Viral production, decay rates, and life strategies along a trophic gradient in the North Adriatic Sea

Although the relationships between trophic conditions and viral dynamics have been widely explored in different pelagic environments, there have been few attempts at independent estimates of both viral production and decay. In this study, we investigated factors controlling the balance between viral production and decay along a trophic gradient in the north Adriatic basin, providing independent estimates of these variables and determining the relative importance of nanoflagellate grazing and viral life strategies. Increasing trophic conditions induced an increase of bacterioplankton growth rates and of the burst sizes. As a result, eutrophic waters displayed highest rates of viral production, which considerably exceeded observed rates of viral decay (up to 2.9 x 10(9) VLP liter(-1) h(-1)). Viral decay was also higher in eutrophic waters, where it accounted for ca. 40% of viral production, and dropped significantly to 1.3 to 10.7% in oligotrophic waters. These results suggest that viral production and decay rates may not necessarily be balanced in the short term, resulting in a net increase of viruses in the system. In eutrophic waters nanoflagellate grazing, dissolved-colloidal substances, and lysogenic infection were responsible together for the removal of ca. 66% of viral production versus 17% in oligotrophic waters. Our results suggest that different causative agents are primarily responsible for the removal of viruses from the water column in different trophic conditions. Factors other than those considered in the past might shed light on processes responsible for the removal and/or decay of viral particles from the water column.     

Rapid Virus Production and Removal as Measured with Fluorescently Labeled Viruses as Tracers

Pelagic marine viruses have been shown to cause significant mortality of heterotrophic bacteria, cyanobacteria, and phytoplankton. It was previously demonstrated, in nearshore California waters, that viruses contributed to up to 50% of bacterial mortality, comparable to protists. However, in less productive waters, rates of virus production and removal and estimates of virus-mediated bacterial mortality have been difficult to determine. We have measured rates of virus production and removal, in nearshore and offshore California waters, by using fluorescently labeled viruses (FLV) as tracers. Our approach is mathematically similar to the isotope dilution technique, employed in the past to simultaneously measure the release and uptake of ammonia and amino acids. The results indicated overall virus removal rates in the dark ranging from 1.8 to 6.2% h⁻¹ and production rates in the dark ranging from 1.9 to 6.1% h⁻¹, corresponding to turnover times of virus populations of 1 to 2 days, even in oligotrophic offshore waters. Virus removal rates determined by the FLV tracer method were compared to rates of virus degradation, determined at the same locations by radiolabeling methods, and were similar even though the current FLV method is suitable for only dark incubations. Our results support previous findings that virus impacts on bacterial populations may be more important in some environments and less so in others. This new method can be used to determine rates of virus degradation, production, and turnover in eutrophic, mesotrophic, and oligotrophic waters and will provide important inputs for future investigations of microbial food webs.     

Viral Activity in Two Contrasting Lake Ecosystems

For aquatic systems, especially freshwaters, there is little data on the long-term (i.e., >6-month period) and depth-related variability of viruses. In this study, we examined virus-induced mortality of heterotrophic bacteria over a 10-month period and throughout the water column in two lakes of the French Massif Central, the oligomesotrophic Lake Pavin and the eutrophic Lake Aydat. Concurrently, we estimated nonviral mortality through heterotrophic nanoflagellate and ciliate bacterivory. Overall, viral infection parameters were much less variable than bacterial production. We found that the frequency of visibly infected cells (FVIC), estimated using transmission electron microscopy, peaked in both lakes at the end of spring (May to June) and in early autumn (September to October). FVIC values were significantly higher in Lake Pavin (mean [M] = 1.6%) than in Lake Aydat (M = 1.1%), whereas the opposite trend was observed for burst sizes, which averaged 25.7 and 30.2 virus particles bacterium⁻¹, respectively. We detected no significant depth-related differences in FVIC or burst size. We found that in both lakes the removal of bacterial production by flagellate grazing (M_Pavin = 37.7%, M_Aydat = 18.5%) was nearly always more than the production removed by viral lysis (M_Pavin = 16.2%, M_Aydat = 19%) or ciliate grazing (M_Pavin = 2.7%, M_Aydat = 8.8%). However, at specific times and locations, viral lysis prevailed over protistan grazing, for example, in the anoxic hypolimnion of Lake Aydat. In addition, viral mortality represented a relatively constant mortality source in a bacterial community showing large variations in growth rate and subject to large variations in loss rates from grazers. Finally, although viruses did not represent the main agent of bacterial mortality, our data seem to show that their relative importance was higher in the less productive system.     

