Grazing rates of bacterivorous protists inhabiting diverse marine planktonic microenvironments

Abstract

Bacterivory on aggregates and bulk seawater by three nanoflagellates and a microciliate was investigated. A new approach to measure bacterivory on the attached bacterial community is proposed. Macroaggregates containing attached fluorescently labeled bacteria (FLB) were obtained by adding labeled planktonic bacteria to rolling tanks filled with natural seawater, and they were used to measure protistan grazing rates on the bacteria attached to aggregates. Protistan grazing on free-suspended bacteria was measured according to the monodispersed FLB uptake technique. The four protists showed low grazing rates when feeding on free-suspended bacteria at the low densities usually found in seawater. Surprisingly, the analyzed protists also showed low grazing rates when feeding on aggregates despite the very high bacterial densities inhabiting them. The analysis of the influence of prey density on bacterial grazing rates suggests that feeding on bacteria attached to aggregates involves a relevant additional effort when compared to feeding on free-suspended bacteria. The paradox between these results and the very high abundance of protistan communities usually found in aggregates is discussed attending to the view of aggregates as biological reactors. Therefore, marine macroaggregates can be considered as microniches able to support the maintenance of a protistan community without food limitation, as well as to attract protistan colonizers by generating a water layer enriched in organic matter and bacteria.

Ross Sea deep-ocean and epipelagic microzooplankton during the summer-autumn transition period

“…In contrast, both small and large heterotrophic dinoflagellates declined rapidly with depth. We also observed that a number of the smaller flagellates, especially among the kinetoplastidae, appeared to be associated with (marine snow) aggregates, an observation consistent with other studies (Artolozaga et al 2002, Kiørboe et al 2004. Among the HNF genera, especially in bathypelagic waters, smaller unidentified species became increasingly important, as has also been reported elsewhere (Arndt et al 2003.…”

“…Similarly, the larger heterotrophic (and mixotrophic) dinoflagellates known to be capable of grazing larger prey also declined rapidly with depth. The loss of these populations was directly associated with the decline of phytoplankton prey, while smaller HNF and oligotrichs have a greater affinity for bacterial and detrital material and are known to colonise descending aggregates (Artolozaga et al 2002, Kiørboe et al 2004. Spatial changes in composition related to epipelagic waters only occurred in the upper mesopelagic zone; beyond these depths, due to the in creasing importance of small unidentified HNF, we were unable to identify clear changes in terms of speciation.…”

Microbial dynamics on diatom aggregates in Øresund, Denmark

“…The size-specific abundance of flagellates showed the strongest decrease with increasing aggregate size, whereas that of ciliates decreased less. The grazing rates of protists were calculated using the relationships between bacteria and protozoa abundance to estimate grazing rates of protozoa (Artolozaga et al 2002). The average cell-specific grazing rate of protists was 2.8 ± 1.9 bacteria flagellate , whereas the fraction of free-living bacteria grazed was 27 ± 18% d -1…”

“…In an inter-ecosystem comparison, Sanders et al (1992) found a strong correlation between specific community bacterivory and bacterial production as well as bacterial numbers. Considering the high densities of attached bacteria relative to that in the surrounding water, grazing rates of bacterivorous protists on these have previously been shown to be surprisingly low compared to those on free-living bacteria (Artolozaga et al 2002). Low grazing rates of aggregate-associated protists, however, may partly be compensated by the higher cell-specific biomass and nutrient content of aggregate-associated microorganisms .…”

Mesocosm study of particle dynamics and control of particle-associated bacteria by flagellate grazing

“…data). Despite this evidence for its importance, quantification of grazing mortality of marine snow bacteria is rare (Caron 1987, Lawrence & Snyder 1998, Riemann et al 2000, Artolozaga et al 2002, 2004.…”