Diffusional Interactions among Marine Phytoplankton and Bacterioplankton: Modelling H₂O₂ as a Case Study

Area of Science:

• Marine microbial ecology
• Biogeochemical cycling
• Cellular biophysics

Background:

• Marine phytoplankton and bacterioplankton produce and consume reactive oxygen species (ROS) to maintain homeostasis.
• Hydrogen peroxide (H2O2) is a key ROS with significant extracellular lifetimes and membrane permeability.
• Cell size and density critically influence solute exchange dynamics in aquatic microbial communities.

Purpose of the Study:

• To quantify the influence of phytoplankton cell size and density on diffusional interactions mediated by hydrogen peroxide (H2O2).
• To model the spatial extent of H2O2 gradients originating from individual phytoplankton and bacterioplankton cells.
• To assess the implications of these gradients for cell-to-cell communication and ecological processes.

Main Methods:

• Simulations of hydrogen peroxide (H2O2) concentration ([H2O2]) gradients extending from representative phytoplankton and bacterioplankton cells.
• Modeling incorporated cell size, suspension density, and H2O2 decay rates.
• Analysis focused on the diffusion distances of H2O2 from cell surfaces under varying conditions.

Main Results:

• H2O2 diffusion distances were highly dependent on cell size, extending ~3.1 µm from *Prochlorococcus* and 90 µm from diatoms.
• Bacterioplankton influenced local H2O2 levels within ~1.2 µm of their cells.
• Volumetric dilution was the primary factor limiting H2O2 gradients, more so than decay, over short distances.

Conclusions:

• Direct H2O2 exchange is unlikely in oligotrophic waters with sparse, small cells.
• Cell-to-cell H2O2 exchange becomes more significant in eutrophic conditions with closer spacing.
• Phytoplankton colony formation creates environments conducive to extensive H2O2-mediated interactions.