Marine gel production in coastal diatoms studied by experimental and modeling approaches under varying light conditions

Marine gels are organic polymers spanning from dissolved Exopolymeric Substances (EPS; nm to µm) to Transparent Exopolymer Particles (TEP; µm to mm), playing a key role in marine ecosystems by enhancing flocculation between organic and mineral particles. This process significantly affects the size distribution, density, and vertical transport of Suspended Particulate Matter (SPM) as well as the carbon cycle in the ocean. In turbid coastal environments, TEP produced by phytoplankton determines the seasonality of SPM concentration and influences the export of particulate organic matter. Although the biogeochemical importance of TEP and EPS is now recognized, the factors controlling their production by phytoplankton remain poorly understood and their dynamics is seldom included in biogeochemical models.

This study combines experimental laboratory approaches with mechanistic numerical modeling to decipher the complex relationships between light intensity, interspecific variation, and marine gel production in a turbid coastal zone. Laboratory experiments were conducted on six representative marine diatom strains isolated from the coastal Belgian Part of the North Sea. Following the carbon overflow hypothesis, which suggests that excess cellular internal carbon compared to nutrients leads to EPS excretion and subsequent TEP formation, the strains were subjected to varying light intensities. EPS and TEP concentrations were measured along with phytoplankton and bacterial abundances, as well as particulate organic carbon and nitrogen concentrations during exponential and stationary growth phases. This set of experiments is used to further develop a zero-dimensional biogeochemical model initially designed to simulate dissolved organic matter production and TEP formation during a mesocosm diatom bloom.

Analysis of EPS production revealed distinct patterns across strains, with maximum specific EPS production rates varying by 35%. No correlation between mean cell volume and specific EPS or TEP production was found. Despite the absence of a systematic correlation, specific production rates and TEP formation were generally higher under high light conditions, supporting the carbon overflow hypothesis. The smallest taxon Skeletonema sp. exhibited irregular EPS dynamics with significant losses, suggesting interspecific differences in EPS reactivity and bacterial activity. Cellular C:N ratios remained stable (5-7 mol C:mol N) across all conditions, indicating maintained internal stoichiometry in stationary phase, with no clear relationship to EPS or TEP production.

Preliminary model simulations showed increased TEP:phytoplankton biomass ratios under high irradiance conditions compared to moderate and low light conditions, agreeing with the more similar and lower TEP production reached in medium and low light experiments. Yet, despite the low interspecific variation in maximum EPS production rates suggesting that a homogeneous parameterization could be used, other resource acquisition parameters are known to vary with cell size, and the current constant model parameterization could not allow the adequate simulation of all experiments. This set of experimental data and simulations shows that the underlying mechanisms controlling marine gel production require further investigation and improvement of our biogeochemical models to better represent particle and carbon dynamics in coastal systems.