1CNRS/INSU, Observatoire Océanologique de Roscoff, UMR 7144, Groupe Plancton Océanique, 29680 Roscoff, France
2UPMC-Université Paris 06, Station Biologique, Place Georges Teissier, 29680 Roscoff, France
3School of Life Sciences, University of Warwick, Coventry CV4 7AL, UK
4CNRS/INSU and UPMC-Université Paris 06, Laboratoire Arago, UMS 2348, Observatoire Océanologique, 66651 Banyuls-sur-mer, France
5CNRS/INSU and Université Paris 06, UMR 7093, Laboratoire d'Océanographie de Villefranche, 06234 Villefranche-sur-mer, France
6Laboratoire d'océanographie physique et biogéochimique, Centre d'océanologie de Marseille, case 901, campus de Luminy, 13288 Marseille cedex 09, France
*These authors contributed equally to this work.
Abstract. Biological communities populating the Mediterranean Sea, which is situated at the northern boundary of the subtropics, are often claimed to be particularly affected by global warming. This is indicated, for instance, by the introduction of (sub)tropical species of fish or invertebrates that can displace local species. This raises the question of whether microbial communities are similarly affected, especially in the Levantine basin where sea surface temperatures have risen in recent years. In this paper, the genetic diversity of the two most abundant members of the phytoplankton community, the picocyanobacteria Prochlorococcus and Synechococcus, was examined on a transect from the South coast of France to Cyprus in the summer of 2008 (BOUM cruise). Diversity was studied using dot blot hybridization with clade-specific 16S rRNA oligonucleotide probes and clone libraries of the 16S–23S ribosomal DNA Internal Transcribed Spacer (ITS) region. Data were compared with those obtained during the PROSOPE cruise held almost a decade earlier, with a focus on the abundance of clades that may constitute bioindicators of warm waters. During both cruises, the dominant Prochlorococcus clade in the upper mixed layer at all stations was HLI, a clade typical of temperate waters, whereas the HLII clade, the dominant group in (sub)tropical waters, was only present at very low concentrations. The Synechococcus community was dominated by clades I, III and IV in the northwestern waters of the Gulf of Lions and by clade III and groups genetically related to clades WPC1 and VI in the rest of the Mediterranean Sea. In contrast, only a few sequences of clade II, a group typical of warm waters, were observed. These data indicate that local cyanobacterial populations have not yet been displaced by their (sub)tropical counterparts. This is discussed in the context of the low phosphorus concentrations found in surface waters in the eastern Mediterranean basin, as this may constitute a barrier to the colonization of these waters by alien picocyanobacterial groups.