The distribution of aerobic anoxygenic phototrophs (AAPs) was surveyed in various regions of the Mediterranean Sea in spring and summer. These phototrophic bacteria were present within the euphotic layer at all sampled stations. The AAP abundances increased with increasing trophic status ranging from 2.5 × 10(3) cells per ml in oligotrophic Eastern Mediterranean up to 90 × 10(3) cells per ml in the Bay of Villefranche. Aerobic anoxygenic phototrophs made up on average 1-4% of total prokaryotes in low nutrient areas, whereas in coastal and more productive stations these organisms represented 3-11% of total prokaryotes. Diel bacteriochlorophyll a decay measurements showed that AAP community in the Western Mediterranean grew rapidly, at rates from 1.13 to 1.42 day(-1). The lower AAP abundances registered in the most oligotrophic waters suggest that they are relatively poor competitors under nutrient limiting conditions. Instead, AAPs appear to be metabolically active organisms, which thrive better in more eutrophic environments providing the necessary substrates to maintain high growth rates.

Departament de Biologia Marina i Oceanografia Institut de Ciències del Mar CSIC Pg Marítim de la Barceloneta 37 49 E08003 Barcelona Catalunya Spain

Institute of Microbiology CAS Opatovický mlýn 379 81 Třeboň Czechia

Institute of Physical Biology University of South Bohemia Zámek 36 373 33 Nové Hrady Czechia

Laboratory of Plankton Ecology and Evolution Stazione Zoologica ‘A Dohrn' Villa Comunale 1 80121 Napoli Italy

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