Dynamics and Distribution of Cyanophages and Their Effect on Marine Synechococcus spp.

Cyanophages infecting marine Synechococcus cells were frequently very abundant and were found in every seawater sample along a transect in the western Gulf of Mexico and during a 28-month period in Aransas Pass, Tex. In Aransas Pass their abundance varied seasonally, with the lowest concentrations coincident with cooler water and lower salinity. Along the transect, viruses infecting Synechococcus strains DC2 and SYN48 ranged in concentration from a few hundred per milliliter at 97 m deep and 83 km offshore to ca. 4 × 10⁵ ml^-1 near the surface at stations within 18 km of the coast. The highest concentrations occurred at the surface, where salinity decreased from ca. 35.5 to 34 ppt and Synechococcus concentrations were greatest. Viruses infecting strains SNC1, SNC2, and 838BG were distributed in a similar manner but were much less abundant (<10 to >5 × 10³ ml^-1). When Synechococcus concentrations exceeded ca. 10³ ml^-1, cyanophage concentrations increased markedly (ca. 10² to > 10⁵ ml^-1), suggesting that a minimum host density was required for efficient viral propagation. Data on the decay rate of viral infectivity d (per day), as a function of solar irradiance I (millimoles of quanta per square meter per second), were used to develop a relationship (d = 0.2610I - 0.00718; r² = 0.69) for conservatively estimating the destruction of infectious viruses in the mixed layer of two offshore stations. Assuming that virus production balances losses and that the burst size is 250, ca. 5 to 7% of Synechococcus cells would be infected daily by viruses. Calculations based on contact rates between Synechococcus cells and infectious viruses produce similar results (5 to 14%). Moreover, balancing estimates of viral production with contact rates for the farthest offshore station required that most Synechococcus cells be susceptible to infection, that most contacts result in infection, and that the burst size be about 324 viruses per lytic event. In contrast, in nearshore waters, where ca. 80% of Synechococcus cells would be contacted daily by infectious cyanophages, only ca. 1% of the contacts would have to result in infection to balance the estimated virus removal rates. These results indicate that cyanophages are an abundant and dynamic component of marine planktonic communities and are probably responsible for lysing a small but significant portion of the Synechococcus population on a daily basis.     

Viral Abundance, Decay, and Diversity in the Meso- and Bathypelagic Waters of the North Atlantic

To elucidate the potential importance of deep-water viruses in controlling the meso- and bathypelagic picoplankton community, the abundance, decay rate, and diversity of the virioplankton community were determined in the meso- and bathypelagic water masses of the eastern part of the subtropical North Atlantic. Viral abundance averaged 1.4 × 10⁶ ml⁻¹ at around 100 m of depth and decreased only by a factor of 2 at 3,000 to 4,000 m of depth. In contrast, picoplankton abundance decreased by 1 order of magnitude to the Lower Deep Water (LDW; 3,500- to 5,000-m depth). The virus-to-picoplankton ratio increased from 9 at about 100 m of depth to 110 in the LDW. Mean viral decay rates were 3.5 × 10⁻³ h⁻¹ between 900 m and 2,750 m and 1.1 × 10⁻³ h⁻¹ at 4,000 m of depth, corresponding to viral turnover times of 11 and 39 days, respectively. Pulsed-field gel electrophoresis fingerprints obtained from the viral community between 2,400 m and 4,000 m of depth revealed a maximum of only four bands from 4,000 m of depth. Based on the high viral abundance and the low picoplankton production determined via leucine incorporation, we conclude that the viral production calculated from the viral decay is insufficient to maintain the high viral abundance in the deep North Atlantic. Rather, we propose that substantial allochthonous viral input or lysogenic or pseudolysogenic production is required to maintain the high viral abundance detected in the meso- and bathypelagic North Atlantic. Consequently, deep-water prokaryotes are apparently far less controlled in their abundance and taxon richness by lytic prokaryotic phages than the high viral abundance and the virus-to-picoplankton ratio would suggest.     

