Spatio-Temporal Variations of High and Low Nucleic Acid Content Bacteria in an Exorheic River

Bacteria with high nucleic acid (HNA) and low nucleic acid (LNA) content are commonly observed in aquatic environments. To date, limited knowledge is available on their temporal and spatial variations in freshwater environments. Here an investigation of HNA and LNA bacterial abundance and their flow cytometric characteristics was conducted in an exorheic river (Haihe River, Northern China) over a one year period covering September (autumn) 2011, December (winter) 2011, April (spring) 2012, and July (summer) 2012. The results showed that LNA and HNA bacteria contributed similarly to the total bacterial abundance on both the spatial and temporal scale. The variability of HNA on abundance, fluorescence intensity (FL1) and side scatter (SSC) were more sensitive to environmental factors than that of LNA bacteria. Meanwhile, the relative distance of SSC between HNA and LNA was more variable than that of FL1. Multivariate analysis further demonstrated that the influence of geographical distance (reflected by the salinity gradient along river to ocean) and temporal changes (as temperature variation due to seasonal succession) on the patterns of LNA and HNA were stronger than the effects of nutrient conditions. Furthermore, the results demonstrated that the distribution of LNA and HNA bacteria, including the abundance, FL1 and SSC, was controlled by different variables. The results suggested that LNA and HNA bacteria might play different ecological roles in the exorheic river.     

Phylogenetic characterisation of picoplanktonic populations with high and low nucleic acid content in the North Atlantic Ocean

In flow cytometric analyses of marine prokaryotic picoplankton often two populations with distinct differences in their apparent nucleic acid content are discernable, one with a high and one with a low nucleic acid content (HNA and LNA, respectively). In this study we determined the phylogenetic composition of flow cytometrically sorted HNA and LNA populations, collected at six stations along a transect across three oceanic provinces from Iceland to the Azores. Catalysed reporter deposition fluorescence in situ hybridisation (CARD-FISH) analysis of sorted cells revealed distinct differences in phylogenetic composition between the LNA and HNA populations with only little overlap. At all stations the LNA population was dominated by the alphaproteobacterial clade SAR11 (45–74%). Also, Betaproteobacteria were always present at 2–4%. While the LNA composition was rather stable, the HNA populations were composed of distinct phylogenetic clades in the different oceanic provinces of Arctic and Tropics. For example Cyanobacteria dominated the North Atlantic Gyre HNA population (29–44%) with Prochlorococcus as the major clade (34–44%), but were low in Arctic and Polar waters (1% and 5%, respectively). In contrast, Bacteroidetes accounted for the majority of HNA cells in the Polar and Arctic province (26% and 32%, respectively), but were low in the Gyre region (3–10%). The DNA content of the HNA population was about 3.5 times higher than that of the LNA populations. This reflects differences in the genome sizes of closely related cultured representatives of HNA clades (3–6 Mbp) and LNA clades (1.3–1.5 Mbp).     

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.     

An annual cycle of dimethylsulfoniopropionate-sulfur and leucine assimilating bacterioplankton in the coastal NW Mediterranean

The contribution of major phylogenetic groups to heterotrophic bacteria assimilating sulfur from dissolved dimethylsulfoniopropionate (DMSP) and assimilating leucine was analysed in surface seawaters from Blanes Bay (NW Mediterranean) over an annual study between March 2003 and April 2004. The percentage of bacteria assimilating DMSP-S showed a strong seasonal pattern, with a steady increase from winter (8 +/- 5%) to summer (23 +/- 3%). The same seasonal pattern was observed for the rate of DMSP-S assimilation. The annual average percentage of DMSP-S-assimilating bacteria (16 +/- 8%) was lower than the corresponding percentage of leucine-assimilating cells (35 +/- 16%), suggesting that not all bacteria synthesizing protein incorporated DMSP-S. Smaller differences between both percentages were recorded in summer. Members of the Alphaproteobacteria (Roseobacter and SAR11) and Gammaproteobacteria groups accounted for most of bacterial DMSP-S-assimilating cells over the year. All major bacterial groups showed an increase of the percentage of cells assimilating DMSP-S during summer, and contributed to the increase of the DMSP-S assimilation rate in this period. In these primarily P-limited waters, enrichment with P + DMSP resulted in a stimulation of bacterial heterotrophic production comparable to, or higher than, that with P + glucose in summer, while during the rest of the year P + glucose induced a stronger response. This suggested that DMSP was more important a S and C source for bacteria in the warm stratified season. Overall, our results suggest that DMSP-S assimilation is controlled by the contribution of DMSP to S (and C) sources rather than by the phylogenetic composition of the bacterioplankton.     

