New Insights for the Renewed Phytoplankton-Bacteria Coupling Concept: the Role of the Trophic Web.

It is widely accepted that in many aquatic ecosystems bacterioplankton is dependent on and regulated by organic carbon supplied by phytoplankton, leading to coupled algae-bacteria relationship. In this study, an in-depth analysis of this relationship has been carried out by combining two approaches: (i) a correlation analyses between heterotrophic bacterial production (BP) vs. primary production (PP) or algal excretion of organic carbon (EOC), (ii) the balance between bacterial carbon demands (BCD) and the supply of C as EOC, measured as BCD:EOC ratio. During the study period (2013-2016), the algae-bacteria relationship was constantly changing from a coupling in 2013, uncoupling in 2014 and 2015, and an incipient return to coupling (in 2016). Our results show that top-down control (bacterivory) by algal mixotrophy acts as a decoupling force since it provides a fresh C source different to algal EOC to satisfy bacterial carbon demands. Notably, a relationship between the BCD:EOC ratio and the ecosystem metabolic balance (Primary production (PP): respiration (R)) was found, suggesting that PP:R may be a good predictor of the algae-bacteria coupling. This analysis, including the comparison between basal and potential ecosystem metabolic balance, can be a tool to improve knowledge on the interaction between both biotics compartments, which the traditional analyses on coupling may not capture.