Relationship between bacteria, phytoplankton and heterotrophic nanoflagellates along the trophic gradient

Bacterial and heterotrophic nanoflagellates (HNF) abundance, as well as bacterial production and chlorophylla levels, were measured at five sites extending from the coastal zone toward the open Adriatic in the period from March to October 1995. The investigated areas were grouped into trophic categories according to concentrations of chlorophylla. All the biotic-para-meters increased along the trophic gradient, leading to eutrophy, but they did not increase at the same rate. The bacterial biomass: phytoplankton biomass (BB: chla) ratio decreased from about 10 in the very oligotrophic area to 0.8 at the eutrophic site. In contrast, the bacterial abundance: HNF abundance ratio (B: HNF) increased from 1000 bacteria per 1 flagellate in the oligotrophic system to 1700 bacteria flagellate⁴ in the eutrophic area. Decreasing BB: chla and increasing B: HNF ratios along the trophic gradient might reflect the different structures of the microbial food web. Relationships between bacterial abundance and production, and chla and HNF showed that bacterial abundance along the trophic gradient was regulated by the interplay between nutrient supply and grazing pressure. But in the oligotrophic system, bacterial abundance was more closely related to bacterial production and chla than in the eutrophic system, suggesting stronger control of bacterial abundance by substrate supply. On the other hand, the coupling between bacteria and HNF, and uncoupling between bacterial abundance and production in the eutrophic system, showed that the importance of bacteriovory increased in richer systems.     

Viral activity in two contrasting lake ecosystems

For aquatic systems, especially freshwaters, there is little data on the long-term (i.e., >6-month period) and depth-related variability of viruses. In this study, we examined virus-induced mortality of heterotrophic bacteria over a 10-month period and throughout the water column in two lakes of the French Massif Central, the oligomesotrophic Lake Pavin and the eutrophic Lake Aydat. Concurrently, we estimated nonviral mortality through heterotrophic nanoflagellate and ciliate bacterivory. Overall, viral infection parameters were much less variable than bacterial production. We found that the frequency of visibly infected cells (FVIC), estimated using transmission electron microscopy, peaked in both lakes at the end of spring (May to June) and in early autumn (September to October). FVIC values were significantly higher in Lake Pavin (mean [M] = 1.6%) than in Lake Aydat (M = 1.1%), whereas the opposite trend was observed for burst sizes, which averaged 25.7 and 30.2 virus particles bacterium(-1), respectively. We detected no significant depth-related differences in FVIC or burst size. We found that in both lakes the removal of bacterial production by flagellate grazing (M(Pavin) = 37.7%, M(Aydat) = 18.5%) was nearly always more than the production removed by viral lysis (M(Pavin) = 16.2%, M(Aydat) = 19%) or ciliate grazing (M(Pavin) = 2.7%, M(Aydat) = 8.8%). However, at specific times and locations, viral lysis prevailed over protistan grazing, for example, in the anoxic hypolimnion of Lake Aydat. In addition, viral mortality represented a relatively constant mortality source in a bacterial community showing large variations in growth rate and subject to large variations in loss rates from grazers. Finally, although viruses did not represent the main agent of bacterial mortality, our data seem to show that their relative importance was higher in the less productive system.     

