SUMMARY A trade-off between strategies maximizing growth and minimizing losses appears to be a fundamental property of evolving biological entities existing in environments with limited resources. In the special case of unicellular planktonic organisms, the theoretical framework describing the trade-offs between competition and defense specialists is known as the “killing the winner” hypothesis (KtW). KtW describes how the availability of resources and the actions of predators (e.g., heterotrophic flagellates) and parasites (e.g., viruses) determine the composition and biogeochemical impact of such organisms. We extend KtW conceptually by introducing size- or shape-selective grazing of protozoans on prokaryotes into an idealized food web composed of prokaryotes, lytic viruses infecting prokaryotes, and protozoans. This results in a hierarchy analogous to a Russian doll, where KtW principles are at work on a lower level due to selective viral infection and on an upper level due to size- or shape-selective grazing by protozoans. Additionally, we critically discuss predictions and limitations of KtW in light of the recent literature, with particular focus on typically neglected aspects of KtW. Many aspects of KtW have been corroborated by in situ and experimental studies of isolates and natural communities. However, a thorough test of KtW is still hampered by current methodological limitations. In particular, the quantification of nutrient uptake rates of the competing prokaryotic populations and virus population-specific adsorption and decay rates appears to be the most daunting challenge for the years to come.

Diel variations in substrate availability (largely due to variations in phytoplankton production) can lead to pronounced diel patterns in prokaryotic activity in the euphotic zone. We examined short-term changes in viral infection of bacterioplankton and its relation to bacterial activity in 3 distinct masses of North Sea surface waters marked by drifting buoys in June 2001 and April 2002. The water masses were sampled every 4 to 6 h for a period of 20 to 36 h. The frequency of infected cells (FIC) estimated by a virus dilution approach varied from 5 to 64% at the western site, 17 to 55% at the southern site, and 10 to 22% at the northern site and was generally higher during the night than at daytime. Furthermore, FIC was negatively related to bacterial activity at all sites. Bacterial activity, measured via [14 C]-leucine incorporation, was ca. 1.5-to 5-fold higher during the day than at night. Our results indicate that viral lysis of bacteria occurs around noon to afternoon, and infection mainly during the night. Moreover, lysis and viral production seem to take place during high bacterial activity; this could be a strategy to increase the number of newly produced viruses. KEY WORDS: Viral infection · Viral lysis · Bacteria · Frequency of viral infectionResale or republication not permitted without written consent of the publisher Aquat Microb Ecol 35: 207-216, 2004 by the availability of dissolved organic carbon (DOC) during the night in the surface layers of the subtropical Atlantic Ocean, again implying a coupling of phytoplankton extracellular release and bacterial activity. These examples indicate that phyto-and bacterioplankton activity can change at the scale of hours, while cell abundance and biomass are often more constant. Such cycles result in efficient recycling mechanisms of carbon and nutrients within the microbial food web.In situ studies assessing changes in microbiological processes at high temporal resolution are relatively rare, especially for virioplankton. Suttle & Chen (1992) have argued that the concentration of infectious viruses should display a strong diel signal in surface waters due to sunlight-mediated viral decay. While a number of studies have found that total viral abundance in surface waters varies only moderately at the scale of hours to days (Wommack & Colwell 2000), other authors have found more pronounced changes. For example, viral abundance increased within 2 d during a coastal phytoplankton bloom in the North Atlantic (Bratbak et al. 1990). By following a defined water mass in the northern Adriatic Sea, Weinbauer et al. (1995) found changes in viral and bacterial abundance that resembled predator-prey oscillations; however, no diel rhythm was observed. Bratbak et al. (1996) found significant temporal fluctuations of viral abundance in bottle incubations and in situ within 10 to 20 min. Bettarel et al. (2002) reported diel periodicity in viral abundance and the frequency of visibly infected cells in the Bay of Villefranche (France, Mediterranean Sea) with ...

The frequency of virus infected bacterial cells (FIC) was estimated in surface waters of the Mediterranean Sea, the Baltic Sea and the North Sea using the frequency of visibly infected cells (FVIC) as determined by transmission electron microscopy (TEM) and published average conversion factors (average 5.42, range 3.7 to 7.14) to relate FVIC to FIC. A virus dilution approach was used to obtain an independent estimation of FIC in bacterioplankton, and we provide evidence for the reliability of this approach. Across all investigated environments, FIC ranged from 2.4 to 43.4%. FIC data using both approaches were well correlated; however, the values were higher using the virus dilution approach. This indicates that the TEM approach has the potential to reveal spatiotemporal trends of viral infection; however, it may underestimate the significance of viral infection of bacteria when average conversion factors are used. Using data from the virus dilution approach and the TEM approach, we calculated new conversion factors for relating FVIC to FIC (average 7.11, range 4.34 to 10.78). Virally caused mortality of bacteria estimated from published FVIC data of marine and freshwater systems and using the new conversion factors ranged from not detectable to 129%, thus confirming that viral infection is a significant and spatiotemporally variable cause of bacterial cell death.

