The results of empirical studies have revealed links between phytoplankton and bacterioplankton, such as the frequent correlation between chlorophyll a and bulk bacterial abundance and production. Nevertheless, little is known about possible links at the level of specific taxonomic groups. To investigate this issue, seawater microcosm experiments were performed in the northwestern Mediterranean Sea. Turbulence was used as a noninvasive means to induce phytoplankton blooms dominated by different algae. Microcosms exposed to turbulence became dominated by diatoms, while small phytoflagellates gained importance under still conditions. Denaturing gradient gel electrophoresis (DGGE) of 16S rRNA gene fragments showed that changes in phytoplankton community composition were followed by shifts in bacterioplankton community composition, both as changes in the presence or absence of distinct bacterial phylotypes and as differences in the relative abundance of ubiquitous phylotypes. Sequencing of DGGE bands showed that four Roseobacter phylotypes were present in all microcosms. The microcosms with a higher proportion of phytoflagellates were characterized by four phylotypes of the Bacteroidetes phylum: two affiliated with the family Cryomorphaceae and two with the family Flavobacteriaceae. Two other Flavobacteriaceae phylotypes were characteristic of the diatom-dominated microcosms, together with one Alphaproteobacteria phylotype (Roseobacter) and one Gammaproteobacteria phylotype (Methylophaga). Phylogenetic analyses of published Bacteroidetes 16S rRNA gene sequences confirmed that members of the Flavobacteriaceae are remarkably responsive to phytoplankton blooms, indicating these bacteria could be particularly important in the processing of organic matter during such events. Our data suggest that quantitative and qualitative differences in phytoplankton species composition may lead to pronounced differences in bacterioplankton species composition.

The searching trajectories of different animals can be described with a broad class of flight length (l j) distributions with P(lj) ‫؍‬ lj ؊ . Theoretical studies have shown that changes in these distributions (i.e., different values) are key to optimizing the long-term encounter statistics under certain searcher-resource scenarios. In particular, they predict the advantage of Lé vy searching ( Ϸ 2) over Brownian motion ( > 3) for low-prey-density scenarios. Here, we present experimental evidence of predicted optimal changes in the flight-time distribution of a predator's walk in response to gradual density changes of its moving prey. Flight times of the dinoflagellate Oxyrrhis marina switched from an exponential to an inverse square power-law distribution when the prey (Rhodomonas sp.) decreased in abundance. Concomitantly, amplitude and frequency of the short-term helical path increased. The specific biological mechanisms involved in these searching behavioral changes are discussed. We suggest that, in a threedimensional environment, a stronger helical component combined with a Lé vy walk searching strategy enhances predator's encounter rates. Our results support the idea of universality of the statistical laws in optimal searching processes despite variations in the biological details of the organisms. R andom walks based on Lévy flight distributions P(l j ) ϭ l j Ϫ in concrete ''Lévy walks'' with Ϸ 2, are the optimal searching strategy for scarce fixed targets that are randomly located (1). A Lévy walk could be more efficient than the usual Gaussian (i.e., Brownian) motion as suggested by early works on microzooplankton (2-4), although Brownian motion is generally assumed in reaction-diffusion predator-prey models. Recently, ecological examples of Lévy walks have been provided for a wide range of animal species (5-10). However, the theoretical study of more complex scenarios has shown that the advantage of Lévy searching over other types of motion is restricted to a set of prey densities, and mobility and size of the predator relative to the prey (11-13). Therefore, natural selection should favor flexible behaviors, combining different searching strategies (i.e., searching statistics) under different conditions. Here, we present experimental evidence that changes occur in both the short-and long-term searching statistics of a predator (Oxyrrhis marina), coinciding with density changes of its moving prey (Rhodomonas sp.). The specific biological mechanisms involved are also identified.The marine heterotrophic dinoflagellate O. marina has two flagella, one transversal and one longitudinal, providing three types of movement: rotation, translation, and sudden directional changes (14, 15). The flagellar apparatus of O. marina has been well studied at both the cellular (15) and the ultrastructural (16) level. Continuous flagellar movements are responsible for simultaneous rotation and translation of organisms, giving rise to a helical path during movement. Normal helical motion is interrupted by sudden (60-100 ms)...

Total bacterial abundances estimated with different epifluorescence microscopy methods (4′,6-diamidino-2-phenylindole [DAPI], SYBR Green, and Live/Dead) and with flow cytometry (Syto13) showed good correspondence throughout two microcosm experiments with coastal Mediterranean water. In the Syto13-stained samples we could differentiate bacteria with apparent high DNA (HDNA) content and bacteria with apparent low DNA (LDNA) content. HDNA bacteria, “live” bacteria (determined as such with the Molecular Probes Live/Dead BacLight bacterial viability kit), and nucleoid-containing bacteria (NuCC) comprised similar fractions of the total bacterial community. Similarly, LDNA bacteria and “dead” bacteria (determined with the kit) comprised a similar fraction of the total bacterial community in one of the experiments. The rates of change of each type of bacteria during the microcosm experiments were also positively correlated between methods. In various experiments where predator pressure on bacteria had been reduced, we detected growth of the HDNA bacteria without concomitant growth of the LDNA bacteria, such that the percentage contribution of HDNA bacteria to total bacterial numbers (%HDNA) increased. This indicates that the HDNA bacteria are the dynamic members of the bacterial assemblage. Given how quickly and easily the numbers of HDNA and LDNA bacteria can be obtained, and given the similarity to the numbers of “live” cells and NuCC, the %HDNA is suggested as a reference value for the percentage of actively growing bacteria in marine planktonic environments.