Investigation of 0.2 µm filterable bacteria from the Western Mediterranean Sea using a molecular approach: dominance of potential starvation forms.

Although the existence of 0.2 µm filterable bacteria has been known since the early 80's, they are not taken into consideration when modeling microbial food webs, due to an overall lack of information concerning this specific size class. According to physiological studies on starvation forms and investigations on small bacterial cells in marine ecosystems, a 0.2 µm filtrate may consist of different phenotypes: starvation forms of typical marine bacteria, ultramicrobacteria or bacterial cells, even larger than 0.2 µm, but flexible enough to pass the nominal filter pore-size. In this pilot study we examined three filtered seawater fractions from the Western Mediterranean Sea (Bay of Calvi, Corsica/France) - the total bacterial population, the bacterial fraction above 0.2 µm and the 0.2 µm filtrate - to investigate the bacterial community structure of each of those fractions by the molecular approach of denaturing gradient gel electrophoresis (DGGE) of 16S rDNA fragments. The analysis of the resulting DGGE profiles revealed different patterns of dominant bands for the 0.2 µm filterable and the total bacterial populations within the samples. Additionally the 0.2 µm filterable bacterial compartment exhibited obvious differences in band patterns for winter and summer samples, which were not observed for the total bacterial fraction. According to the current knowledge concerning the status of 0.2 µm filterable bacteria, DGGE patterns indicate that most of the fragments representing 0.2 µm filterable bacteria were rather starvation forms of marine bacteria than ultramicrobacteria. The sequencing of excised and cloned DNA bands of the DGGE profiles characterized the phylogenetic affiliation of the corresponding 0.2 µm filterable bacteria, clustering mainly with known, typical marine isolates of both alpha-subclass and gamma-subclass of the Proteobacteria and the Cytophaga-Flavobacterium-Bacteroides branch.