High Nucleic Acid Cells Research Articles

Changes in biofilm composition and microbial water quality in drinking water distribution systems by temperature increase induced through thermal energy recovery

Drinking water distribution systems (DWDSs) have been thoroughly studied, but the concept of thermal energy recovery from DWDSs is very new and has been conceptualized in the past few years. Cold recovery results in a temperature increase of the drinking water. Its effects on drinking water quality and biofilm development are unclear. Hence, we studied both bulk water and biofilm phases for 232 days in two parallel pilot scale distribution systems with two temperature settings after cold recovery, 25 °C and 30 °C, and compared these with a reference pilot system without cold recovery. In all three pilot distributions systems (DSs) our results showed an initial increase in biomass (ATP) in the biofilm phase, along with occurrence of primary colonizers (Betaproteobacteriales) and subsequently a decrease in biomass and an increasing relative abundance of other microbial groups (amoeba resisting groups; Xanthobacteraceae, Legionellales), including those responsible for EPS formation in biofilms (Sphingomonadaceae). The timeline for biofilm microbial development was different for the three pilot DSs: the higher the temperature, the faster the development took place. With respect to the water phase within the three pilot DSs, major microbial contributions came from the feed water (17–100%) and unkown sources (2–80%). Random contributions of biofilm (0–70%) were seen between day 7–77. During this time period six-fold higher ATP concentration (7–11 ng/l) and two-fold higher numbers of high nucleic acid cells (5.20–5.80 × 104 cells/ml) were also observed in the effluent water from all three pilot DSs, compared to the feed water. At the end of the experimental period the microbial composition of effluent water from three pilot DSs revealed no differences, except the presence of a biofilm related microbial group (Sphingomonadaceae), within all three DSs compared to the feed water. In the biofilm phase higher temperatures initiated the growth of primary colonizing bacteria but this did not lead to differences in microbial diversity and composition at the end of the experimental period. Hence, we propose that the microbiological water quality of DWDSs with cold recovery should be monitored more frequently during the first 2–3 months of operation.

Elevated Contribution of Low Nucleic Acid Prokaryotes and Viral Lysis to the Prokaryotic Community Along the Nutrient Gradient From an Estuary to Open Ocean Transect.

Prokaryotes represent the largest living biomass reservoir in aquatic environments and play a crucial role in the global ocean. However, the factors that shape the abundance and potential growth rate of the ecologically distinct prokaryotic subgroups [i.e., high nucleic acid (HNA) and low nucleic acid (LNA) cells] along varying trophic conditions in the ocean remain poorly understood. This study conducted a series of modified dilution experiments to investigate how the abundance and potential growth rate of HNA and LNA prokaryotes and their regulating factors (i.e., protozoan grazing and viral lysis) change along a cross-shore nutrient gradient in the northern South China Sea. The results showed that the abundance of both HNA and LNA cells was significantly positively correlated with the abundance of heterotrophic nanoflagellates and viruses, whereas only HNA abundance exhibited a significant positive correlation with nutrient level. With a decreasing nutrient concentration, the potential growth rate of the HNA subgroup declined significantly, while that of the LNA subgroup was significantly enhanced, leading to an elevated relative potential growth rate of the LNA to HNA subgroup under decreasing nutrient levels. Furthermore, our data revealed different regulatory roles of protozoan grazing and viral lysis on the HNA and LNA subgroups, with HNA suffering higher mortality pressure from grazing than from lysis in contrast to LNA, which experienced equivalent pressures. As the nutrient levels declined, the relative contribution of lysis to the mortality of the HNA subgroup increased significantly, in contrast to the insignificant change in that of the LNA subgroup. Our results indicated the elevated role of LNA cells in the prokaryotic community and the enhanced viral lysis pressure on the total prokaryotes under oligotrophic conditions. This implies a weakened efficiency of carbon cycling within the microbial loop and enhanced viral lysis to shunt more carbon and energy flow in the future ocean, in which oligotrophication will be strengthened due to global warming.

