Vibrios are ubiquitous marine bacteria that have long served as models for heterotrophic processes and have received renewed attention because of the discovery of increasing numbers of facultatively pathogenic strains. Because the occurrence of specific vibrios has frequently been linked to the temperature, salinity, and nutrient status of water, we hypothesized that seasonal changes in coastal water bodies lead to distinct vibrio communities and sought to characterize their level of differentiation. A novel technique was used to quantify shifts in 16S rRNA gene abundance in samples from Barnegat Bay, N.J., collected over a 15-month period. Quantitative PCR (QPCR) with primers specific for the genus Vibrio was combined with separation and quantification of amplicons by constant denaturant capillary electrophoresis (CDCE). Vibrio populations identified by QPCR-CDCE varied between summer and winter samples, suggesting distinct warm-water and year-round populations. Identification of the CDCE populations by cloning and sequencing of 16S rRNA genes from two summer and two winter samples confirmed this distinction. It further showed that CDCE populations corresponded in most cases to ϳ98% rRNA similarity groups and suggested that the abundance of these follows temperature trends. Phylogenetic comparison yielded closely related cultured and often pathogenic representatives for most sequences, and the temperature ranges of these isolates confirmed the trends seen in the environmental samples. Overall, this suggests that temperature is a good predictor of the occurrence of closely related vibrios but that considerable microdiversity of unknown significance coexists within this trend.The genus Vibrio encompasses a diverse group of heterotrophic marine bacteria including many facultative symbiotic and pathogenic strains. The latter include Vibrio cholerae, the causative agent of cholera, and V. parahaemolyticus and V. vulnificus, which together are responsible for most cases of fatal seafood poisoning (31). Vibrio infections are not limited to humans, as recently highlighted by reports of Vibrio species capable of killing coral tissue (3, 27), and vibrios represent a major source of concern in aquaculture facilities and marine aquaria (11,40,47). Because all of these pathogens appear to maintain planktonic populations, considerable interest exists in understanding the prevalence and dynamics of specific Vibrio populations in the environment.Most studies on Vibrio ecology to date have focused on specific members of the genus, leading to an extensive body of literature on their genetics and ecology. However, the diversity and dynamics of co-occurring Vibrio populations have only rarely been addressed (e.g., see references 1a, 4, 8, 17, 18, and 38) and more rarely still by using quantitative culture-independent methods (8,17,18,38). All quantitative surveys (by molecular techniques) have confirmed the ubiquity of vibrios but have, with the exception of one study (38), also suggested that Vibrio populations are generally Ͻ1% ...

The mixotrophic (bacterivorous), freshwater chrysophyte Dinobryon cylindricum was cultured under a variety of light regimes and in bacterized and axenic cultures to investigate the role of phototrophy and phagotrophy for the growth of this alga. D. cylindricum was found to be an obligate phototroph. The alga was unable to survive in continuous darkness even when cultures were supplemented with high concentrations of bacteria, and bacterivory ceased in cultures placed in the dark for a period longer than one day. Axenic growth of the alga was poor even in an optimal light regime. Live bacteria were required for sustained, vigorous growth of the alga in the light. Carbon (C), nitrogen (N), and phosphorus (P) budgets determined for the alga during growth in bacterized cultures indicated that bacterial biomass ingested by the alga may have contributed up to 25% of the organic carbon budget of the alga. Photosynthesis was the source of most ([Symbol: see text]75%) of the organic carbon of the alga. D. cylindricum populations survived but did not grow when cultured in a continuous low light intensity (30 μE m(-2) sec(-1)), or in a light intensity of 150 μE m(-2) sec(-1) for only two hours each day. Net efficiency of incorporation of bacterial C, N, and P into algal biomass under these two conditions was zero (i.e., no net algal population growth). We conclude that the primary function of bacterivorous behavior in D. cylindricum may be to provide essential growth factor(s) or major nutrients for photosynthetic growth, or to allow for the survival of individuals during periods of very low light intensity or short photoperiod.

