Abstract
This study explores the role of transparent exopolymer particles (TEP) as an additional carbon source for heterotrophic microbial activity in the eutrophic Qishon estuary. From the coastal station and upstream the estuary; TEP concentrations, β-glucosidase activity, bacterial production and abundance have gradually increased. TEP were often found as bio-aggregates, scaffolding algae, detritus matter and bacteria that likely formed “hotspots” for enhance microbial activity. To further demonstrate the link between TEP and heterotrophic bacterial activity, confined incubations with ambient and polysaccharide-enriched estuary water were carried out. Following polysaccharide addition, elevated (~50%) β-glucosidase activity rates were observed, leading to TEP hydrolysis. This newly formed bioavailable carbon resulted in significantly higher growth rates, with up to a 5-fold increase in heterotrophic bacterial biomass, comprising mostly high nucleic acid content bacteria. Taking together the findings from this research, we conclude that even in highly eutrophic environments heterotrophic bacteria may still be carbon limited. Further, TEP as a polysaccharide matrix can act as a metabolic surrogate, adding fresh bioavailable carbon through tight associations with bacteria in eutrophic ecosystems such as the Qishon estuary.
Highlights
Transparent exopolymer particles (TEP) are planktonic, acidic-polysaccharide hydrogels that are ubiquitous in various marine and fresh water environments (Passow, 2002; Bar-Zeev et al, 2015)
We suggest that the higher transparent exopolymer particles (TEP) concentrations at the bottom of the estuary were possibly due to polysaccharide release by benthic suspension feeders (McKee et al, 2005; Heinonen et al, 2007) and sedimentation processes, as previously shown for other estuaries and aquatic environments (Logan et al, 1995; Passow et al, 2001; Simon et al, 2002; Beauvais et al, 2006)
We suggest that heterotrophic bacterial abundance (BA) was higher at the end of the control incubations than initial values, the low growth rates relative to other environments indicate that metabolic activity was mostly tunneled to maintain cellular energetic requirements rather than growth and propagation
Summary
Transparent exopolymer particles (TEP) are planktonic, acidic-polysaccharide hydrogels that are ubiquitous in various marine and fresh water environments (Passow, 2002; Bar-Zeev et al, 2015). In the last two decades, TEP were shown to mediate diverse biochemical cycles in aquatic environments; TEP promote aggregate formation and sedimentation (Passow et al, 2001; Simon et al, 2002; Engel, 2004) by providing a scaffold for marine or lake “snow” (Mari and Kirboe, 1996; Grossart et al, 1997, 2006; Berman and Viner-mozzini, 2001). Semi-enclosed coastal environments where open seawater is partly diluted by fresh river water (Pritchard, 1967; Elliott and McLusky, 2002). Convergence of estuaries and open seawater often result in some of the steepest physiochemical gradients, including light penetration and oxygen concentration (Malone et al, 1996; Hall and Pearl, 2011)
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