Characterization of heterotrophic prokaryote subgroups in the Sfax coastal solar salterns by combining flow cytometry cell sorting and phylogenetic analysis

Here, we combined flow cytometry (FCM) and phylogenetic analyses after cell sorting to characterize the dominant groups of the prokaryotic assemblages inhabiting two ponds of increasing salinity: a crystallizer pond (TS) with a salinity of 390 g/L, and the non-crystallizer pond (M1) with a salinity of 200 g/L retrieved from the solar saltern of Sfax in Tunisia. As expected, FCM analysis enabled the resolution of high nucleic acid content (HNA) and low nucleic acid content (LNA) prokaryotes. Next, we performed a taxonomic analysis of the bacterial and archaeal communities comprising the two most populated clusters by phylogenetic analyses of 16S rRNA gene clone library. We show for the first time that the presence of HNA and LNA content cells could also be extended to the archaeal populations. Archaea were detected in all M1 and TS samples, whereas representatives of Bacteria were detected only in LNA for M1 and HNA for TS. Although most of the archaeal sequences remained undetermined, other clones were most frequently affiliated to Haloquadratum and Halorubrum. In contrast, most bacterial clones belonged to the Alphaproteobacteria class (Phyllobacterium genus) in M1 samples and to the Bacteroidetes phylum (Sphingobacteria and Salinibacter genus) in TS samples.     

Activity and phylogenetic diversity of bacterial cells with high and low nucleic acid content and electron transport system activity in an upwelling ecosystem

We evaluated whether bacteria with higher cell-specific nucleic acid content (HNA) or an active electron transport system, i.e., positive for reduction of 5-cyano-2,3-ditolyl tetrazolium chloride (CTC), were responsible for the bulk of bacterioplankton metabolic activity. We also examined whether the phylogenetic diversity of HNA and CTC-positive cells differed from the diversity of Bacteria with low nucleic acid content (LNA). Bacterial assemblages were sampled both in eutrophic shelf waters and in mesotrophic offshore waters in the Oregon coastal upwelling region. Cytometrically sorted HNA, LNA, and CTC-positive cells were assayed for their cell-specific [3H]leucine incorporation rates. Phylogenetic diversity in sorted non-radioactively labeled samples was assayed using denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rRNA genes. Cell-specific rates of leucine incorporation of HNA and CTC-positive cells were on average only slightly greater than the cell-specific rates of LNA cells. HNA cells accounted for most bacterioplankton substrate incorporation due to high abundances, while the low abundances of CTC-positive cells resulted in only a small contribution by these cells to total bacterial activity. The proportion of the total bacterial leucine incorporation attributable to LNA cells was higher in offshore regions than in shelf waters. Sequence data obtained from DGGE bands showed broadly similar phylogenetic diversity across HNA, LNA, and CTC-positive cells, with between-sample and between-region variability in the distribution of phylotypes. Our results suggest that LNA bacteria are not substantially different from HNA bacteria in either cell-specific rates of substrate incorporation or phylogenetic composition and that they can be significant contributors to bacterial metabolism in the sea.     

Activity and Phylogenetic Diversity of Bacterial Cells with High and Low Nucleic Acid Content and Electron Transport System Activity in an Upwelling Ecosystem

We evaluated whether bacteria with higher cell-specific nucleic acid content (HNA) or an active electron transport system, i.e., positive for reduction of 5-cyano-2,3-ditolyl tetrazolium chloride (CTC), were responsible for the bulk of bacterioplankton metabolic activity. We also examined whether the phylogenetic diversity of HNA and CTC-positive cells differed from the diversity of Bacteria with low nucleic acid content (LNA). Bacterial assemblages were sampled both in eutrophic shelf waters and in mesotrophic offshore waters in the Oregon coastal upwelling region. Cytometrically sorted HNA, LNA, and CTC-positive cells were assayed for their cell-specific [³H]leucine incorporation rates. Phylogenetic diversity in sorted non-radioactively labeled samples was assayed using denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rRNA genes. Cell-specific rates of leucine incorporation of HNA and CTC-positive cells were on average only slightly greater than the cell-specific rates of LNA cells. HNA cells accounted for most bacterioplankton substrate incorporation due to high abundances, while the low abundances of CTC-positive cells resulted in only a small contribution by these cells to total bacterial activity. The proportion of the total bacterial leucine incorporation attributable to LNA cells was higher in offshore regions than in shelf waters. Sequence data obtained from DGGE bands showed broadly similar phylogenetic diversity across HNA, LNA, and CTC-positive cells, with between-sample and between-region variability in the distribution of phylotypes. Our results suggest that LNA bacteria are not substantially different from HNA bacteria in either cell-specific rates of substrate incorporation or phylogenetic composition and that they can be significant contributors to bacterial metabolism in the sea.     

