No abstract

Quantitative and qualitative changes in bacterial communities from the Mediterranean Sea were analysed under eutrophication conditions simulated in batch mesocosms (addition of inorganic nutrients or phytoplanktonic lysate). A wide variety of methods including traditional microbial ecology techniques, n~olecular biology and flow cytometry were combined to determine abundances, production, cell size, activity, culturability, and genetic and taxonomic diversity. In all mesocosms, the increase in biomass was rapidly controlled by protozoan grazing. Morphologcal and physiological changes were observed during the growth phase of bacteria and under grazing pressure. The proportion of medium-size and culturable cells increased during the growth phase. Grazing eliminates preferentially active and medium-sized cells withln communities regulating bacterial productivity. Small and large cells were produced as a consequence of grazing pressure, and the large active cells contributed to the remaining productivity after grazing. Although grazing had an effect on the genetic diversity of bacterial communities by eliminating some populations, other species were preserved. It seems that some species such as Alterornonas macleodii may have developed defence strategies to escape predation. We hypothesize that such species may escape grazing by producing small andlor large cells during their growing phase.

Three nucleic acid dyes (SYTO-13, TOTO-1, and YOYO-1) were tested on cultures of Escherichia coli and marine prokaryote populations. These dyes stain the RNA and DNA in E. coli but only respond to DNA in marine populations, according to the histograms obtained after DNase and RNase treatments.

Small subunit rDNA clone libraries were generated from amplified DNA of bacterioplankton taken at different time points from a mesocosm containing eutrophied Mediterranean seawater and made eutrophic by the addition of N and P. Analysis of 96 partial sequences indicated that 22% of the clones formed four clusters which showed the highest sequence similarity with the 16S rDNA sequence of Alteromonas macleodii DSM 6062T. A fifth cluster, comprising 31% of the clone sequences is moderately related to the A. macleodii sequence. Similarity between the almost complete sequences of two representatives of clone clusters 1, 2 and 3 and A. macleodii ranged between 97.7 and 98.1%. Four oligonucleotide probes, representing four clone clusters, were developed on the basis of partial clone sequences. Dot blot hybridization with PCR‐amplified 16S rDNA from 739 clones revealed that 24% of clones belong to one of these clusters. Dot blot hybridization between the four probes and PCR‐amplified 16S rDNA from 128 strains isolated from the mesocosm identified 21% of the isolates possessing the probe target region. While probes GP‐1 and GP‐4 unambiguously identified 0.8 and 4.0% of the strains, respectively, probes GP‐2 and GP‐3 showed cross hybridization with 16% of the strains. Analysis of the probe target region of the 16S rDNA of one of the isolates indeed demonstrated the presence of double peaks in the relevant region of the sequence which is indicative of microheterogeneity at the rrn operon level. Although some of the diversity can be attributed to intra‐strain variation, the data indicate that the phylogenetic diversity of A. macleodii is higher than represented by the type strain of this species.

Quantitative and qualitative changes in bacterial communities from the Mediterranean Sea were compared in duplicate batch mesocosms with or without addition of inorganic nutrients. Methods including traditional microbial ecology techniques, molecular biology and flow cytometry were combined to determine abundances, production, cell size, activity, culturability and taxonomic diversity of bacterial cells. Addition of nutrients and confinement resulted in an increase of bacterial densities which were rapidly controlled by protozoan grazing. Changes in bacterial activity and morphology were observed during the growth phase of bacteria and under grazing pressure. The proportion of medium-size and culturable cells increased during the growth phase. These cells were preferentially consumed by grazers resulting in a strong limitation of bacterial production. As a consequence of the grazing pressure, large cells were produced and contributed to the remaining bacterial productivity after grazing. Grazing had an effect on the taxonomic composition of bacterial communities by preferentially eliminating gamma-Proteobacteria, alpha-Proteobacteria were preserved. It seems that some species from the genera Ruegeria and Cytophaga may have developed defence strategies to escape predation.