Abstract
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 significantly risen over the last 25 years (0.50 ± 0.11 °C in average per decade, P < 0.01). In this paper, the genetic diversity of the two most abundant members of the phytoplankton community, the picocyanobacteria Prochlorococcus and Synechococcus, was examined during two cruises through both eastern and western Mediterranean Sea basins held in September 1999 (PROSOPE cruise) and in June–July 2008 (BOUM cruise). Diversity was studied using dot blot hybridization with clade-specific 16S rRNA oligonucleotide probes and/or clone libraries of the 16S-23S ribosomal DNA Internal Transcribed Spacer (ITS) region, 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.
Highlights
Due to their ubiquity and abundance, marine cyanobacteria are widely recognized as being major contributors to global photosynthetic biomass and primary production (Campbell et al, 1994; Goericke and Welschmeyer, 1993; Li, 1994; Partensky et al, 1999)
These nutrients are rapidly consumed by phytoplankton during late winter/early spring blooms, which occur in surface waters in the northern Mediterranean Sea and at depth further south (Crombet et al, 2011)
The upper mixed layer becomes oligotrophic and the chlorophyll biomass reaches annual minima over the whole Mediterranean Sea (D’Ortenzio and Ribera d’Alcala, 2009)
Summary
Due to their ubiquity and abundance, marine cyanobacteria are widely recognized as being major contributors to global photosynthetic biomass and primary production (Campbell et al, 1994; Goericke and Welschmeyer, 1993; Li, 1994; Partensky et al, 1999). Two genera dominate the picophytoplankton size fraction (0.2–2 μm), Prochlorococcus and Synechococcus. These genera co-occur in tropical and temperate areas, their abundance patterns differ spatially and seasonally (Johnson et al, 2006; Tai and Palenik, 2009; Zwirglmaier et al, 2008). Synechococcus prefers well-lit, nutrient-rich waters and usually blooms in spring when the water column is mixed, whereas Prochlorococcus is most abundant in oligotrophic waters and reaches maximal concentrations in summer and fall when waters are stratified, with a deeper sub-surface maximum than Synechococcus Mella-Flores et al.: Diversity of picocyanobacteria in the Mediterranean Sea
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