Contribution of virus-induced lysis and protozoan grazing to benthic bacterial mortality estimated simultaneously in microcosms

In contrast to the water column, the fate of bacterial production in freshwater sediments is still a matter of debate. Thus, the importance of virus-induced lysis and protozoan grazing of bacteria was investigated for the first time simultaneously in a silty sediment layer of a mesotrophic oxbow lake. Microcosms were installed in the laboratory in order to study the dynamics of these processes over 15 days. All microbial and physicochemical parameters showed acceptable resemblance to field data observed during a concomitant in situ study, and similar conclusions can be drawn with respect to the quantitative impact of viruses and protozoa on the bacterial compartment. Viral decay rates ranged from undetectable to 0.078 h⁻¹ (average, 0.033 h⁻¹), and the control of bacterial production from below the detection limit to 36% (average, 12%). The contribution of virus-induced lysis of bacteria to the dissolved organic matter pool as well as to benthic bacterial nutrition was low. Ingestion rates of protozoan grazers ranged from undetectable to 24.7 bacteria per heterotrophic nanoflagellate (HNF) per hour (average, 4.8 bacteria HNF⁻¹ h⁻¹) and from undetectable to 73.3 bacteria per ciliate per hour (average, 11.2 bacteria ciliate⁻¹ h⁻¹). Heterotrophic nanoflagellate and ciliates together cropped up to 5% (average, 1%) of bacterial production. The viral impact on bacteria prevailed over protozoan grazing by a factor of 2.5–19.9 (average, 9.5). In sum, these factors together removed up to 36% (average, 12%) of bacterial production. The high number of correlations between viral and protozoan parameters is discussed in view of a possible relationship between virus removal and the presence of protozoan grazers.     

The significance of viruses to mortality in aquatic microbial communities

A variety of approaches including enumeration of visibly infected microbes, removal of viral particles, decay of viral infectivity, and measurements of viral production rates have been used to infer the impact of viruses on microbial mortality. The results are surprisingly consistent and suggest that, on average, about 20% of marine heterotrophic bacteria are infected by viruses and 10–20% of the bacterial community is lysed daily by viruses. The effect of viruses on phytoplankton is less certain, but ca. 3% of Synechococcus biomass may be lysed daily. The fraction of primary productivity this represents depends upon the relative biomass and growth rate of Synechococcus. Virus enrichment experiments suggest that the productivity of eukaryotic phytoplankton would be ca. 2% higher in the absence of viruses. Overall, probably about 2–3% of primary productivity is lost to viral lysis. There is considerable variation about these estimates; however, they represent a starting point for incorporating viral-mediated processes into aquatic ecosystem models.     

Role of planktonic bacteria in productivity and cycling of organic matter in the Eastern Pacific Ocean

Total number, biomass, production, and respiration of bacterioplankton were measured in oligotrophic, mesotrophic and eutrophic waters of the Eastern Pacific. Total number of bacteria in the upper mixed layer and in the upper thermocline boundary layers varied from 30–60.10³ ml^-1 in oligotrophic waters to 100–400.10³ ml^-1 in mesotrophic waters of fronts and divergences, and to 1–2,5.10⁶ ml^-1 in eutrophic waters of coastal upwellings. Wet biomass varied from 5–10 mg l^-1 in oligotrophic waters, to 50–200 mg l^-1 in mesotrophic waters, and to 1–2 g m^-3 in eutrophic waters. Below the layer of maximum temperature gradient i.e. below 35–50 m, bacterioplankton density decreased 5–10 times. P/B coefficients per day were highest in the oligotrophic surface water ( 1), and lowest in the eutrophic ones (0.2–0.4). In mesotrophic waters they were intermediate (0.4–1.0). the stock of labile organic matter (LOM) accessible to microbial action varied from 0.3 to 1.6 mg Cl^-1. Its highest value occurred in the upwelling area. The stock of LOM does not noticeably decrease from the euphotic zone to a depth of 2 000 m. Its turnover time varied from 5 to 45 days in surface waters, and 30–50 years in deep oceanic waters. The role of bacterioplankton in productivity and in cycling of organic matter in surface — and deep oceanic waters is discussed.     

