Bacterioplankton from a meso-eutrophic dam reservoir was size fractionated to reduce (<0.8-m treatment) or enhance (<5-m treatment) protistan grazing and then incubated in situ for 96 h in dialysis bags. Time course samples were taken from the bags and the reservoir to estimate bacterial abundance, mean cell volume, production, protistan grazing, viral abundance, and frequency of visibly infected cells. Shifts in bacterial community composition (BCC) were examined by denaturing gradient gel electrophoresis (DGGE), cloning and sequencing of 16S rDNA genes from the different treatments, and fluorescence in situ hybridization (FISH) with previously employed and newly designed oligonucleotide probes. Changes in bacterioplankton characteristics were clearly linked to changes in mortality rates. In the reservoir, where bacterial production about equaled protist grazing and viral mortality, community characteristics were nearly invariant. In the "grazerfree" (0.8-m-filtered) treatment, subject only to a relatively low mortality rate (ϳ17% day ؊1 ) from viral lysis, bacteria increased markedly in concentration. While the mean bacterial cell volume was invariant, DGGE indicated a shift in BCC and FISH revealed an increase in the proportion of one lineage within the beta proteobacteria. In the grazing-enhanced treatment (5-m filtrate), grazing mortality was ϳ200% and viral lysis resulted in mortality of 30% of daily production. Cell concentrations declined, and grazing-resistant flocs and filaments eventually dominated the biomass, together accounting for >80% of the total bacteria by the end of the experiment. Once again, BCC changed strongly and a significant fraction of the large filaments was detected using a FISH probe targeted to members of the Flectobacillus lineage. Shifts of BCC were also reflected in DGGE patterns and in the increases in the relative importance of both beta proteobacteria and members of the Cytophaga-Flavobacterium cluster, which consistently formed different parts of the bacterial flocs. Viral concentrations and frequencies of infected cells were highly significantly correlated with grazing rates, suggesting that protistan grazing may stimulate viral activity.

We studied the effects of nutrient availability and protistan grazing on bacterial dynamics and community composition (BCC) in different parts of the canyon-shaped Ř ímov reservoir (Czech Republic). The effects of protistan grazing on BCC were examined using a size fractionation approach. Water from the dam area with only bacteria (<0.8 m), bacteria and heterotrophic nanoflagellates (<5 m), or whole water were incubated in situ inside dialysis bags. Top-down or predator manipulations (size fractionation) were also combined with bottom-up or resource manipulations, i.e., transplantation of samples to the middle and upper inflow parts of the reservoir with increased phosphorus availability. Significant genotypic shifts in BCC occurred with transplantation as indicated by denaturing gradient gel electrophoresis. Using different probes for fluorescence in situ hybridization, we found that 10 to 50% of total bacteria were members of the phylogenetically small cluster of ␤-proteobacteria (targeted with the probe R-BT065). These rod-shaped cells of very uniform size were vulnerable to predation but very fast growing and responded markedly to the different experimental manipulations. In all the grazer-free treatments, the members of the R-BT065 cluster showed the highest net growth rates of all studied bacterial groups. Moreover, their relative abundance was highly correlated with bacterial bulk parameters and proportions of bacteria with high nucleic acid (HNA) content. In contrast, increasing protistan bacterivory yielded lower proportions of R-BT065-positive and HNA bacteria substituted by increasing proportions of the class Actinobacteria, which profited from the enhanced protistan bacterivory.

We analysed bacteriochlorophyll diel changes to assess growth rates of aerobic anoxygenic phototrophs in the euphotic zone across the Atlantic Ocean. The survey performed during Atlantic Meridional Transect cruise 16 has shown that bacteriochlorophyll in the North Atlantic Gyre cycles at rates of 0.91-1.08 day(-1) and in the South Atlantic at rates of 0.72-0.89 day(-1). In contrast, in the more productive equatorial region and North Atlantic it cycled at rates of up to 2.13 day(-1). These results suggest that bacteriochlorophyll-containing bacteria in the euphotic zone of the oligotrophic gyres grow at rates of about one division per day and in the more productive regions up to three divisions per day. This is in striking contrast with the relatively slow growth rates of the total bacterial community. Thus, aerobic anoxygenic phototrophs appear to be a very dynamic part of the marine microbial community and due to their rapid growth, they are likely to be larger sinks for dissolved organic matter than their abundance alone would predict.

The community of aerobic anoxygenic phototrophs was investigated in the Baltic Sea using infrared epifluorescence microscopy from September 2004 to October 2005. The majority of these bacteriochlorophyll-containing organisms exhibited a specific sickle-shaped morphology, with rods or other morphotypes observed only occasionally. Aerobic anoxygenic phototrophs were observed mostly from April to September (1 to 12% of total prokaryotes), peaking in May 2005 at a concentration of up to 0.38 × 10 6 cells ml -1. This peak was associated with the later phase of the spring bloom. In the later months, the amount of phototrophic bacteria gradually declined until the beginning of the fall mixing, and remained low from November to March, contributing only 0 to 2% of total prokaryotes. A novel technique combining fluorescent in situ hybridization (FISH) and infrared epifluorescence microscopy indicated that the Baltic aerobic anoxygenic phototrophs were mostly Gammaproteobacteria, with a smaller fraction of Alphaproteobacteria. KEY WORDS: Aerobic photosynthetic bacteria · Bacteriochlorophyll a · Photoheterotrophy · Epifluorescence microscopy Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 45: [247][248][249][250][251][252][253][254] 2006 thermocline structure gradually dissolves due to the decrease in irradiance and intensified wind mixing. The autumn bloom (September-October) is dominated by cyanobacteria along with dinophytes and chlorophytes as other main contributors (Sto? et al. 2002, Wasmund & Uhlig 2003. By the end of the fall season, the water column becomes homogenous down to the halocline, with mean water temperatures of 3 to 4°C.Earlier studies suggested that Baltic primary production is mostly limited by nitrogen availability, but the activity of nitrogen-fixing cyanobacteria in the summer period may cause limitation by phosphorus (Kivi et al. 1993, Nausch et al. 2004. Knowledge of Baltic bacterioplankton dynamics pattern is fragmentary. In the coastal zone offshore of Sweden, bacterial cell numbers display a relatively simple pattern, with a minimum in winter and a maximum in summer (Hagström et al. 1979). In early spring, the bacterial community is predominantly controlled by nitrogen availability and nanoflagellate grazing (Kuupo et al. 1998). In some studies, a stimulation of bacterial growth by phosphorus was observed in late spring, whereas in summer a great stimulation was induced by the combined addition of nitrogen and phosphorus (Kivi et al. 1993).In a previous study, we surveyed the presence of AAPs in the Baltic Sea in late summer (August -September) 2003 by IR kinetic fluorometry; at that time, Bchl a concentration varied between 8 and 50 ng l -1 . Interestingly, during our next survey in April 2004, the activity of AAPs was below the detection limit of the instrument (~2 ng Bchl a l -1 ; M. Koblí=ek unpubl. data). This result suggested that the community of AAPs in the Baltic Sea undergoes seasonal changes. For this reason, we followed ...