Role of bacterial phenotypic traits in selective feeding of the heterotrophic nanoflagellate Spumella sp.

Abstract

The influence of different bacterial phenotypic traits on the feeding selectivity of a bacterivorous nanoflagellate was investigated in laboratory experiments. Twelve bacterial isolates from freshwater habitats were characterized in terms of cell size, morphology, capsule formation, surface hydrophobicity and charge, and swimming behavior. Mechanisms of differential flagellate feeding on these isolates were studied in short-term grazing experiments by high-resolution video microscopy using the nanoflagellate Spumella sp. as a model interception-feeding predator. The differentiation into distinct stages of the feeding process (contact, capture, ingestion) revealed a complex selection behavior illustrated by the relatively small proportion of ingested bacteria over cell contacts. We found that bacterial swimming speed increased contact rates but also decreased flagellate capture efficiency, which had a compensating effect on overall ingestion rates. Bacterial cell size revealed no correlation with flagellate contact rates but an effective inhibition of ingestion rates when exceeding a critical size limit. Correlation analysis presented no bacterial property to account for the residual variability of flagellate ingestion efficiencies. Experiments with specifically coated artificial particles provided evidence for the relative importance of the biochemical surface composition in flagellate food selection compared to physicochemical interaction forces. Only extreme charges beyond -45 mV reduced flagellate ingestion rates. Our results reinforce the idea that bacterial cell size strongly affects the feeding success of bacterivorous flagellates, and further implicates size-independent bacterial traits such as swimming speed and biochemical surface composition in this feeding success.

The Selective Value of Bacterial Shape

“…Protists face the same choices when grazing, and the results are similar: quite often, bacteria too large or too small are rejected in favor of a "just-right" range of interme- diate-sized cells (114,150,206,246,248,249). Hydrodynamic calculations indicate that bacteria smaller than 0.5 m in diameter encounter grazing protists four to six times less often than do larger cells (ϳ1 m), and filamentous cells or cells with diameters greater than 3 m are often too large for protists to ingest (204,206,246). Therefore, cells in the intermediate size range are consumed more rapidly, very often creating a "bimodal effect" that selects for the very large or the very small (Fig.…”

Enterococci in the Environment

“…Digestion rates of both flagellated and ciliated protozoa increased exponentially at temperatures between 12°C and 22°C (294), and a direct correlation between rates of predation and temperature was found in a variety of environments, with more vigorous grazing and an increase in protozoan concentrations at higher temperatures (7,9,19,225,294). Prey characteristics such as cell wall morphology and the physiological state may also influence the magnitude and efficiency of protozoan grazing (23,127,220,300,343). Notably, lower rates of grazing were observed for Gram-positive organisms (including E. faecalis) than for E. coli (75,126,169,170,253).…”

Marine bacterial community structure resilience to changes in protist predation under phytoplankton bloom conditions

“…The physiological state of bacterial cells has also been suggested as a key factor in grazing selectivity (Del Giorgio and Gasol, 2008), and preferential grazing of the more active cells within a community by protist grazers has been repeatedly observed (Del Giorgio et al, 1996;Pernthaler et al, 1997;Simek et al, 1997;Tadonléké et al, 2005;Sintes and Del Giorgio, 2014). This is probably related to the general positive relation between cell size and activity in marine bacteria (Gasol et al, 1995;Hahn and Höfle, 2001;Matz and Jürgens, 2001;Matz et al, 2002;Corno and Jürgens, 2006), suggesting that larger bacterioplankton cells are also usually the most active ones. Moreover, there could be a concentration-dependent component, where grazing might be different on abundant versus non-abundant populations .…”