Direct and Indirect Effects of Protist Predation on Population Size Structure of a Bacterial Strain with High Phenotypic Plasticity

Abstract

We studied the impact of grazing and substrate supply on the size structure of a freshwater bacterial strain (Flectobacillus sp.) which showed pronounced morphological plasticity. The cell length varied from 2 to >40 m and encompassed rods, curved cells, and long filaments. Without grazers and with a sufficient substrate supply, bacteria grew mainly in the form of medium-sized rods (4 to 7 m), with a smaller proportion (<10%) of filamentous forms. Grazing experiments with the bacterivorous flagellate Ochromonas sp. showed that freely suspended cells of <7 m were highly vulnerable to grazers, whereas filamentous cells were resistant to grazing and became enriched during predation. A comparison of long-term growth in carbon-limited chemostats with and without grazers revealed that strikingly different bacterial populations developed: treatments with flagellates were composed of >80% filamentous cells. These attained a biomass comparable to that of populations in chemostats without grazers, which were composed of medium-sized rods and c-shaped cells. Carbon starvation resulted in a fast decrease in cell length and a shift towards small rods, which were highly vulnerable to grazing. Dialysis bag experiments in combination with continuous cultivation revealed that filament formation was significantly enhanced even without direct contact of bacteria with bacterivores and was thus probably stimulated by grazer excretory products.

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 .…”

High-resolution imaging of pelagic bacteria by Atomic Force Microscopy and implications for carbon cycling

“…High-resolution imaging of pelagic bacteria by AFM F Malfatti et al Bacterial biovolume and its implication for the C cycle Cell biovolume and the associated C distribution are functions of community diversity (Straza et al, 2009), metabolic activity level (Mitchell, 1991), grazing pressure (Simek and Chrzanowski, 1992;Corno and Jurgens, 2006) and phage infection (Dubrovin et al, 2008). As expected, AFM has shown large variations in biovolume classes of natural mixed assemblages.…”

Protists have divergent effects on bacterial diversity along a productivity gradient

“…Bacterial communities undergo morphological and physiological changes when subjected to protist predators by forming predation-resistant filaments (Hahn et al 2000;Jurgens & Sala 2000;Hahn & Hofle 2001) or by increasing cell size (Corno & Jurgens 2006), but there has been less experimental effort focused on the ecological effects of protist predators on bacterial community diversity. Studies have tended to either involve intraspecific morphological diversity (Meyer & Kassen 2007;Friman et al 2008) simplified, constructed bacterial communities (Jiang & Morin 2005;Jiang & Krumins 2006;Meyer & Kassen 2007;Friman et al 2008), or have been observational studies of natural environments (Hahn & Hofle 2001).…”