Remineralisation of Organic Carbon by Marine Bacterioplankton (REMAIN) - Reducing the Known Unknown

Microbial community respiration (MCR), and particularly the apportionment of this respiration to size classes or functional groups within the plankton, is important for an understanding of the cycling of carbon through the marine foodweb. In addition, since the respiratory production of CO₂ during the remineralisation of dissolved organic material (DOM) by marine bacterioplankton is a major constraint on the storage of carbon in the ocean and thus climate mediation, it is important to determine the MCR attributable to the bacterioplankton size fraction and the factors influencing its variability. The validation of the reduction of the tetrazolium salt INT (INT_R) as a proxy for plankton respiration, is an exciting step forward in determining the temporal and spatial variability in bacterioplankton respiration. However, recent INT_R data have suggested that the proportion of MCR attributable to the 0.2-0.8 μm bacterioplankton size class is consistently low (30-40%), even in open ocean communities where bacterioplankton are dominant. This unexpected result led us to ask: which size class contributes most to MCR if not the bacterial size class, and what influences the variability in respiration if not the type of plankton present. To answer these intriguing questions, we measured MCR and size fractionated (0.1-0.8, 0.8-2 and >2μm) respiration for the first time using 3 methods: dissolved oxygen consumption, INT_R and the enzyme kinetics of the electron transport system (ETS) alongside plankton community structure and DOM during an Atlantic Meridional Transect covering six biogeographic regimes. INT_R data indicates that, throughout the transect, the majority of the MCR was attributable to 0.8 – 2.0 µm cells while plankton < 0.8 µm contributed only ~30% to MCR. Here, we will interpret the INT_R and ETS size fractionated data to determine which plankton size class is the major producer of CO₂ in the Atlantic Ocean, and what causes this to vary.