Functional Responses of Bacterioplankton Diversity and Metabolism to Experimental Bottom-Up and Top-Down Forcings.

We conducted an experimental approach using microcosms to simultaneously examine the functional response of natural freshwater bacterial assemblages to the impact of resources (nutrients) and top-down factors (viruses and grazers) on bacterial physiological state and their community structure. Addition of organic and inorganic nutrients led to the proliferation of high nucleic acid content bacterial cells accompanied by high bacterial growth efficiency (considered as proxy of bacterial carbon metabolism) estimates, suggesting that this subgroup represented the most active fraction of bacterial community and had a high capacity to incorporate carbon into its biomass. However, their rapid growth induced the pressure of viral lytic infection which led to their lysis toward the end of the experiment. In microcosms with flagellates plus viruses, and with viruses alone, the selective removal of metabolically active high nucleic acid cells through viral lysis benefitted the less active low nucleic acid content cells, perhaps via the use of lysis products for its growth and survival. Changes in bacterial physiological state in microcosms were reflected in their community structure which was examined using 16S ribosomal RNA (rRNA) gene sequencing by Illumina's Miseq platform. Chao estimator and Shannon diversity index values suggested that bacterial species richness was highest in the presence of both the top-down factors, indicating a tighter control of bacterioplankton dominants within a relatively stable bacterial community. The increase in bacterial metabolism with nutrient addition followed by subsequent lysis of bacterial dominants indicate that both resources and top-down factors work in concert for the sustenance of stable bacterial communities.

Linking host prokaryotic physiology to viral lifestyle dynamics in a temperate freshwater lake (Lake Pavin, France).

In aquatic ecosystems, fluctuations in environmental conditions and prokaryotic host physiological states can strongly affect the dynamics of viral life strategies. The influence of prokaryote physiology and environmental factors on viral replication cycles (lytic and lysogeny) was investigated from April to September 2011 at three different strata (epi, meta, and hypolimnion) in the mixolimnion of deep volcanic temperate freshwater Lake Pavin (France). Overall, the euphotic region (epi and metalimnion) was more dynamic and showed significant variation in microbial standing stocks, prokaryotic physiological state, and viral life strategies compared to the aphotic hypolimnion which was stable within sampled months. The prokaryotic host physiology as inferred from the nucleic acid content of prokaryotic cells (high or low nucleic acid) was strongly regulated by the chlorophyll concentration. The predominance of the high nucleic acid (HNA) prokaryotes (cells) over low nucleic acid (LNA) prokaryotes (cells) in the spring (HNA/LNA = 1.2) and vice versa in the summer period (HNA/LNA = 0.4) suggest that the natural prokaryotic communities underwent major shifts in their physiological states during investigated time period. The increase in the percentage of inducible lysogenic prokaryotes in the summer period was associated with the switch in the dominance of LNA over HNA cells, which coincided with the periods of strong resource (nutrient) limitation. This supports the idea that lysogeny represents a maintenance strategy for viruses in unproductive or harsh nutrient/host conditions. A negative correlation of percentage of lysogenic prokaryotes with HNA cell abundance and chlorophyll suggest that lysogenic cycle is closely related to prokaryotic cells which are stressed or starved due to unavailability of resources for its growth and activity. Our results provide support to previous findings that changes in prokaryote physiology are critical for the promotion and establishment of lysogeny in aquatic ecosystems, which are prone to constant environmental fluctuations.

Modeling the winter-to-summer transition of prokaryotic and viral abundance in the Arctic Ocean.

