Diel,seasonal, and depth-related variability of viruses and dissolved DNA in the northern Adriatic Sea

The depth-dependent, seasonal, and diel variability of virus numbers, dissolved DNA (D-DNA), and other microbial parameters was investigated in the northern Adriatic Sea. During periods of water stratification, we found higher virus abundances and virus/bacterium ratios (VBRs) as well as a larger variability of D-DNA concentrations at the thermocline, probably as a result of higher microbial biomass. At the two investigated stations, virus densities were highest in summer and autumn (up to 9.5 × 10¹⁰ 1^–1) and lowest in winter (< 10⁹ 1^–1); D-DNA concentrations were highest in summer and lowest in winter. The VBR as well as an estimated proportion of viral DNA on total D-DNA showed a strong seasonal variability. VBR averaged 15.0 (range, 0.9–89.1), and the percentage of viral DNA in total D-DNA averaged 18.3% (range, 0.1–96.1%). An estimation of the percentage of bacteria lysed by viruses, based on 2-h sample intervals in situ, ranged from 39.6 to 212.2% d^–1 in 5 m and from 19.9 to 157.2% d^–1 in 22 m. The estimated contribution of virus-mediated bacterial DNA release to the D-DNA pool ranged from 32.9 to 161% d^–1 in 5 m and from 10.3 to 74.2% d^–1 in 22 m. Multiple regression analysis and the diel dynamics of microbial parameters indicate that viral lysis occasionally could be more important in regulating bacterial abundances than grazing by heterotrophic nanoflagellates. Correspondence to: M.G. Weinbauer     

Distribution of viruses and their impact on bacterioplankton in mesotrophic and eutrophic reservoirs

Spatial distribution of planktonic viral particles (virioplankton) and mortality of heterotrophic bacteria caused by viral lysis were studied in the eutrophic Ivan’kovskoe and mesotrophic Uglichskoe reservoirs (the Upper Volga). During the summer peak of phytoplankton, the number of viral particles was higher in the Ivan’kovskoe Reservoir ((55.1 ± 9.5) × 10⁶ ml⁻¹ on average) than in the Uglichskoe Reservoir ((42.9 ± 5.1) × 10⁶ ml⁻¹ on average). The ratio of viral to bacterial abundances ranged from 2.5 to 7.0. The average number of mature phages in infected heterotrophic bacteria varied from 17 to 109 particles/cell. Most of the infected bacterial cells in the Ivan’kovskoe Reservoir were rod-shaped, and, in the Uglichskoe Reservoir, they were mainly vibrio-shaped. In the Ivan’kovskoe Reservoir, from 8.3 to 22.4% of planktonic bacteria were infected by phages, suggesting phage-induced mortality of bacterioplankton equal to 10.5–34.8% (19.1% on average) of the daily bacterial production. In the Uglichskoe Reservoir, from 9.4 to 33.5% of bacteria were phage-infected, suggesting phage-induced bacterial mortality of 13.7–40.2% (23.5% on average) of the daily bacterial production. The obtained results testify to an important role of autochthonous viruses in the regulation of bacterioplankton abundance and production in the reservoirs.     

Complex interactions and different possible pathways among functional components of the aquatic microbial world in Farasan Archipelago,Southern Red Sea,Saudi Arabia

This work aims to outline the dynamics of trophic links between the three main microbial components (bacteria, nanoflagellates, and ciliates) of the Farasan Archipelago in order to establish a baseline for future research in this area. The Farasan Archipelago lies along the southwestern coast of the Saudi Arabia, southern Red Sea between 16°20′–17°10′N and 41°30′–42°30′E and had been declared as marine and terrestrial reserve by the year 1996. Three different sites were chosen for this study, with each site visited bimonthly for 18 months from September 2016 to February 2018. Bacteria, nanoflagellates and ciliates were enumerated in order to explore the complex interactions between the main microbial categories in sea waters of the Farasan Archipelago. High abundances were recorded during the present study for bacteria (8.7 × 10⁶ bacteria ml⁻¹), nanoflagellates (3.7 × 10⁴ TNAN ml⁻¹) and ciliates (40.4 ciliates ml⁻¹). The paper discusses the various potential pathways controlling the complex interactions between these microbial groups in this part of the southern Red Sea. It is concluded that a linear trophic chain consisting of bacteria; heterotrophic nanoflagellates; filter feeding ciliates is a major route by which the production of bacteria is transferred to the higher consuming levels, thereby confirming the high importance of t bottom-up control (food supply), alongside top-down control (predation) in regulating bacterial abundances in the Farasan Archipelago. During the present investigation, each nanoflagellate ingested between 11 and 87 bacteria in one hour, while each ciliate consumed between 20 and 185 nanoflagellates every hour. These calculated grazing rates of protistan eukaryotes confirmed the role of heterotrophic nanoflagellates as the main consumers of bacteria, and the role of ciliates as the major control for the heterotrophic nanoflagellate population dynamics, and thus the top predators within the microbial plankton assemblage in the Farasan Archipelago.     

