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

Although the existence of 0.2 mu m filterable bacteria has been known since the early 80's, they are not taken into consideration when modeling microb ial food webs, due to an overall lack of information concerning this specif ic size class. According to physiological studies on starvation forms and i nvestigations on small bacterial cells in marine ecosystems, a 0.2 mu m fil trate may consist of different phenotypes: starvation forms of typical mari ne bacteria, ultramicrobacteria or bacterial cells. even larger than 0.2 mu m, but flexible enough to pass the nominal filter pore-size. In this pilot study we examined three filtered seawater fractions from the Western Medit erranean Sea (Bay of Calvi, Corsica/France) - the total bacterial populatio n, the bacterial fraction above 0.2 mu m and the 0.2 mu m filtrate - to inv estigate the bacterial community structure of each of those fractions by th e molecular approach of denaturing gradient gel electrophoresis (DGGE) of 1 6S rDNA fragments. The analysis of the resulting DGGE profiles revealed dif ferent patterns of dominant bands for the 0.2 mu m Filterable and the total bacterial populations within the samples. Additionally the 0.2 mu m filter able bacterial compartment exhibited obvious differences in band patterns f or a inter and summer samples, which were not observed for the total bacter ial fraction. According to the current knowledge concerning the status of 0 .2 mu m filterable bacteria, DGGE patterns indicate that most of the fragme nts representing 0.2 mu m filterable bacteria were rather starvation forms of marine bacteria than ultramicrobacteria. The sequencing of excised and c loned DNA bands of the DGGE profiles characterized the phylogenetic affilia tion of the corresponding 0.2 mu m filterable bacteria, clustering mainly w ith known, typical marine isolates of both alpha-subclass and gamma-subclas s of the Proteobacteria and the Cytophaga-Flavobacterium-Bacteroides branch . (C) 1999 Published by Elsevier Science B.V. All rights reserved.