Biomedical subjects

Francesc Peters

Publications and source records attributed to Francesc Peters.

4 recordsLinked to original sources

PubMed2005

Experimental analysis of coagulation of particles under low-shear flow.

The aggregation and breakup of particle flocs were investigated by monitoring the size distribution of a suspension of aggregates, with diameter d(o), under shear flow created by two mixing systems. The aggregation behavior was studied in 63 experiments under various conditions of induced shear rate and particle volume concentration for particle aggregates smaller than the Kolmogorov scale. Despite small shear rates being characteristics of natural systems, only experiments with comparatively high shear rates have been conducted to date. Because of this reason, in this study, the shear rates were chosen to mimic those found in natural systems. In the first set of experiments the aggregate size, d, was analyzed by changing the mean shear, G (ranging from 0.70 to 27.36 s(-1)) created in a tank with a grid oscillating through the whole suspension volume. In the second set of experiments, a spherical flask was placed in an orbital shaking table where G ranged from 0.45 to 2.40s(-1). In all the cases there was an increase of d at increasing G . The dependence on d was found to be identical for the particle volume concentrations investigated, phi=0.2,0.8,2,4,6,8 and 10 x 10(-5), with the stable aggregate size shifting towards aggregate growth as particle volume concentration increased. These results demonstrate that shear provided a means to keep the particle number count high for collisions to occur but it is small enough that the aggregation-breakup balance is dominated by aggregation.

Flocculation↗

PubMed2004

Turbulence increases the average settling velocity of phytoplankton cells.

It is a well known fact that stirring keeps particles suspended in fluids. This is apparent, for instance, when shaking medicine flasks, when agitating tea deposits in a mug, or when heavy winds fill the air with dust particles. The commonplace nature of such observations makes it easy to accept that this feature will apply to any natural phenomenon as long as the flow is turbulent enough. This has been the case for phytoplankton in the surface mixed layers of lakes and oceans. The traditional view assumes that an increase in turbulence bears ecological advantages for nonmotile groups like diatoms that, otherwise, would settle in deep and unlit waters. However, this assumption has no theoretical ground, and the experimental results we present here point in the opposite direction. Phytoplankton settling velocity increases when turbulence intensifies from the low to the higher values recorded in the upper mixed layers of lakes and oceans. Consequently, turbulence does not favor phytoplankton remaining in lit waters but is rather an environmental stress that can only be avoided through morphological and/or physiological adaptations.

Acoustics↗

PubMed2004

Changes in bacterioplankton composition under different phytoplankton regimens.

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.

Animals↗

PubMed2003

Helical Lévy walks: adjusting searching statistics to resource availability in microzooplankton.

The searching trajectories of different animals can be described with a broad class of flight length (lj) distributions with P(lj) = lj-mu. Theoretical studies have shown that changes in these distributions (i.e., different mu values) are key to optimizing the long-term encounter statistics under certain searcher-resource scenarios. In particular, they predict the advantage of Lévy searching (mu approximately 2) over Brownian motion (mu > or = 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 three-dimensional environment, a stronger helical component combined with a Lévy 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.

Animals↗