093) and the Oceanic Interdisciplinary Program of Shanghai Jiao Tong University (grant no. SL2022PT203).

Paper link:
https://doi.org/10.5194/bg-22-5961-2025