Low and high nucleic acid content bacteria play discrepant roles in response to various carbon supply modes

Planktonic bacteria can be grouped into ‘high nucleic acid content (HNA) bacteria’ and ‘low nucleic acid content (LNA) bacteria.’ Nutrient input modes vary in environments, causing nutrient availability heterogeneity. We incubated them with equal amounts of total glucose added in a continuous/pulsed mode. The pulse-treated LNA bacteria exhibited twice the cell abundance and four times the viability of the continuous-treated LNA, while HNA did not show an adaptation to pulsed treatment. In structural equation modelling, LNA bacteria had higher path coefficients than HNA, between growth and carbon-saving metabolic pathways, intracellular ATP and the inorganic energy storage polymer, polyphosphate, indicating their low-cost growth, and flexible energy storage and utilisation. After incubation, the pulse-treated LNA bacteria contained more proteins and polysaccharides (0.00064, 0.0012 ng cell⁻¹) than the continuous-treated LNA (0.00014, 0.00014 ng cell⁻¹), conferring endurance and rapid response to pulses. Compared to LNA, HNA keystone taxa had stronger correlations with the primary glucose metabolism step, glycolysis, and occupied leading positions to explain the random forest model. They are essential to introduce glucose into the element cycling of the whole community under both treatments. Our work outlines a systematic bacterial response to carbon input.     

More,smaller bacteria in response to ocean's warming?

Heterotrophic bacteria play a major role in organic matter cycling in the ocean. Although the high abundances and relatively fast growth rates of coastal surface bacterioplankton make them suitable sentinels of global change, past analyses have largely overlooked this functional group. Here, time series analysis of a decade of monthly observations in temperate Atlantic coastal waters revealed strong seasonal patterns in the abundance, size and biomass of the ubiquitous flow-cytometric groups of low (LNA) and high nucleic acid (HNA) content bacteria. Over this relatively short period, we also found that bacterioplankton cells were significantly smaller, a trend that is consistent with the hypothesized temperature-driven decrease in body size. Although decadal cell shrinking was observed for both groups, it was only LNA cells that were strongly coherent, with ecological theories linking temperature, abundance and individual size on both the seasonal and interannual scale. We explain this finding because, relative to their HNA counterparts, marine LNA bacteria are less diverse, dominated by members of the SAR11 clade. Temperature manipulation experiments in 2012 confirmed a direct effect of warming on bacterial size. Concurrent with rising temperatures in spring, significant decadal trends of increasing standing stocks (3% per year) accompanied by decreasing mean cell size (−1% per year) suggest a major shift in community structure, with a larger contribution of LNA bacteria to total biomass. The increasing prevalence of these typically oligotrophic taxa may severely impact marine food webs and carbon fluxes by an overall decrease in the efficiency of the biological pump.     

深圳近海表层浮游细菌分布特征及其环境影响因素

于2015年3月、5月、8月和10月在深圳市近岸海域(珠江口、深圳湾和大亚湾)采集表层水样,利用流式细胞仪测定总浮游细菌、高DNA含量亚群细菌(HNA)、低DNA含量亚群细菌(LNA)的丰度,分析它们的时空分布特点,阐释环境因子对浮游细菌时空分布格局的影响。结果表明,珠江口、深圳湾和大亚湾海域表层浮游细菌的平均丰度依次降低,分别为3.82×10~6个/mL、7.67×10~6个/mL和3.38×10~6个/mL。珠江口海域浮游细菌丰度由远岸到近岸递增,深圳湾海域湾内各站位浮游细菌丰度差异较小,大亚湾海域浮游细菌丰度空间差异不显著(P0.05)。浮游细菌丰度时间差异主要受温度影响,空间差异主要受营养盐和叶绿素a影响。HNA亚群丰度时空差异性比LNA亚群的大,HNA亚群受温度影响显著(P0.01),而LNA亚群与温度相关性不显著(P0.05)。环境对HNA和LNA亚群丰度的影响有许多相似之处,但两者对某些环境因子有着不同的响应,说明它们在近海表层生态系统中可能扮演着部分重叠但略有不同的角色。     

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.     

Does the High Nucleic Acid Content of Individual Bacterial Cells Allow Us To Discriminate between Active Cells and Inactive Cells in Aquatic Systems?

