Revising the release of fixed carbon in coastal phytoplankton: the role of transparent exopolymer particles (TEP)

Historically, dissolved organic carbon production by phytoplankton in laboratory cultures has been measured to be 10% or less of total production, yet this percentage becomes more variable and generally higher in the field. Reconciling these differences is challenging, with implications for carbon modeling, carbon sequestration, and microbial loop processes. We modeled how carbon flows in upwelling systems among various carbon pools (particulate, dissolved, respired CO₂) by growing three coastal diatom species of varied sizes and life histories under three types of nutrient stress (silicic acid, nitrate or simultaneous silicic acid and nitrate) in simulated upwelling conditions (i.e. batch cultures). We tracked transparent exopolymer particles (TEP) (particulate carbon assumed to form from dissolved precursors) and bacterial production (BP) (bacterial carbon formed from dissolved carbon substrate) in addition to dissolved (DOCp) and particulate (POCp) production. By taking into account the range of conversion factors used to convert these measures to estimates of TEP carbon production and bacterial respiration (BR) from the literature, we found that while traditional estimates of dissolved carbon production relative to total production (i.e. DOCp/(DOCp + POCp)) were consistent with previous studies (5% or less), actual extracellular release (defined here as DOCp + TEP production (TEPp) + BR) relative to total production averaged 40% (Range: 12-70%). This percentage varied most among species, with the type of nutrient stress and growth stage (i.e. exponential, stationary) playing secondary roles. The results imply that that a greater fraction of fixed carbon is excreted by phytoplankton than previously thought with the majority forming TEP.