Bacterial abundance

Root exudation and microbial colonization have both been shown to change with plant age and stage of development. The quantity of both proteins (54) and carbohydrates (55) released by herbaceous plants has been shown to decrease with increasing plant age. Liljeroth and Baath (56) found bacterial abundance on the... 

FIGURE 2 Aquatic systems organized as an aqueous gel. The propensity of diverse macromolecules to coalesce imparts a gellike structure on aqueous environments. Bacteria and nutrients associate with this lattice to create microenvironments that enhance community metabolism and diversity. As a result, bacterial abundance may decline more rapidly with dilution than DOM. 

Junge K (2002) Bacterial abundance, activity, and diversity at extremely cold temperatures in Arctic sea ice, Ph.D. dissertation, University of Washington, Seattle... 

Bacteria were inoculated in a 1 10 volume ratio to each prepared media of organic matter (2 1). Bioassays were then incubated at 2°C in the dark for about 30 days. Sub-samples were collected to monitor the time evolution of bacterial abundance, biomass, genetic diversity, 5-cyano-2,3-ditotyl tetrazolium... 

Fig. 2 Time course of bacterial abundance (109 cells 1 1) (a, b), CTC-positive cell ratio (%) (c, d) and bacterial biovolume (pm3) (e, f) during 30 days incubation in low Fe (<1 nM Fe) bioassay and Fe amended bioassay...

The estimation of bacterial growth efficiency from the variation of bacterial biomass and organic carbon consumption could be underestimated due to continual recycling of organic matter by successive bacterial lysis and consumption of this lysed organic matter in our bioassays. In both Fe conditions, the increase of bacterial abundance is controlled by mortality processes. No protozoa were observed by microscopy. Mortality by protozoan grazing can thus be excluded. 

Viral abundance tends to be positively correlated with both chlorophyll concentrations and bacterial abundance (Maranger and Bird, 1995). Data on viral-mediated mortality are highly variable. A number of different approaches have been used to quantify phage-induced mortality of bacteria and phytoplankton but the task has proved challenging. Best estimates are that viruses kill an estimated 20-40% of bacteria daily (Sutde, 2005). There are even fewer estimates of phytoplankton mortality but these too cluster in the 10—40% range with extreme examples where 100% of the mortality of a phytoplankton bloom was attributed to viruses (reviewed in Brussaard, 2004). 

Figure 14.12 Typical profiles of (A) beam attenuation coefficient (BAG, from Naqvi et al., 1993) and (B) bacterial abundance (10 cells from Ducklow, 1993) within the denitrifying zone. The sampling sites, located close to each other (around 15°N,67°E) had similar N02 distributions (pM) (C, open circles corresponding to microbial data). Subsequent measurements have confirmed the co-occurrence of these features (Naqvi etal., in preparation).

Early studies of marine bacteria via culture-based methods indicated a low abundance in seawater (Sverdrup et ah, 1942 ZoBeU, 1941), and that their assemblages were not diverse (Waksman, 1934). Rehable enumeration techniques developed in the late 1970s indicated that typical bacterial abundances in marine surface waters exceeded colony-forming unit (cfu) estimates by over two orders of magnitude (Hobbieeia/., 1977), with abundances of typically 10 cells in surface waters. 

With the present lack of synoptic tools for surveying bacterial abundance, data on the abundance and production of heterotrophic bacteria in the Eastern Mediterranean are more sporadic than for phytoplankton chlorophyll and depend on discrete depth sampling and analysis. The data available are for the Cyprus Eddy in summer (Zohary Robarts, 1992) and in winter (Zohary etal., 1998), and in the Cretan Sea in March and September (van Wambeke etal., 2000), and a general survey of the Levantine Basin in fall (Robarts etal., 1996). Some additional data exists for the Western Mediterranean which will not be reviewed here. 

Bacterial cells were more or less uniformly distributed with depth between the surface and the DCM. Below the DCM the abundance declined with depth. The very few data points available for deep water of the Eastern Mediterranean (>500 m depth) show that bacterial numbers are at least an order of magnitude lower than at the surface water, although there are occasional hot spots of higher bacterial abundance and activity at those great depths. 

As has been noted above, the Eastern Mediterranean is characterised by many eddies and jets (POEM, 1992). Indeed there are almost no areas of the basin which are not part of some mesoscale feature or other (Fig. 4.3). Yet the nutrient distribution (Kress Herat, 2001) and many of the plankton features such as bacterial abundance and activity and chlorophyll content (Yacobi etal., 1995) seem to be nearly constant across large parts of the basin except for those locations where they intersect major and persistent mesoscale features (Fig. 4.5). Under those circumstances major changes in nutrient distribution and productivity can be seen. The Rhodes Gyre and the Cyprus Eddy (aka Shikmona Gyre) are permanent features which always have an effect on the local biogeochemistry and have been studied in some detail. 

Robarts, R. D., Zohary, T., Waiser, M.J. and Yacobi, Y.Z. (1996) Bacterial abundance, biomass, and production in relation to phytoplankton biomass in the Levantine Basin of the southeastern Mediterranean Sea. Marine Ecology Progress Series, 137, 273-281. 

There have been several studies of bacterial abundance and productivity in the Arabian Sea (Ducklow, 1993 Ramaiah etal., 1996 Wiehinga etal., 1997 Campbell etal., 1998 Garrison etal., 2000 Pomroy Joint, 1999 Ducklow etal., 2001). These have revealed that the heterotrophic bacteria, expectedly most abundant in the surface layer, numerically dominate microbial population also, their abundance in the region (>5 x 108 cells l-1) is generally higher throughout the year as compared to other tropical regions. 

Fig. 6.10 Distributions of bacterial abundance at the sea surface during various seasons. Two different methods - epifluorescence microscopy (upper panels) and flow cytometry (lower panels) - were used to enumerate bacteria. Redrawn from Ducklow etal. (2001) with permission from Elsevier Science.

Ramaiah, N., Raghukumar, S. and Gauns, M. (1996) Bacterial abundance and production in the central and eastern Arabian Sea. Current Science (Bangalore), 71, 878—882. 

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