Effects of Deposit-Feeding Macrofauna on Benthic Bacteria,Viruses, and Protozoa in a Silty Freshwater Sediment

In microcosm experiments, we simultaneously tested the effects of increased numbers of deposit-feeding macrofauna (chironomids, oligochaetes and cladocerans) on the standing stock, activities and interactions of heterotrophic bacteria, viruses, and bacterivorous protozoa (heterotrophic nanoflagellates and ciliates) in the aerobic layer of a silty littoral freshwater sediment. On average, bacterial secondary production was stimulated between 11 and 29% by all macrofaunal groups compared to control experiments without macrofauna addition. Bacterial standing stock increased significantly by 8 and 13% in case of chironomids and cladocerans, respectively. Oligochaetes and chironomids produced significant negative effects on viral abundance while the results with cladocerans were inconsistent. The addition of oligochaetes and chironomids resulted in a significant decrease by on average 68 and 32% of viral decay rates, respectively, used as a measure of viral production. The calculated contribution of virus-induced lysis to benthic bacterial mortality was low, with 2.8 to 11.8% of bacterial secondary production, and decreased by 39 to 81% after the addition of macrofauna compared to the control. The abundances of heterotrophic nanoflagellates were significantly reduced by 20% by all tested macrofauna groups, while ciliates showed inconsistent results. The importance of heterotrophic nanoflagellate grazing on benthic bacteria was very low (<1% of bacterial secondary production) and was further reduced by elevated numbers of macrofauna. Thus, the selected deposit feeding macrofauna groups seem to have several direct and indirect and partly antagonistic effects on the benthic bacterial compartment through the enhancement of bacterial production and the reduction of virus-induced cell lysis and protozoan grazing.     

Growth and grazing losses of prokaryotes in the central Atlantic Ocean

The trophic relation between prokaryotes and heterotrophic nanoflagellateswas studied during two latitudinal cruises in the central AtlanticOcean. The losses to predation on prokaryotes were determinedin 12 locations covering a wide range of trophic situations,from ultraoligotrophic [<0.05 mg chlorophyll a (Chl a) m^–3]to moderately eutrophic waters (>1 mg Chl a m^–3). Inthese locations, the abundance of prokaryotes (P) covaries withthat of heterotrophic nanoflagellates, thus suggesting thatresources controlled the abundance of heterotrophic nanoflagellates(HNF). Besides, the losses to predation were positively relatedto prokaryotic and heterotrophic nanoflagellate biomass, whichpoints toward higher consumption rates associated with largerconcentrations of preys and predators. Conversely, decliningtrends between prokaryotic production (P_P) and the fractionof this production lost to predation revealed higher relativelosses in the environments with lower productions. Our studyshows for the central Atlantic that 35% of prokaryotic biomass(B_P), equating to between 40 and 83% of P_P can be ingested dailyand that 55% of the variability observed in the rate of prokaryoticloss to predation was related with the HNF. As predators grazeon many prey types, in an oligotrophic system containing manyprey species but little numeric loading, there will still beprey for predators but not enough hosts for viruses. In thissense, our study confirms the importance of the prey–predatorrelationship between prokaryotes and heterotrophic nanoflagellatesin the flow of carbon of the less productive regions of theocean.     

Effects of UV-A (320 to 399 Nanometers) on Grazing Pressure of a Marine Heterotrophic Nanoflagellate on Strains of the Unicellular Cyanobacteria Synechococcus spp.