One of the challenges in oceanography is to understand the influence of environmental factors on the abundances of prokaryotes and viruses. Generally, conventional statistical methods resolve trends well, but more complex relationships are difficult to explore. In such cases, Artificial Neural Networks (ANNs) offer an alternative way for data analysis. Here, we developed ANN-based models of prokaryotic and viral abundances in the Arctic Ocean. The models were used to identify the best predictors for prokaryotic and viral abundances including cytometrically-distinguishable populations of prokaryotes (high and low nucleic acid cells) and viruses (high- and low-fluorescent viruses) among salinity, temperature, depth, day length, and the concentration of Chlorophyll-a. The best performing ANNs to model the abundances of high and low nucleic acid cells used temperature and Chl-a as input parameters, while the abundances of high- and low-fluorescent viruses used depth, Chl-a, and day length as input parameters. Decreasing viral abundance with increasing depth and decreasing system productivity was captured well by the ANNs. Despite identifying the same predictors for the two populations of prokaryotes and viruses, respectively, the structure of the best performing ANNs differed between high and low nucleic acid cells and between high- and low-fluorescent viruses. Also, the two prokaryotic and viral groups responded differently to changes in the predictor parameters; hence, the cytometric distinction between these populations is ecologically relevant. The models imply that temperature is the main factor explaining most of the variation in the abundances of high nucleic acid cells and total prokaryotes and that the mechanisms governing the reaction to changes in the environment are distinctly different among the prokaryotic and viral populations.

Dynamics of heterotrophic bacteria in temperate coastal waters: similar net growth but different controls in low and high nucleic acid cells

Heterotrophic bacteria in aquatic environments are almost universally distributed into 2 distinct flow cytometric clusters based on their relative nucleic acid content: low (LNA) and high (HNA). The dynamics and possible regulation processes of both subgroups in southern Bay of Biscay coastal waters were examined by comparing weekly measurements over 2 yr at a shallow environment (L’Arbeyal beach) with monthly samples at 3 continental shelf stations off Xixon (Spain). Similar seasonal variations of temperature and chlorophyll a characterized inshore and offshore waters. Total bacterial abundances were also similar in shallow and shelf waters (from 0.22 to 3.72 × 106 cells ml−1), with the most remarkable difference being the higher abundance and larger size of HNA bacteria at L’Arbeyal, probably related to greater substrate availability. There, apparent net growth rates estimated from coherent periods (6 to 26 wk) of sustained increase or decrease (−0.02 to 0.04 d−1) were similar for LNA and HNA cells, although related to different factors. LNA bacteria net growth rates were strongly positively correlated with temperature (r = 0.95, p < 0.01, n = 6) and negatively with chlorophyll a (r = −0.90, p = 0.01, n = 6), supporting the hypothesis that they are independent from phytoplankton, which was recently suggested for this and other coastal sites using different approaches. We also show an opposite relationship between cell size and apparent net growth rates of LNA and HNA bacteria, providing further evidence of fundamentally different ecological roles.

Vertical and longitudinal gradients in HNA-LNA cell abundances and cytometric characteristics in the Mediterranean Sea

Abstract. Heterotrophic bacterioplankton abundance and production were investigated with depth (down to bathypelagic layers) and with longitude (from 4.9° E to 32.7° E) along a cruise track across the Mediterranean Sea in early summer 2008. Abundances and flow cytometric characteristics (green fluorescence and side scatter signals) of high nucleic acid (HNA) and low nucleic acid (LNA) bacterial cells were determined using flow cytometry. Contrary to what is generally observed, the relative importance of HNA cells, as a percent of total cells, (%HNA, range 30–69 %) was inversely related to bacterial production (range 0.15–44 ng C l−1 h−1) although the negative relation was weak (log–log regression r2=0.19). The %HNA as well as the mean side scatter of HNA group increased significantly with depth in the meso and bathypelagic layers. Vertical stratification played an important role in influencing the distribution and characteristics of bacterial cells especially with regard to layers located above, within or below the deep chlorophyll maximum. Within a given layer, the relationships between the flow cytometric characteristics and environmental variables such as chlorophyll-a, nutrients or bacterial production changed. Overall, the relationships between HNA and LNA cells and environmental parameters differed vertically more than longitudinally.