Qualitative importance of the microbial loop and plankton community structure in a eutrophic lake during a bloom of cyanobacteria

Plankton community structure and major pools and fluxes of carbon were observed before and after culmination of a bloom of cyanobacteria in eutrophic Frederiksborg Slotssø, Denmark. Biomass changes of heterotrophic nanoflagellates, ciliates, microzooplankton (50 to 140 μm), and macrozooplankton (larger than 140 μm) were compared to phytoplankton and bacterial production as well as micro- and macrozooplankton ingestion rates of phytoplankton and bacteria. The carbon budget was used as a means to examine causal relationships in the plankton community. Phytoplankton biomass decreased and algae smaller than 20 μm replacedAphanizomenon after the culmination of cyanobacteria. Bacterial net production peaked shortly after the culmination of the bloom (510 μg C liter^?1 d^?1 and decreased thereafter to a level of approximately 124 μg C liter^?1 d^?1. Phytoplankton extracellular release of organic carbon accounted for only 4–9% of bacterial carbon demand. Cyclopoid copepods and small-sized cladocerans started to grow after the culmination, but food limitation probably controlled the biomass after the collapse of the bloom. Grazing of micro- and macrozooplankton were estimated from in situ experiments using labeled bacteria and algae. Macrozooplankton grazed 22% of bacterial net production during the bloom and 86% after the bloom, while microzooplankton (nauplii, rotifers and ciliates larger than 50 μm) ingested low amounts of bacteria and removed 10–16% of bacterial carbon. Both macro-and microzooplankton grazed algae smaller than 20 μm, although they did not control algal biomass. From calculated clearance rates it was found that heterotrophic nanoflagellates (40–440 ml^?1) grazed 3–4% of the bacterial production, while ciliates smaller than 50 μm removed 19–39% of bacterial production, supporting the idea that ciliates are an important link between bacteria and higher trophic levels. During and after the bloom ofAphanizomenon, major fluxes of carbon between bacteria, ciliates and crustaceans were observed, and heterotrophic nanoflagellates played a minor role in the pelagic food web.     

Increased Diversity of Predacious <Emphasis Type="Italic">Bdellovibrio</Emphasis>-Like Organisms (BLOs) as a Function of Eutrophication in Kumaon Lakes of India

Predation by Bdellovibrio-like organisms (BLOs) results in bacterial community succession in aquatic ecosystems. The effects of nutrient loading on the distribution and phylogeny of BLOs remain largely unknown. To this end, we present our findings on BLO diversity from four north-Indian lakes that are variable in their trophic status; Nainital is eutrophic, both, Bhimtal and Naukuchiatal are mesotrophic and Sattal remains oligotrophic, respectively. Initially, total heterotrophic bacteria and BLOs were quantified by most probable number (MPN) analyses using Pseudomonas putida and Escherichia coli as prey bacteria. Total bacterial numbers were at least two-logs higher in the eutrophic lake samples compared with oligotrophic lake. Similarly, BLO numbers were approximately 39-fold higher using Pseudomonas sp., which is likely the preferred prey within these lakes. Conversely, significant differences were not observed between mesotrophic and oligotrophic BLO numbers when E. coli was used as the prey. PCR-RFLP of small subunit rDNA (SSU rDNA) of BLOs, followed by cloning, sequencing, and taxonomic categorization revealed distinct differences such that, eutrophic lake consisted of higher BLO diversity compared with mesotrophic and oligotrophic lake, most likely due to both, higher numbers and availability of a diverse population of prey bacteria resulting from nutrient loading in this ecosystem.     