In the open ocean, where turbidity is very low, UV radiation may be an important factor regulating interactions among planktonic microorganisms. The effect of exposure to UV radiation on grazing by a commonly isolated marine heterotrophic nanoflagellate, Paraphysomonas bandaiensis, on two strains of the cyanobacteria Synechococcus spp. was investigated. Laboratory cultures were exposed to a range of irradiances of artificially produced UV-B (290 to 319 nm) and UV-A (320 to 399 nm) for up to 10 h. At a UV-B irradiance of 0.19 W m⁻², but not 0.12 W m⁻², grazing mortality of Synechococcus spp. and nanoflagellate-specific grazing rates were reduced compared to mortality and grazing rates with UV-A treatment. Within 6 h of exposure, UV-A alone suppressed grazing mortality at irradiances as low as 3.02 W m⁻². The extent to which grazing mortality and nanoflagellate-specific grazing rates were suppressed by UV-A increased with both irradiance and duration of exposure. Over a 6-h exposure period, differences in grazing mortality were largely attributable to differential survival of nanoflagellates. Over a longer period of exposure, there was impairment by UV-A alone of nanoflagellate-specific grazing rates. Rates of primary productivity of Synechococcus spp. were also reduced by UV-A. The extent to which Synechococcus productivity was reduced, compared to the reduction in Synechococcus grazing mortality, depended on the duration of UV-A exposure. These results support the hypothesis that UV-A alone influences the composition and biomass of marine microbial communities by affecting predator-prey interactions and primary production.     

Distribution of Viruses and Dissolved DNA along a Coastal Trophic Gradient in the Northern Adriatic Sea

The distribution of viral and other microbial abundances as well as the concentrations of dissolved DNA (D-DNA) along a trophic gradient in the northern Adriatic Sea were determined. Virus abundances, covering a range of 1.2 × 10⁹ to 8.7 × 10¹⁰ liter^-1 were on average 2.5-fold higher in eutrophic than in mesotrophic stations. A 2.5-fold enrichment was also measured for chlorophyll a concentrations, whereas the densities of bacteria and heterotrophic nanoflagellates were only approximately 1.5-fold higher. The frequency of bacteria containing mature phage increased linearly with bacterial abundance. Assuming that mature phage is only visible during the last 14 to 27% of the latent period (L. M. Proctor, A. Okubo, and J. A. Fuhrman, Microb. Ecol. 25:161-182, 1993), we estimated that between 3.5 and 7.3% of the bacterial population was infected at mesotrophic stations versus between 7.0 and 19.5% at eutrophic stations, indicating that the bacterial mortality due to viral lysis might increase with the degree of eutrophication. The frequency of bacteria with mature phage and the burst size varied significantly with the bacterial morphotype; rod-shape cells, the most abundant morphotype, showed low infection rates but a high burst size. Concentrations of D-DNA varied significantly with season but not with trophic conditions. The estimated percentage of viral DNA on total D-DNA concentrations averaged 17.1% (range, 0.7 to 88.3%). Some kind of interaction between heterotrophic nanoflagellates and viruses is proposed. We conclude (i) that the significance of viruses varies with changing trophic conditions and (ii) that viral activity may play a significant role in food web structure under changing trophic conditions.     

Rapid virus production and removal as measured with fluorescently labeled viruses as tracers

Pelagic marine viruses have been shown to cause significant mortality of heterotrophic bacteria, cyanobacteria, and phytoplankton. It was previously demonstrated, in nearshore California waters, that viruses contributed to up to 50% of bacterial mortality, comparable to protists. However, in less productive waters, rates of virus production and removal and estimates of virus-mediated bacterial mortality have been difficult to determine. We have measured rates of virus production and removal, in nearshore and offshore California waters, by using fluorescently labeled viruses (FLV) as tracers. Our approach is mathematically similar to the isotope dilution technique, employed in the past to simultaneously measure the release and uptake of ammonia and amino acids. The results indicated overall virus removal rates in the dark ranging from 1.8 to 6.2% h(-1) and production rates in the dark ranging from 1.9 to 6.1% h(-1), corresponding to turnover times of virus populations of 1 to 2 days, even in oligotrophic offshore waters. Virus removal rates determined by the FLV tracer method were compared to rates of virus degradation, determined at the same locations by radiolabeling methods, and were similar even though the current FLV method is suitable for only dark incubations. Our results support previous findings that virus impacts on bacterial populations may be more important in some environments and less so in others. This new method can be used to determine rates of virus degradation, production, and turnover in eutrophic, mesotrophic, and oligotrophic waters and will provide important inputs for future investigations of microbial food webs.     