Relationships Between Coastal Bacterioplankton Growth Rates and Biomass Production: Comparison of Leucine and Thymidine Uptake with Single-Cell Physiological Characteristics

Specific growth rates of heterotrophic bacterioplankton have been frequently estimated from in situ bacterial production (BP) to biomass (BB) ratios, using a series of assumptions that may result in serious discrepancies with values obtained from predator-free cultures. Here, we used both types of approaches together with a comprehensive assessment of single-cell physiological characteristics (membrane integrity, nucleic acid content, and active respiration) of coastal bacterioplankton during a complete annual cycle (February 2007-January 2008) in the southern Bay of Biscay off Xixón, Spain. Both leucine and thymidine incorporation rates were used in conjunction with empirical tracer to carbon or cells conversion factors (eCFs) to accurately derive BP. Leu and TdR incorporation rates covaried year-round, as did the corresponding eCFs at 0 and 50 m depth. eCFs peaked in autumn, with mean annual values close to the theoretical ones (3.4 kg C mol Leu(-1) and 2.0 × 10(18) cells mol TdR(-1)). Bacterial abundance (0.2-1.5 × 10(6) cells L(-1)) showed a bimodal distribution with maxima in May and October and minima in March. Live (membrane-intact) cells dominated year-round (79-97%), with high nucleic acid cells (42-88%) and actively respiring bacteria (CTC+, 1-16%) showing distinct surface maxima in April and July, respectively. BB (557-1,558 mg C m(-2)) and BP (7-139 mg C m(-2) day(-1)) presented two distinct peaks in spring and autumn, both of similar size due to a strong upwelling event observed in September. Specific growth rates (0.35-3.8 day(-1)) were one order of magnitude higher in predator-free incubations than bacterial turnover rates derived from integrated BP:BB ratios (0.01-0.16 and 0.01-0.09 day(-1), for Leu and TdR, respectively) and were not correlated, probably due to a significant contribution of low activity cells to total standing stocks. The Leu:TdR molar ratio averaged for the water column (6.6-25.5) decreased significantly with higher integrated BB, indicating that low standing stocks tend to present unbalanced growth. Discrepancies about the true magnitude of specific growth rates must be solved before fully appreciating the role of bacteria in the ocean carbon cycle.

Bacterial single-cell activities along the nutrient availability gradient in a canyon-shaped reservoir: a seasonal study

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 60:215-225 (2010) - DOI: https://doi.org/10.3354/ame01425 Bacterial single-cell activities along the nutrient availability gradient in a canyon-shaped reservoir: a seasonal study Karel Horňák*, Jan Jezbera, Karel Šimek Biology Centre of the Academy of Sciences of the Czech Republic, v.v.i., Institute of Hydrobiology, Na Sádkách 7, 370 05 Ceské Budejovice, Czech Republic *Email: hornak@hbu.cas.cz ABSTRACT: We investigated the linkages between the composition and the activities of bacterioplankton assemblages in the meso-eutrophic Římov reservoir (Czech Republic). We examined bacterial fractions with visible leucine and glucose incorporation using microautoradiography, and proportions of bacteria with an intact membrane (live), high nucleic acid (HNA) and low nucleic acid (LNA) content, and an active electron transport system (CTC+ cells; CTC is 5-cyano-2,3-ditolyl-tetrazolium chloride) by flow cytometry. Three stations along the longitudinal axis of the reservoir (designated Dam, Middle, and River) with contrasting phosphorus concentrations and bacterial and extracellular phytoplankton production were sampled at intervals of 3 wk from 29 March to 15 November 2005. At all stations, Betaproteobacteria (BET) or Actinobacteria (ACT) dominated the bacterial community composition, while Cytophaga-Flavobacteria (CF) accounted for smaller proportions. ACT showed high proportions of cells incorporating both leucine and glucose in all samples, whereas only small fractions of CF were scored positive for the incorporation of these substrates. BET incorporated leucine preferentially to glucose in all samples. We identified bacterial phylogenetic groups that correlated with different bacterial populations as determined by flow cytometry: CF and BET significantly contributed to the pool of HNA cells at Stn River, whereas the opposite was found for ACT, forming the pool of LNA cells at all stations. Since ACT efficiently incorporated organic substrates, the LNA fraction represented a highly active component of bacterial assemblages. At Stns Dam and Middle, the dynamics of CF also correlated with the population of CTC+ cells and an enhanced extracellular phytoplankton production. KEY WORDS: Reservoir · Bacterial activity · Leucine and glucose incorporation · Live cells · HNA cells · CTC+ cells Full text in pdf format NextCite this article as: Horňák K, Jezbera J, Šimek K (2010) Bacterial single-cell activities along the nutrient availability gradient in a canyon-shaped reservoir: a seasonal study. Aquat Microb Ecol 60:215-225. https://doi.org/10.3354/ame01425Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 60, No. 3. Online publication date: June 16, 2010 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2010 Inter-Research.

