A "green" synthesis alternative for Au nanoparticles (NPs) is reported using an aqueous media, environmental friendly reducing agents and the replacement of alkytrimethylammonium halides by biocompatibles molecules. The usage of an extract from the alga Egregia sp. intrinsically provides with biomolecules for the shell formation around the Au NPs to aid for their biocompatibility. The detailed analysis presented here allows to elucidate the double role played by the algae extract as the reducing agent and as the stabilizing capping shell for the Au NPs colloid. The optimized parameters to obtain a narrow size distribution of Au NPs are elucidated.