海洋微型和微微型浮游生物的区域分布与影响因素

微型和微微型浮游生物几乎存在于所有的海洋生态系统中,其群落遍布世界各大洋。在大部分海域,微型浮游生物的优势类群为鞭毛藻;原绿球藻是贫营养海域的优势类群,而聚球藻、微微型真核生物和异养细菌主要在营养盐丰富的海域出现,在热带、亚热带和温带的富营养区域占优势。温度、盐度、光照、营养盐可得性、水体稳定性和摄食压力是影响微型和微微型浮游生物的主要因素,各海域主要调控因子的不同造成了微型和微微型浮游生物类群和丰度分布的差异。本文主要综述海洋微型和微微型浮游生物类群检测方法、区域分布特点及其受环境影响的研究概况,并提出了今后的重点研究内容和发展方向。     

Photoreactivation compensates for UV damage and restores infectivity to natural marine virus communities.

We investigated the potential for photoreactivation to restore infectivity to sunlight-damaged natural viral communities in offshore (chlorophyll a, < 0.1 microgram liter-1), coastal (chlorophyll a, ca. 0.2 microgram liter-1), and estuarine (chlorophyll a, ca. 1 to 5 micrograms liter-1) waters of the Gulf of Mexico. In 67% of samples, the light-dependent repair mechanisms of the bacterium Vibrio natriegens restored infectivity to natural viral communities which could not be repaired by light-independent mechanisms. Similarly, exposure of sunlight-damaged natural viral communities to > 312-nm-wavelength sunlight in the presence of the natural bacterial communities restored infectivity to 21 to 26% of sunlight-damaged viruses in oceanic waters and 41 to 52% of the damaged viruses in coastal and estuarine waters. Wavelengths between 370 and 550 nm were responsible for restoring infectivity to the damaged viruses. These results indicate that light-dependent repair, probably photoreactivation, compensated for a large fraction of sunlight-induced DNA damage in natural viral communities and is potentially essential for the maintenance of high concentrations of viruses in surface waters.     

Does Virus-Induced Lysis Contribute Significantly to Bacterial Mortality in the Oxygenated Sediment Layer of Shallow Oxbow Lakes?

Despite the recognition that viruses are ubiquitous components of aquatic ecosystems, the number of studies on viral abundance and the ecological role of viruses in sediments is scarce. In this investigation, the interactions between viruses and bacteria were studied in the oxygenated silty sediment layer of a mesotrophic oxbow lake. A long-term study (13 months) and a diel study revealed that viruses are a numerically important and dynamic component of the microbial community. The abundance and decay rates ranged from 4.3 × 10⁹ to 7.2 × 10⁹ particles ml of wet sediment⁻¹ and from undetectable to 22.2 × 10⁷ particles ml⁻¹ h⁻¹, respectively, and on average the values were 2 orders of magnitude higher than the values for the overlying water. In contrast to our expectations, viruses did not contribute significantly to the bacterial mortality in the sediment, since on average only 6% (range, 0 to 25%) of the bacterial secondary production was controlled by viruses. The low impact of viruses on the bacterial community may be associated with the quantitatively low viral burden that benthic bacteria have to cope with compared to the viral burden with which bacterial assemblages in the water column are confronted. The virus-to-bacterium ratio of the sediment varied between 0.9 and 3.2, compared to a range of 5.0 to 12.4 obtained for the water column. We speculate that despite high numbers of potential hosts, the possibility of encountering a host cell is limited by the physical conditions in the sediment, which is therefore not a favorable environment for viral proliferation. Our data suggest that viruses do not play an important role in the processing and transfer of bacterial carbon in the oxygenated sediment layer of the environment investigated.     