Effect of top-down control on cell-specific activity and diversity of active marine bacterioplankton

Mortality processes such as grazing and viral lysis can alter both phylogenetic diversity and cell-specific activity of bacterioplankton. We conducted experiments to examine the effect of re- duction in grazer or viral abundance on metabolically active bacterioplankton at a eutrophic shelf station and an oligotrophic basin station. Leucine assimilation was used as a proxy to characterize metabolically active bacterial cells. The phylogenetic affiliation of marine bacterioplankton assimilat- ing leucine was identified with fluorescence in situ hybridization and microautoradiography. Flow cytometric sorting of leucine-labeled cells quantified cell-specific activity of high nucleic acid (HNA) and low nucleic acid cells. The activity and diversity of the bacterial community at the oligotrophic station was more responsive to a reduction in mortality compared to the community at the eutrophic station. HNA cells at the oligotrophic station showed a 4-fold increase in cell-specific leucine incorpo- ration when the abundance of flagellates was reduced and an 8-fold increase when the abundance of flagellates, viruses, and bacterial cells was reduced. The abundance of active Alphaproteobacteria increased at the oligotrophic station when either grazer or viral abundance was reduced. Activity re- sponses were less striking at the eutrophic station, and the abundance of active Gammaproteobacte- ria showed a greater increase with reduced flagellates compared to the treatment with reduced fla- gellates, viruses, and bacterioplankton. Our results indicate both the presence and type of mortality process play a key role in structuring a bacterial community, and this effect varies in ecosystems of differing trophic state.

Assessing the relevance of nucleic acid content as an indicator of marine bacterial activity

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 46:141-152 (2007) - doi:10.3354/ame046141 Assessing the relevance of nucleic acid content as an indicator of marine bacterial activity Xosé Anxelu G. Morán1,*, Antonio Bode2, Luis Ángel Suárez1, Enrique Nogueira1 1Centro Oceanográfico de Xixón, IEO, Camín de L’Arbeyal, s/n, 33212 Xixón, Spain 2Centro Oceanográfico de A Coruña, IEO, Muelle de Ánimas, s/n, Apdo. 130, 15080 A Coruña, Spain *Email: xelu.moran@gi.ieo.es ABSTRACT: Current flow cytometry techniques allow the rapid estimation of the abundance of 2 distinct groups of heterotrophic bacteria, characterized by their relative nucleic acid content. High nucleic acid (HNA) bacteria are, at least in coastal environments, usually regarded as more active than the low nucleic acid (LNA) group. We tested the effects of substrate supply and bacterial cell size on the relationship between bacterial activity and the abundance of HNA bacteria by simultaneous measurements of LNA and HNA cell distributions, chlorophyll a and 3H-leucine uptake rates in temperate shelf waters of the northern Iberian Peninsula. We considered 3 zones based on hydrological properties. Significant correlations were found between bacterial activity (range 0.1 to 80 pmol Leu l–1 h–1) and both total and relative (range 28 to 84%) HNA cell abundance for pooled data, but the ready use of HNA bacterial abundance as a proxy for activity in natural systems was questioned by the low percentage of variance explained (16%). However, a detailed regional study of bottom-up effects revealed that the strength of this relationship increased significantly when bacteria were apparently controlled by phytoplankton substrate supply. Moreover, the relationship between mean biomass (overall range 12.4 to 21.2 fg C cell–1) and abundance-activity correlation coefficients in the 3 zones (r = 0.94, p = 0.005, n = 6) suggests that only at large cell sizes can we expect bacterial activity and production to be reasonably predicted by the abundance of HNA cells. KEY WORDS: Bacterioplankton · Flow cytometry · Nucleic acid content · HNA cells · LNA cells · Leucine uptake · Bacterial production Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 46, No. 2. Online publication date: February 02, 2007 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2007 Inter-Research.