Seasonal Variation of Virioplankton in a Eutrophic Shallow Lake

Lake Donghu is a typical eutrophic freshwater lake in which high abundance of planktonic viruses was recently revealed. In this study, seasonal variation of planktonic viruses were observed at three different trophic sites, hypertrophic, eutrophic, and mesotrophic regions, and the correlation between their abundances and other aquatic environmental components, such as bacterioplankton, chlorophyll a, burst size, pH, dissolved oxygen, and temperature, was analyzed for the period of an year. Virioplankton abundance detected by transmission electron microscope (TEM) ranged from 5.48 × 10⁸ to 2.04 × 10⁹ ml⁻¹ in all the sites throughout the study, and the high abundances and seasonal variations of planktonic viruses were related to the trophic status at the sampled sites in Lake Donghu. Their annual mean abundances were, the highest at the hypertrophic site (1.23×10⁹ ml⁻¹), medium at the eutrophic site (1.19×10⁹ ml⁻¹), and the lowest at the mesotrophic site (1.02×10⁹ ml⁻¹). The VBR (virus-to-bacteria ratio) values were high, ranging from 49 to 56 on average at the three sampled sites. The data suggested that the high viral abundance and high VBR values might be associated with high density of phytoplankton including algae and cyanobacteria in this eutrophic shallow lake, and that planktonic viruses are important members of freshwater ecosystems.     

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.     

Effect of grazers and viruses on bacterial community structure and production in two contrasting trophic lakes

Background

Over the last 30 years, extensive studies have revealed the crucial roles played by microbes in aquatic ecosystems. It has been shown that bacteria, viruses and protozoan grazers are dominant in terms of abundance and biomass. The frequent interactions between these microbiological compartments are responsible for strong trophic links from dissolved organic matter to higher trophic levels, via heterotrophic bacteria, which form the basis for the important biogeochemical roles of microbial food webs in aquatic ecosystems. To gain a better understanding of the interactions between bacteria, viruses and flagellates in lacustrine ecosystems, we investigated the effect of protistan bacterivory on bacterial abundance, production and structure [determined by 16S rRNA PCR-DGGE], and viral abundance and activity of two lakes of contrasting trophic status. Four experiments were conducted in the oligotrophic Lake Annecy and the mesotrophic Lake Bourget over two seasons (early spring vs. summer) using a fractionation approach. In situ dark vs. light incubations were performed to consider the effects of the different treatments in the presence and absence of phototrophic activity.

Results

The presence of grazers (i.e. < 5-μm small eukaryotes) affected viral production positively in all experiments, and the stimulation of viral production (compared to the treatment with no eukaryotic predators) was more variable between lakes than between seasons, with the highest value having been recorded in the mesotrophic lake (+30%). Viral lysis and grazing activities acted additively to sustain high bacterial production in all experiments. Nevertheless, the stimulation of bacterial production was more variable between seasons than between lakes, with the highest values obtained in summer (+33.5% and +37.5% in Lakes Bourget and Annecy, respectively). The presence of both predators (nanoflagellates and viruses) did not seem to have a clear influence upon bacterial community structure according to the four experiments.

Conclusions

Our results highlight the importance of a synergistic effect, i.e. the positive influence of grazers on viral activities in sustaining (directly and indirectly) bacterial production and affecting composition, in both oligotrophic and mesotrophic lakes.     

Seasonal dynamics of crustacean zooplankton, heterotrophic nanoflagellates and bacteria in a shallow, eutrophic lake

1. The seasonal development of crustacean zooplankton, heterotrophic nanoflagellates (HNF) and bacteria was examined in Grosser Binnensee, a shallow, eutrophic lake in northern Germany. The grazing impact of Daphnia on bacteria and nanoflagellates was estimated from field data on population abundances and from clearance rates obtained in laboratory experiments. 2. The seasonal succession of zooplankton showed distinct peaks of Daphnia magna, cyclopopid copepods, Bosmina longirostris and Daphnia galeata and D. hynlina. The population dynamics of Dapfinia had the strongest impact on all sestonic components. Daphnia maxima coincided with clearwater phases, and were negatively correlated with particulate organic carbon (POC), HNF and phytoplankton. Bacterial abundance was only slightly affected although daphnids were at times more important as bacterial consumers than HNF, as estimated from measured bacterial clearance rates. Other crustaceans (copepods, Bosmina) were probably of minor importance as grazers of bacteria and nanoplankton. 3. HNF abundance varied from 550 ml^?1 to more than 30000 ml^?1. HNF appeared to be suppressed by daphnids and reached highest densities when copepods dominated the metazooplankton. The variation in HNF abundance was not reflected in the concentration of heterotrophic bacteria, which fluctuated rather irregularly between 5 and 20 ± 10⁶ ml^?1. Long filamentous bacteria which were probably resistant to protozoan grazing, however, appeared parallel to the development of HNF. These bacterial cells, although small in number, could comprise more than 30% of the total bacterial biomass.     

