Effects of the green alga Dictyosphaeria ocellata on its surrounding bacterial community Marine macroalgae are widespread throughout the world’s oceans. They are important primary producers and have been used for millennia as food products for humans. The health and ecology of these organisms are greatly affected by the bacterial communities surrounding them. Because bacteria are so abundant in the oceans as many as 2.5 x 108 bacterial cells per milliliter of seawater it is likely that macroalgae have evolved mechanisms to regulate the composition of the bacterial community surrounding them. In fact recent studies indicate that a certain degree of host specificity exists among bacterial communities on the surface of algae. In addition it has been known for some time that algae produce compounds with bacteriostatic or bacteriolytic properties. However a good understanding of the ecological roles of these compounds has yet to be established. In order to better understand the effects of macroalgae on their surrounding bacterial community I performed a series of field and laboratory experiments using a green siphonous alga Dictyosphaeria ocellata as a model organism. The results presented here indicate that D. ocellata regulates the composition of the bacterial community on its surface and in the closely surrounding seawater. Furthermore that the alga produces organic compounds that are involved in the bacterial community regulation.