Heterotrophic marine bacteria play key roles in remineralizing organic matter 
generated from primary production. However, far more is known about which groups 
are dominant than about the cellular processes they perform in order to become 
dominant. In the Southern Ocean, eukaryotic phytoplankton are the dominant 
primary producers. In this study we used metagenomics and metaproteomics to 
determine how the dominant bacterial and archaeal plankton processed bloom 
material. We examined the microbial community composition in 14 metagenomes and 
found that the relative abundance of Flavobacteria (dominated by Polaribacter) 
was positively correlated with chlorophyll a fluorescence, and the relative 
abundance of SAR11 was inversely correlated with both fluorescence and 
Flavobacteria abundance. By performing metaproteomics on the sample with the 
highest relative abundance of Flavobacteria (Newcomb Bay, East Antarctica) we 
defined how Flavobacteria attach to and degrade diverse complex organic 
material, how they make labile compounds available to Alphaproteobacteria 
(especially SAR11) and Gammaproteobacteria, and how these heterotrophic 
Proteobacteria target and utilize these nutrients. The presence of 
methylotrophic proteins for archaea and bacteria also indicated the importance 
of metabolic specialists. Overall, the study provides functional data for the 
microbial mechanisms of nutrient cycling at the surface of the coastal Southern 
Ocean.