"A drop of seawater reveals a complex network of life teeming with bacteria that simultaneously consume and synthesize material and energy with a relentless efficiency and thus serve other organisms in the marine food web. The metabolisms of marine bacteria represent an important source and sink for compounds of environmental significance such as climatically active one-carbon (C1) compounds. Yet, despite their importance in the global carbon cycle and climate regulation, bacteria involved in C1 cycling are still poorly understood.While microbial oceanographers are slowly starting to recognize the possibilities of cooperation in the oceanic microbial world, I hypothesize that beneficial interactions between bacteria involved in C1 cycling, may have tremendous impact on marine, and even global, carbon cycling. Therefore, this project seeks to gain insight into the sociobiology of bacterioplankton in relation to the cycling of C1 compounds through the following objectives: i) identify bacteria consuming C1 compounds and determine their spatial variability and coupling with other marine bacteria in contrasting oceanic regions; ii) study the temporal variations and co-occurrence of C1 compounds producers and consumers under defined experimental conditions and iii) gain insight into their interactions using model organisms and specific substrates in controlled experiments. Through combination of multidisciplinary techniques in controlled experiments and in situ, the proposed project will help understand the impact of bacteria on ecosystem functioning.The proposed research follows recommendations from the European marine board on key questions in aquatic microbial ecology; by helping to predict how environmental factors influence co-regulations of metabolisms between bacteria and how microscale synergy influences ecosystem functioning. Moreover, this project will enhance the quality of research in Europe through the transfer of knowledge and collaborations with the US."