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.     

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.     

Seasonal and spatial variability of virio-, bacterio-, and picophytoplanktonic abundances in three peri-alpine lakes

Flow cytometry (FCM) was used to assess microbial community abundances and patterns in three natural, large and deep peri-alpine hydrosystems, i.e., lakes Annecy (oligotrophic), Bourget, and Geneva (mesotrophic). Picocyanobacteria, small eukaryotic autotrophs, heterotrophic prokaryotes, and viruses were studied in the 0–50 m surface layers to highlight the impact of both physical and chemical parameters as well as possible biotic interactions on the functioning of microbial communities. Some specificities were recorded according to the trophic status of each ecosystem such as the higher number of viruses and heterotrophic bacteria in mesotrophic environments (i.e., Lakes Geneva and Bourget) or the higher abundance of picocyanobacteria in the oligotrophic Lake Annecy. However, both seasonal (temperature) and spatial (depth) variations were comparatively more important than the trophic status in driving the microbial communities’ abundances in these three lakes, as revealed by principal component analysis (PCA). A strong viral termination of the heterotrophic bacterial blooms could be observed in autumn for each lake, in parallel to the mixing of the upper lit layers. As virus to bacteria ratio (VBR) was indeed very high at this period with values varying between 87 and 114, such important relationships between viruses and bacteria were likely. The magnitudes of seasonal variations in VBR, with the highest values ever reported so far, were largely greater than the magnitude of theoretical variations due to the trophic status, suggesting also a strong seasonality in virioplankton production associated to prokaryotic dynamics. FCM analyses allowed discriminating several viral groups. Virus-Like Particles group 1 (VLP1) and group 2 (VLP2) were always observed and significantly correlated to bacteria for the former and chlorophyll a and picocyanobacteria for the latter, suggesting that most of VLP1 and VLP2 could be bacteriophages and cyanophages, respectively. On the basis of these results, new ways of investigation emerge concerning the study of relationships between specific picoplanktonic groups; and overall these results provide new evidence of the necessity to consider further viruses for a better understanding of lake plankton ecology. Handling editor: Luigi Naselli-Flores     

Budding and prosthecate bacteria from freshwater habitats of various trophic states

Budding and prosthecate bacteria were enumerated in spring and summer by viable counting procedures in several freshwater habitats in Australia including oligotrophic lakes, a mesotrophic lake, and eutrophic ponds.Caulobacter spp. were the most numerous type encountered. They were present in the highest concentrations (exceeding 1000/ml) in the mesotrophic lake during the summer. Their proportion to total viable heterotrophic bacteria was also highest (35.1 to 37.7) in this habitat. From 17 to 330/mlCaulobacter spp. were counted in the eutrophic habitats where their proportion to total viable numbers was less than 1.0%. In the oligotrophic lakes they varied from 5 to 23/ml and comprised greater than 5% of the total viable count.Hyphomicrobium- like bacteria were also numerous in the mesotrophic lake and in one oligotrophic lake during the summer sampling period.Ancalomicrobium spp. occurred in high concentrations (130/ml) in the mesotrophic lake. Budding bacteria of thePlanctomyces-Pasteuria group were most numerous in the eutrophic habitats where as many as 240/ml were counted; their proportion to total heterotrophs remained relatively constant regardless of trophic state, however. A similar pattern was observed withProsthecobacter spp.     

