Life is not aseptic! In their natural environment all microorganisms are part of a complex biological network. Their relationships range from mutualistic interactions to obligate symbioses. Heterotrophic bacteria of the phycosphere are only loosely associated with their algal and cyanobacterial hosts, in contrast to phototrophic consortia that represent case examples of a tight mutual interdependency. We investigate the bacterial community composition of these consortia via (meta-) genome analyses to identify and characterize the key players. Specific gene knock-outs, various OMICs-techniques and co-cultivation experiments help to understand the modes of phototroph-heterotroph interaction.