The community structure of attached and free-living bacteria in the Aegean Sea (eastern Mediterranean Sea) was analyzed with use of terminal-restriction fragment length polymorphism (T-RFLP) fingerprinting. Since the Aegean Sea is characterized by rather small temperature fluctuations between surface and deep-water layers, it represents an ideal study site to determine the variations in the community structure of bacteria with depth, since environmental factors other than temperature are likely to determine depth zonation of bacteria. The analysis of 132 T-RFLP electropherograms indicated pronounced differences among the attached and free-living bacterial communities defined as operational taxonomic units (OTUs). Distinct vertical differences of attached and free-living OTUs were found between mesopelagic waters (Ͼ200 m depth) and the upper mixed water column (ϳ10-200 m). Attached and free-living OTUs differed considerably throughout the water column, with only ϳ35% for the South Aegean and ϳ24% for the North Aegean of all OTUs in both free-living and attached OTUs. Approximately 50% of attached and free-living OTUs were present throughout the water column. Fingerprinting analysis using 16S rRNA indicated that only ϳ14% of the attached and ϳ33% of the free-living OTUs were identical to the 16S rDNA fingerprints. The distribution of free-living versus attached bacteria as obtained in this study suggests that even in the absence of temperature as a major selective factor, a distinct deep-water bacterial community exists (particularly in the free-living mode). The deep-water free-living bacterial community appears to be as compositionally complex as the surface water free-living bacterial community.

A three-dimensional theory is described for field-scale Fickian dispersion in anisotropic porous media due to the spatial variability of hydraulic conductivities. The study relies partly on earlier work by the authors the attributes of which are briefly reviewed. It leads to results which differ in important ways from earlier theoretical conclusions about dispersion in anisotropic media. We express the dispersion tensor D as the sum of a local component d and a field-scale component A. The local component is assumed to be independent of velocity (which is most appropriate if it represents molecular diffusion) and its principal terms are taken to act parallel and normal to the mean velocity vector 1•. The field-scale component is written as 0t/•, where 0t is a dispersivity tensor and/• = [1•[. We show that at large Peclet numbers P, the dispersivity tensor reduces to a single principal component parallel to the mean velocity, regardless of how 1• is oriented. This result, valid for arbitrary covariance functions of log-hydraulic conductivity, differs from that of L. W. Gelhar and C. L. Axness (1983), according to whom the asymptotic dispersivity tensor may possess more than one nonzero eigen value. They calculate the direction of the largest principal dispersivity to be offset from the mean velocity toward the direction of least spatial correlation (or away from the stratification in typical layered media). We show that this principal dispersivity is offset in the opposite direction at small and intermediate Peclet numbers but rotates toward the mean velocity as P increases. The largest eigen value is constant and dominated by field-scale velocity fluctuations at large P values. The other two eigen values diminish asymptotically in proportion to P-x and are controlled by d as well as by field-scale differential convection. The range of small Peclet numbers has not been previously investigated under anisotropic conditions yet is of much importance for transport in low-permeability rocks or soils. We show that at low P values all three principal dispersivities are proportional to P and thus A is proportional to /•2 (a phenomenon reminiscent of Taylor diffusion). When the mean velocity is inclined to the axes of anisotropy, the eigen values of A are neither parallel nor normal to 1•. However, since D is dominated by d at small Peclet numbers, the principal dispersion coefficients are asymptotically (as P--• 0) parallel and normal to the mean velocity just like when P is large; their maximum deviation from these directions occurs at intermediate P values.Subsurface solute transport has traditionally been described by the convection-dispersion equation [Scheidegger, 1954;Bear, 1972;Fried, 1975] Oc/Ot = (V. dV -V. v)c c(x, 0) = Co(X ) (1) where c(x, t) is the concentration per unit volume or mass of groundwater; x is a position vector; t is time; d is a hydrodynamic dispersion tensor; v(x) is a seepage velocity vector; and V is a gradient operator. This equation is derived from mass balance considerations and the assump...