The potential importance of grazing-resistant bacteria in planktonic systems

Abstract

The paradigm of the 'microbial loop' has became increasingly important for understanding the structure and function of aquatic ecosystems. Most of the microbial loop studies have focused on energy flow and nutrient cycling. Much less is known, however, about the importance of grazing as a force shaping the structure and community composition of planktonic bacteria. Theoretical considerations of predator-prey interactions suggest that predator evasion mechanisms should have evolved for bactena in the same way as in other predator-prey systems (e.g. zooplankton-phytoplankton). Consistent with this hypothesis, field data show that bacteria are often the most stable component of planktonic communities. Refuges from grazing are one of the possible mechanisms buffering bacterioplankton against strong seasonal fluctuations in abundance. Substantial direct and indirect evidence exists for the occurrence of grazing-resistant bacteria (GRB) in both marine and freshwater habitats. We summarize the potenbal mechanisms for grazing resistance, including morphological, chemcal and behavioral defenses as well as growth in spatial refuges. Cell size appears to be an important factor influencing susceptibility to grazing, with a refuge at the lower and upper ends of the bacterial size range. Thus, a relative grazing resistance can be assumed for the large number of ultramicrobactena as well as for morphologically complex growth forms such as filaments and aggregates. Besides morphological features, resistance may be achieved by other mechanisms for which, however, much less information is available. We describe how GRB can be included in conceptual models of the interactions among metazooplankton, bacterivorous protozoans and bacteria. It is suggested that the relative importance of GRB increases with increasing grazing pressure exerted by protozoans, whereas it decreases with increasing top-down control of protozoans by metazooplankton. GRB may reduce the productivity of planktonic systems through decreased trophic transfer efficiencies and reduced regeneration of bacterially bound nutrients.

Morphology and DNA content of bacterioplankton in the northern Gulf of Mexico: analysis by epifluorescence microscopy and flow cytometry

“…The latter result is consistent with the reported size-and activity-selective grazing behavior of flagellates (Jürgens & Güde 1994, del Giorgio et al 1996, López-Amorós et al 1998. The share of B-III ('high DNA') bacteria shall, therefore, correspond to that of 'active' bacteria as derived from the Live/ Dead fluorescence kit (Molecular Probes), autoradiography and rRNA probes (Gasol & del Giorgio 2000).…”

“…Higher cell-specific uptake rates of 3 H-thymidine (Jellett et al 1996) and leucine ) of high-DNA bacteria might be seen in the context that bacteria with a 5-fold higher DNA content (Gasol & del Giorgio 2000) also need to incorporate 5 times as much thymidine for a single DNA replication. Still, the higher cell-specific growth rates of high-DNA cells in incubation experiments (Li et al 1995) leave grazer impact as an important, community-shaping process (Jürgens & Güde 1994) that deserves further investigation.…”

Changes in the Epilimnetic Bacterial Community Composition, Production, and Protist-Induced Mortality along the Longitudinal Axis of a Highly Eutrophic Reservoir

“…However, depending on which are the dominant protistan bacterivores in a given freshwater system, bacteria in a certain size range are thought to be protected against HNF grazing (usually between 3 and 5 µm long when small bacterivorous HNF dominate [16,19]). Large filaments, other large complex growth forms, bacterial aggregates, and particle-bound bacteria are thus largely resistant to HNF predation [16].…”

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

“…Prey cell size has an important role in determining bacterial susceptibility to bacterivory, as shown by several empirical studies reporting a preference for larger bacterial prey by cultured and natural assemblages of flagellates (Andersson et al, 1986;Gonzalez et al, 1990;Jürgens and Güde, 1994;Kinner et al, 1998) and ciliates (Fenchel, 1980;Epstein et al, 1992;Simek et al, 1995). 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).…”