Variation in cell-specific rates of leucine and thymidine incorporation by marine bacteria with high and with low nucleic acid content off the Oregon coast

The balance between rates of protein synthesis (measured by incorporation of radio- actively labeled leucine) and rates of DNA synthesis (measured by incorporation of radioactively labeled thymidine) has been used to evaluate growth state in marine bacterioplankton. Our objective was to determine if variability in leucine and thymidine incorporation could further elucidate ecolog- ical differences in the role of high nucleic acid (HNA) and low nucleic acid (LNA) cells in marine ecosystems. We report here the first data set in which cell-specific rates of leucine and thymidine incorporation have been compared for HNA and LNA bacterial cells in the open ocean. In general, HNA cells had higher cell-specific incorporation rates of both leucine and thymidine, and had higher leucine:thymidine (Leu:TdR) incorporation ratios than LNA cells. The higher Leu:TdR ratios for HNA cells suggest that increases in the ratio may be associated with increased metabolic activity by marine bacterioplankton. Leu:TdR incorporation ratios were significantly positively correlated to tempera- ture, but not to chlorophyll a concentrations or phytoplankton biomass calculated from specific car- bon:chlorophyll a ratios, indicating that in this region temperature had a greater effect on bacterial growth state than did substrate supply. The proportion of total heterotrophic bacterial activity attrib- utable to LNA cells was significantly greater at slope and basin stations compared to the mesotrophic shelf station. Finally, the difference in incorporation rates between HNA and LNA cells was greater for protein synthesis than for DNA synthesis.

Antarctic marine bacterioplankton subpopulations discriminated by their apparent content of nucleic acids differ in their response to ecological factors

Bacterial abundances determined in Drake Passage and Bransfield and Gerlache Straits (Antarctica) in the Austral summer ranged from 0.78 to 9.4×105 cells ml−1, and were positively correlated with standing stocks of Chl a. Two bacterial subpopulations were discriminated based in their different levels of green fluorescence and wide angle light scatter (SSC) per cell after SYTO-13 staining for the first time in Antarctic waters. High nucleic acid (HNA) and low nucleic acid (LNA) subpopulations differed considerably in their response to changes in environmental variables. The apparent content of nucleic acids per cell for the HNA subpopulation (FL1-HNA) showed vertical profiles similar to those of Chl a, including the presence of a maximum at the subsurface chlorophyll maximum. FL1-HNA was positively correlated with Chl a. No similar trends were observed for the LNA fraction. HNA and LNA subpopulations differed in the response of the wide angle light scatter signal to environmental factors as well. SSC-HNA decreased strongly with depth and was positively correlated with Chl a. Again, no similar trends were observed for the LNA subpopulation. The percentage of HNA cells (%HNA) ranged between 35.0 and 76.7% and showed a general tendency to increase with depth. This increase seemed to be larger when the stratification of the water column was higher. Differences in grazing pressure could be responsible of the unexpected vertical distribution of HNA cells. Our results shows that in situ LNA and HNA bacterioplankton subpopulations are under different ecological controls and likely to play different trophodynamic roles in Antarctic waters.