Dynamics and Distribution of Cyanophages and Their Effect on Marine Synechococcus spp

Cyanophages infecting marine Synechococcus cells were frequently very abundant and were found in every seawater sample along a transect in the western Gulf of Mexico and during a 28-month period in Aransas Pass, Tex. In Aransas Pass their abundance varied seasonally, with the lowest concentrations coincident with cooler water and lower salinity. Along the transect, viruses infecting Synechococcus strains DC2 and SYN48 ranged in concentration from a few hundred per milliliter at 97 m deep and 83 km offshore to ca. 4 x 10 ml near the surface at stations within 18 km of the coast. The highest concentrations occurred at the surface, where salinity decreased from ca. 35.5 to 34 ppt and Synechococcus concentrations were greatest. Viruses infecting strains SNC1, SNC2, and 838BG were distributed in a similar manner but were much less abundant (<10 to >5 x 10 ml). When Synechococcus concentrations exceeded ca. 10 ml, cyanophage concentrations increased markedly (ca. 10 to > 10 ml), suggesting that a minimum host density was required for efficient viral propagation. Data on the decay rate of viral infectivity d (per day), as a function of solar irradiance I (millimoles of quanta per square meter per second), were used to develop a relationship (d = 0.2610I - 0.00718; r = 0.69) for conservatively estimating the destruction of infectious viruses in the mixed layer of two offshore stations. Assuming that virus production balances losses and that the burst size is 250, ca. 5 to 7% of Synechococcus cells would be infected daily by viruses. Calculations based on contact rates between Synechococcus cells and infectious viruses produce similar results (5 to 14%). Moreover, balancing estimates of viral production with contact rates for the farthest offshore station required that most Synechococcus cells be susceptible to infection, that most contacts result in infection, and that the burst size be about 324 viruses per lytic event. In contrast, in nearshore waters, where ca. 80% of Synechococcus cells would be contacted daily by infectious cyanophages, only ca. 1% of the contacts would have to result in infection to balance the estimated virus removal rates. These results indicate that cyanophages are an abundant and dynamic component of marine planktonic communities and are probably responsible for lysing a small but significant portion of the Synechococcus population on a daily basis.     

Viral lysis,flagellate grazing potential,and bacterial production in Lake Pavin

Abundances of different compartments of the microbial loop (i.e., viruses, heterotrophic bacteria, nonpigmented nanoflagellates, and pigmented nanoflagellates), bacterial heterotrophic production (BHP), viral lysis, and potential flagellate grazing impacts on the bacterial assemblages were estimated during a short-term study (24 h) conducted in June 1998 in the epilimnion (5 m) and metalimnion (10 m) of a moderate-altitude oligomesotrophic lake (Lake Pavin, France). Viral and bacterial abundances were higher in the metalimnion than in the epilimnion, whereas pigmented and nonpigmented nanoflagellates were more numerous in the epilimnion. The control of the BHP due to viral lysis (determined by examination of viral-containing bacteria using a transmission electron microscope) was significantly higher in the meta- (range = 6.0-33.7%, mean = 15.6%) than in the epilimnion (3.5-10.3%, 6.4%). The same was for the losses of BHP from the potential predation by nanoflagellates which ranged from 0.5 to 115.4% (mean = 38.7%) in the epilimnion, and from 0.7 to 97.5% (mean = 66.7%) in the metalimnion. Finally, estimated viral mediated mortality rates from the percentage of visibly infected cells and potential nanoflagellate grazing rates based on assumed clearance rates suggest that flagellates consumed a larger proportion of bacterial production than was lost to viral lysis.