Corrosion processes of carbon steel permanently immersed in natural seawater are in any case influenced by microorganisms due to the important biological activity. The influence of sulfide-producing bacteria is well known but various microorganisms, the biofilm itself, and the numerous organic species associated with biological activity may also play a role. Because the sulfate green rust GR_SO4 is one of the main corrosion products of carbon steel in seawater, the role of organic matter/bacteria on its formation and transformation was addressed. GR_SO4 was precipitated from Fe(II) and Fe(III) salts in the presence of various marine bacterial species. Abiotic experiments were performed for comparison, a first one without any organic species, a second one with sodium acetate added. The obtained aqueous suspensions were aged at room temperature for 1 week. The number of bacteria (UFC/mL) was followed over time and the solid phases were characterized by XRD. Whatever the fate of the bacteria (no activity, or activity and growth), the formation of GR_SO4 was favored and its transformation to magnetite completely inhibited. This effect is attributed to the adsorption of organic molecules on the surface of the GR_SO4 crystals. A similar effect, though less important, was observed with acetate.