SHORT COMMUNICATION Antimicrobial Activity of Marine Bacterial Symbionts Retrieved from Shallow Water Hydrothermal Vents Arnheidur Eythorsdottir 1,2 & Sesselja Omarsdottir 2 & Hjorleifur Einarsson 1 Received: 4 February 2015 /Accepted: 19 April 2016 # Springer Science+Business Media New York 2016 Abstract Marine sponges and other sessile macro-organisms were collected at a shallow water hydrothermal site in Eyjafjörður, Iceland. Bacteria were isolated from the organ- isms using selective media for actinomycetes, and the isolates were screened for antimicrobial activity. A total of 111 isolates revealed antimicrobial activity displaying different antimicro- bial patterns which indicates production of various com- pounds. Known test strains were grown in the presence of ethyl acetate extracts from one selected isolate, and a clear growth inhibition of Staphylococcus aureus was observed down to 0.1 % extract concentration in the medium. Identification of isolates shows different species of Actinobacteria with Streptomyces sp. playing the largest role, but also members of Bacilli, Alphaproteobacteria and Gammaproteobacteria. Sponges have an excellent record re- garding production of bioactive compounds, often involving microbial symbionts. At the hydrothermal vents, however, the majority of active isolates originated from other invertebrates such as sea anemones or algae. The results indicate that anti- microbial assays involving isolates in full growth can detect activity not visible by other methods. The macro-organisms inhabiting the Eyjafjörður hydrothermal vent area host diverse microbial species in the phylum Actinobacteria with antimicrobial activity, and the compounds responsible for the activity will be subject to further research. Keywords Marine bacteria . Symbionts . Antimicrobial activity . Hydrothermal vents Introduction Marine hydrothermal vents are unique environmental phe- nomena, located at different oceanic sites, mostly at deep sea mid-ocean ridge locations like in the Pacific or Indian oceans but also on the Mid-Atlantic Ridge. In deep sea vents, precip- itations can form spectacular chimneys, especially at greater depths. The Mohn Ridge located in the north of Jan Mayen also harbours a hydrothermal field with chimney formation in a considerably shallower area or around 500 m (Schander et al. 2010). Iceland is a part of the Mid-Atlantic Ridge system, and shallow water hydrothermal vents have been reported in Icelandic waters (Hannington et al. 2001). Moreover, a cluster of shallow water hydrothermal chimneys has been discovered in the fjord Eyjafjörður, Northern Iceland (Marteinsson et al. 2001). Another cluster at the west side of the fjord—named “Arnarnesstrýtur”—was later discovered at a depth of 25–40 m where the chimneys rise up to 25 m from the sea bed (Fig. 1). These two neighbour vent clusters are apparently parts of two different land-based geothermal systems and are as such de- fined as submarine hot springs. However, the chimney forma- tion is distinctive and morphologically resembles the deep sea fields (Fig. 2). The shallow water position makes the vent site easily accessible by scuba diving, and excursions have re- vealed a rich biodiversity, with benthic invertebrates and algae totally covering the chimneys, except at the top where fresh- water at pH 10 and 70–75 °C flows from the fissures. Microbial ecology and communities have been studied at Electronic supplementary material The online version of this article (doi:10.1007/s10126-016-9695-7) contains supplementary material, which is available to authorized users. * Hjorleifur Einarsson hei@unak.is 1 Faculty of Natural Resource Sciences, University of Akureyri, Borgum, Nordurslod 4, 600 Akureyri, Iceland 2 Faculty of Pharmaceutical Sciences, University of Iceland, Haga, Hofsvallagotu 53, 107 Reykjavík, Iceland Mar Biotechnol DOI 10.1007/s10126-016-9695-7