Experiments were carried out on Georges Bank, a productive coastal region in the northwestern sector of the North Atlantic Ocean, and in the oligotrophic western Sargasso Sea to examine the effects of nutrient (inorganic nitrogen and phosphorus) and organic carbon (glucose) additions on bacterial and phytoplankton growth. Four experiments were conducted in each environment. Phytoplankton growth was monitored over a 36 h period by following changes in the concentration of chlorophyll in unfiltered seawater and in seawater prefiltered through 5 µm screening to reduce grazing pressure. Bacterial production was estimated initially and after 24 h using the 3 Hthymidine (TdR) method in unfiltered seawater and in 1 µm filtrate. Phytoplankton biomass increased significantly in response to nutrient additions in all but 1 experiment, whereas chlorophyll concentrations remained unchanged or decreased in all of the unamended (control) treatments or treatments supplemented with glucose. Responses of the phytoplankton community were similar for the < 5 µm and unfiltered treatments. Bacterial production increased after 24 h in all of the treatments on Georges Bank, and there was little effect of nutrient or glucose addition in unfiltered seawater relative to unamended controls. However, glucose addition to the <1 µm filtrate caused substantial increases in bacterial production relative to controls and N/P-amended treatments in 2 of the experiments from this environment. Glucose had no stimulatory effect (relative to unamended treatments) in 3 of the 4 Sargasso Sea experiments, and only a marginal effect in the fourth. However, the addition of inorganic nitrogen and phosphorus in the latter ecosystem resulted in higher bacterial production (relative to unamended treatments or glucose addition) in 2 of the experiments with unfiltered seawater, and very large increases in 3 of the experiments with 1 µm filtrate. The magnitude of the changes in bacterial production differed greatly between unfiltered and filtered seawater in both ecosystems, indicating an important role for bacterial grazers in controlling bacterial population growth. The results of this study indicate different nutritional restraints on bacterial production in these contrasting environments.

Nanoplankton and picoplankton abundance and community grazing on picoplankton were deterrnined in surnmer and autumn at several stations in a productive coastal environment (Georges Bank. NW Atlantic Ocean) and in an oligotrophic oceanic ecosystem (Sargasso Sea). Ranges of heterotrophic nanoplankton (HNAN) abundance were 1.2 to 3.6 X 103 ceils rnl-' on Georges Bank, and 2.2 to 6.8 X 10' ceiis ml-' in the Sargasso Sea. Ranges of phototrophic nanoplankton (PNAN) abundance in these ecosystems were 1.9 to 6.0 X 103 and 1.3 to 4.7 X 102, respectively. Mixotrophic nanoplankton (MNAN), operationaiiy defined here as chloroplast-bearing nanoplankton that ingested fluorescent tracers, comprised an average of 12 to 17% of PNAN in surface waters in both environments during August and October. Mixotrophs at specific stations constituted as much as 38% of total PNAN abundance on Georges Bank and 30 % in the Sargasso Sea. Mixotrophs represented up to 39 % of the total phagotrophic nanoplankton abundance (MNAN/[MNAN + HNAN]). Community grazing impact was estimated from the disappearance of fluorescent prey surrogates (fluorescently labeled bacteria, FLB; cyanobacteria, FLC; and <3 pm algae, FLA). Absolute grazing rates (total picoplankton cells removed d-') on Georges Bank exceeded those in the Sargasso Sea due to the greater abundances of predators and prey. However, there was overlap in the specific grazing losses at the 2 sites (ranges = 0.08 to 0.38 d-' in the coastal ocean and 0.05 to 0.24 d-' in the oligotrophic ocean). Rates of bactenvory were in approximate balance with rates of bactenal production (3H-thymidine uptake), but production exceeded bacterivory on Georges Bank during the surnmer cruise. These data are among the first documenting the impact of grazing on picoplankton in these environments, and they are consistent with the prediction that nanoplanktonic protists are major predators of picoplankton. While the proportion of phototrophs that are phagotrophic was highly variable, our study indicates that algal mixotrophy is widespread in the marine environment, occurring in both coastal and oligotrophic sites, and should be considered quantitatively in microbial food web investigations.KEY WORDS: Nanoplankton/picoplankton interactions . Mixotrophy . Bacterivory . Herbivory Microbial food w e b . Flagellates . Cyanobacteria

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