Patterns in pelagic and benthic nanoflagellate densities in the coastal upwelling system along the Banc d'Arguin,Mauritania

We studied spatial variation in abundance of marine benthic and pelagic heterotrophic nanoflagellates in relation to abundances of autotrophic flagellates, bacteria and cyanobacteria in an upwelling area off the Banc d'Arguin, Mauritania.There was enormous spatial variation in densities. In the sediments these ranged from 8–219 × 10³ cm^–3 for heterotrophic flagellates. Maximum values are in the range of those for temperate shallow marine bottoms. Low densities (< 20 × 10³) were confined to the deep stations (> 1000 m). Over the shelf (10–100 m depth) densities were high but related to grain size rather than to concurrent upwelling phenomena or to the abundance of benthic macrofauna.Pelagic flagellate abundance appeared to be more indicative of contemporary hydrographic conditions, most obvious by an increase in the ratio heterotrophic/autotrophic nanoflagellates away from the area of most intense upwelling.     

Virus impact on heterotrophic bacterioplankton of water reservoirs

The quantitative distribution of viruses and their impact on heterotrophic bacterioplankton were studied in mesotrophic and eutrophic reservoirs of the Volga and Volga-Baltic waterway. The abundance of planktonic virus particles ranged from 9.4 × 10⁶ to 120 × 10⁶ ml⁻¹ and was from 2.5 to 9 times greater than the bacterial numbers. Production of virioplankton varied from 2.1 × 10⁶ to 132 × 10⁶ particles (ml day)⁻¹ and the population turnover time values were between 0.3 and 11.6 days. The maximum values of numbers and production of virio- and bacterioplankton were observed in the eutrophic Ivan’kovo reservoir. Distribution of the viruses in the Volga reservoirs depended to a significant degree on the number and activity of heterotrophic bacterioplankton. The infected bacteria accounted for 5.5–33.5% of the total bacterial abundance. Phages were an important factor of bacterial mortality. During July to September virus-induced bacterial mortality varied between 6.1 and 40.6% (20.2% on average) of daily bacterioplankton production.     

Nanoflagellate predation on auto- and heterotrophic picoplankton in the oligotrophic Mediterranean Sea

Dynamics of autotrophic and heterotrophic prokaryotes and theirconsumption by nanoflagellates were studied in the euphoticzone at nine stations located from the Levantine Basin (34°E)to the Balearic sea (5°E) in June 1999. Bacterial biomassconstituted the largest proportion of living biomass at allstations. Integrated bacterial production at the furthest eaststation, was sixfold lower than integrated bacterial productionat the furthest west (13 and 75 mg C m^-2 d^-1 respectively).Estimated heterotrophic nanoflagellate bacterivory accountedfor 45–87% of bacterial production. Small protists (<3µm) dominated the bacterivore assemblage and accountedfor more than 90% of the heterotrophic bacterial consumption.Our results indicated that there was no negative selection againstSynechococcus and that both picoplankton groups were grazedaccording to their standing stocks. An estimated consumptionof Synechococcus derived from food vacuole content analysisof nanoflagellates revealed that they consumed from 0.5 to 45%(mean 13%) of Synechococcus stock per day. These data are amongthe first documenting the relative grazing impact of heterotrophicnanoflagellates on bacteria and Synechococcus in situ. Assumingthat there was no selection for or against Prochlorococcus,heterotrophic nanoflagellates could ingest from 1.4 to 21% (mean6%) of Prochlorococcus stock per day. The amount of organiccarbon obtained by heterotrophic nanoflagellates from photosyntheticprokaryotes represented 27% of the total amount of carbon obtainedfrom total prokaryotes     

Relations between bacterioplankton,heterotrophic nanoflagellates,and virioplankton in the littoral zone of a large plain reservoir: Impact of bird colonies

Interactions of the main components of microbial planktonic food web (bacteria, heterotrophic nanoflagellates, and viruses) were studied in a protected overgrown littoral zone of the Rybinsk Reservoir (Upper Volga). The effect of colonial bird settlements (the Laridae family) on these processes was determined. The following systems exhibited significant negative correlations: “heterotrophic nanoflagellates–large rod-shaped bacteria” (“predator–prey”), “viruses-bacteriophages–bacterial products” (“parasite–host”) and “heterotrophic nanoflagellates–viruses-bacteriophages”. Relations between biotic factors controlling bacterial development were more pronounced outside the zone affected by colonial bird settlements. Near the bird colony the role of viruses in mortality of planktonic bacteria increased. Reproduction of bacterial cells accelerated in response to the increase in feeding activity of heterotrophic nanoflagellates. Virusesbacteriophages and heterotrophic nanoflagellates probably eliminate different targets until medium-sized cells become predominant in the bacterial community. Then heterotrophic nanoflagellates consume bacterial cells infected with viruses.     