Coexistence of dominant groups in marine bacterioplankton community—a combination of experimental and modelling approaches

The population dynamics in the coastal bacterioplankton community off Plymouth (UK) was studied in samples proportionally diluted (99%, 90%, 66% and 0%) with sterile seawater, incubated in the dark for 2–4 days and monitored by flow cytometry. Nucleic acid content of cells, stained with SYBR Green I DNA specific dye, was used as an index of a genome size. Using flow sorting and fluorescence in situ hybridization (FISH) with a set of ribosomal RNA targeted oligonucleotide probes, the phylogenetic composition of dominant cytometric groups of bacterioplankton was determined to be similar during growth in the dilution series. The proportion of the low nucleic acid (LNA) group decreased and correspondingly the high nucleic acid (HNA) groups increased with dilution. The assimilation rates of free amino acids, a highly labile nutrient pool, were determined by flow sorting the dominant groups after short incubations with 35S-methionine tracer. The relative cellular amino acid assimilation by the LNA cells increased with dilution, while the activity of the HNA cells either decreased or remained unchanged. However, highly metabolically active LNA bacteria were overgrown by the HNA bacteria, presumably because the small genome size—an adaptation to living in an oligotrophic environment—did not allow the LNA group to grow sufficiently fast to compete with the HNA group under experimentally reduced grazing pressure. To examine the experimental results a numerical model of bacterioplankton population dynamics was formulated based on the hypothesis that the LNA cells consume only a labile fraction of organic nutrients (amino acids etc.), while the HNA cells feed on both the labile and more refractory sources of nutrients, and that in the absence of phytoplankton the labile source of nutrients is produced entirely by the bacterivorous flagellates. The model simulations gave credence to the hypothesized primary dependence of the LNA group on labile organic nutrients recycled within the microbial loop.

Response of bacteria to simulated upwelling phytoplankton blooms

Until recently, studies of the fate of primary production in coastal upwelling systems have focused mainly on export through sinking of particulate organic matter (POM). In week-long deck incubations conducted during the upwelling season off Oregon, a large accumulation of carbon- rich (C:N ≥ 16) dissolved organic matter (DOM) occurred following nitrate depletion by diatom blooms. The response of bacterioplankton to the DOM release in the incubations was observed using flow cytometric analysis of abundances of bacteria with high nucleic acid (HNA) and low nucleic acid (LNA) content. Relatively small increases in the abundance of HNA bacteria were observed in nitrate-replete conditions (<1.0 × 10 6 cells ml -1 ). Coincident with nitrate depletion and accumulation of the DOM, abundances and growth rates of HNA bacteria increased rapidly while little response was observed from LNA bacteria. Although growth rates and abundances of HNA cells increased markedly, a net decrease in dissolved organic carbon (DOC) was observed in only 1 incubation. Within approximately 1 d of nitrate going to depletion, HNA bacterial abundances peaked and then decreased rapidly, possibly due to flagellate grazing or viral lysing. These results indicate that on the timescale of upwelling/relaxation events, which generally last 7 to 10 d, environmental controls on bacterial populations may prevent complete degradation of phytoplankton-derived DOM, thus allowing some of this material to be exported from the system through physical processes following termination of the upwelling event.

Activity and diversity of bacterial cells with high and low nucleic acid content

ABSTRACT: In most aquatic environments, at least 2 subpopulations of bacterial cells can be dis-criminated by flow cytometry based on their nucleic acid content. Recent investigations have shownthat the cells with a high nucleic acid (HNA) content have a higher cell-specific activity (CSA) cellthan those with a low nucleic acid (LNA) content. In this study, the CSA and biomass-specific activi-ties (BSA) of HNA and LNA cells from different aquatic ecosystems, including marine, brackish andfreshwater, were investigated using radioactive leucine incorporation and cell sorting by flow cytom-etry. The genetic diversity of natural assemblages, HNA and LNA cells was investigated using theSSCP (PCR single-strand conformation polymorphism) method. Data showed that both CSA and BSAof HNA cells were always significantly higher than CSA and BSA of LNA cells. In addition, HNA cellshad a dominant contribution to the production of the total community (77 to 98%). For the differentsamples, the SSCP fingerprints from the natural assemblage and from the 2 sorted fractions were notsignificantly different. This clearly suggests that HNA and LNA subpopulations were composed bythe same dominant species and, thus, confirms an important heterogeneity of physiological stateswithin most natural populations.