Marine bacterial community structure resilience to changes in protist predation under phytoplankton bloom conditions

To test whether protist grazing selectively affects the composition of aquatic bacterial communities, we combined high-throughput sequencing to determine bacterial community composition with analyses of grazing rates, protist and bacterial abundances and bacterial cell sizes and physiological states in a mesocosm experiment in which nutrients were added to stimulate a phytoplankton bloom. A large variability was observed in the abundances of bacteria (from 0.7 to 2.4 × 10⁶ cells per ml), heterotrophic nanoflagellates (from 0.063 to 2.7 × 10⁴ cells per ml) and ciliates (from 100 to 3000 cells per l) during the experiment (∼3-, 45- and 30-fold, respectively), as well as in bulk grazing rates (from 1 to 13 × 10⁶ bacteria per ml per day) and bacterial production (from 3 to 379 μg per C l per day) (1 and 2 orders of magnitude, respectively). However, these strong changes in predation pressure did not induce comparable responses in bacterial community composition, indicating that bacterial community structure was resilient to changes in protist predation pressure. Overall, our results indicate that peaks in protist predation (at least those associated with phytoplankton blooms) do not necessarily trigger substantial changes in the composition of coastal marine bacterioplankton communities.     

Utilization of Dissolved Nitrogen by Heterotrophic Bacterioplankton: a Comparison of Three Ecosystems

The contributions of different organic and inorganic nitrogen and organic carbon sources to heterotrophic bacterioplankton in batch cultures of oceanic, estuarine, and eutrophic riverine environments were compared. The importance of the studied compounds was surprisingly similar among the three ecosystems. Dissolved combined amino acids (DCAA) were most significant, sustaining from 10 to 45% of the bacterial carbon demands and from 42 to 112% of the bacterial nitrogen demands. Dissolved free amino acids (DFAA) supplied 2 to 7% of the carbon and 6 to 24% of the nitrogen incorporated into the bacterial biomass, while dissolved DNA (D-DNA) sustained less than 5 and 12% of the carbon and nitrogen requirements, respectively. Ammonium was the second most important source of nitrogen, meeting from 13 to 45% of the bacterial demand in the oceanic and estuarine cultures and up to 270% of the demand in riverine cultures. Nitrate was taken up in the oceanic cultures (uptake equaled up to 46% of the nitrogen demand) but was released in the two others. Assimilation of DCAA, DFAA, and D-DNA combined supplied 43% of the carbon demand of the bacteria in the oceanic cultures, while approximately 25% of the carbon requirements were met by the three substrates at the two other sites. Assimilation of nitrogen from DCAA, DFAA, D-DNA, NH₄⁺, and NO₃^-, on the other hand, exceeded production of particulate organic nitrogen in one culture at 27 h and in all cultures over the entire incubation period (50 h). These results suggest that the studied nutrient sources may fully support the nitrogen needs but only partially support the carbon needs of microbial communities of geographically different ecosystems. Furthermore, a comparison of the initial concentrations of the different substrates indicated that relative pool sizes of the substrates seemed to influence which substrates were primarily being utilized by the bacteria.     

A Dilution Technique For The Direct Measurement Of Viral Production: A Comparison In Stratified And Tidally Mixed Coastal Waters

The abundance of heterotrophic bacteria and viruses, as well as rates of viral production and virus-mediated mortality, were measured in Discovery Passage and the Strait of Georgia (British Columbia, Canada) along a gradient of tidal mixing ranging from well mixed to stratified. The abundances of bacteria and viruses were approximately 10(6) and 10(7) mL(-1), respectively, independent of mixing regime. Viral production estimates, monitored by a dilution technique, demonstrated that new viruses were produced at rates of 10(6) and 10(7) mL(-1)h(-1) across the different mixing regimes. Using an estimated burst size of 50 viruses per lytic event, ca. 19 to 27% of the standing stock of bacteria at the stratified stations and 46 to 137% at the deep-mixed stations were removed by viruses. The results suggest that mixing of stratified waters during tidal exchange enhances virus-mediated bacterial lysis. Consequently, viral lysis recycled a greater proportion of the organic carbon required for bacterial growth under non-steady-state compared to steady-state conditions.