The nucleic acid contents of individual bacterial cells as determined with three different nucleic acid-specific fluorescent dyes (SYBR I, SYBR II, and SYTO 13) and flow cytometry were compared for different seawater samples. Similar fluorescence patterns were observed, and bacteria with high apparent nucleic acid contents (HNA) could be discriminated from bacteria with low nucleic acid contents (LNA). The best discrimination between HNA and LNA cells was found when cells were stained with SYBR II. Bacteria in different water samples collected from seven freshwater, brackish water, and seawater ecosystems were prelabeled with tritiated leucine and then stained with SYBR II. After labeling and staining, HNA, LNA, and total cells were sorted by flow cytometry, and the specific activity of each cellular category was determined from leucine incorporation rates. The HNA cells were responsible for most of the total bacterial production, and the specific activities of cells in the HNA population varied between samples by a factor of seven. We suggest that nucleic acid content alone can be a better indicator of the fraction of growing cells than total counts and that this approach should be combined with other fluorescent physiological probes to improve detection of the most active cells in aquatic systems.     

A comparative study of the cytometric characteristics of high and low nucleic-acid bacterioplankton cells from different aquatic ecosystems

Flow cytometry has revealed the existence of two distinct fractions of bacterioplankton cells, characterized by high and low nucleic acid contents (HNA and LNA cells). Although these fractions seem ubiquitous in aquatic systems, little is known concerning the variation in the cytometric parameters used to characterize them. We have performed cytometric analyses of samples from a wide range of aquatic systems to determine the magnitude and variability in the cytometric characteristics of HNA/LNA. We show that neither group is associated to a fixed level of fluorescence and of light scatter. Rather, the relative position of HNA and LNA in the fluorescence versus side scatter cytograms varies greatly, both within and among ecosystems. Although the cytometric parameters of both groups tend to covary, there is often uncoupling between the two, particularly in light scatter. Our results show that, although the basic HNA/LNA configuration is present in most samples, its cytometric expression changes greatly in different ecosystems and along productivity gradients. The patterns in cytometric parameters do not support the simple, dichotomous view of HNA and LNA as active and inactive cells, or the notion of two distinct and independent communities, but rather suggest that there may be cells that are intrinsic to each fraction, as well as others that may exchange between fractions.     

Does the high nucleic acid content of individual bacterial cells allow us to discriminate between active cells and inactive cells in aquatic systems?

The nucleic acid contents of individual bacterial cells as determined with three different nucleic acid-specific fluorescent dyes (SYBR I, SYBR II, and SYTO 13) and flow cytometry were compared for different seawater samples. Similar fluorescence patterns were observed, and bacteria with high apparent nucleic acid contents (HNA) could be discriminated from bacteria with low nucleic acid contents (LNA). The best discrimination between HNA and LNA cells was found when cells were stained with SYBR II. Bacteria in different water samples collected from seven freshwater, brackish water, and seawater ecosystems were prelabeled with tritiated leucine and then stained with SYBR II. After labeling and staining, HNA, LNA, and total cells were sorted by flow cytometry, and the specific activity of each cellular category was determined from leucine incorporation rates. The HNA cells were responsible for most of the total bacterial production, and the specific activities of cells in the HNA population varied between samples by a factor of seven. We suggest that nucleic acid content alone can be a better indicator of the fraction of growing cells than total counts and that this approach should be combined with other fluorescent physiological probes to improve detection of the most active cells in aquatic systems.     

Seasonal patterns in the sunlight sensitivity of bacterioplankton from Mediterranean surface coastal waters

The sensitivity of coastal marine bacterioplankton to natural photosynthetically active radiation (PAR, 400-700?nm) and ultraviolet radiation (UVR, 280-400?nm) was evaluated in five experiments over a seasonal cycle in the Blanes Bay, NW Mediterranean Sea. Exposure to natural solar radiation generally inhibited bulk bacterial activities or damaged membrane integrity when irradiances were high (i.e. spring and summer experiments) and, in general, UVB (280-320?nm) accounted for most of the inhibition. When assessing activity ((3) H-leucine uptake) at the single-cell level by microautoradiography and rRNA gene probing, seasonally varying responses and sensitivities were found among bacterial groups. While autumn and winter irradiances seemed too low to cause changes in activity, variable effects were found in spring and summer. SAR11 was consistently inhibited by UVR and PAR exposure, whereas Gammaproteobacteria and Bacteroidetes showed higher resistance. Roseobacter, Synechococcus and the NOR5 clade were occasionally photostimulated in their activity, mainly because of PAR. Our results indicate that a component of seasonality exists in the bacterial responses to solar radiation, which vary not only depending on the irradiance and the spectral characteristics, but also on the previous light history and the taxonomic composition of the community.     

Seasonal variations in the contributions of different bacterial groups to the uptake of low-molecular-weight compounds in northwestern Mediterranean coastal waters

We analyzed the contributions of different heterotrophic bacterial groups to the uptake of several low-molecular weight compounds during a seasonal cycle on the northwestern Mediterranean coast (Blanes Bay Microbial Observatory). The bacterial assemblage structure had been shown to change substantially year-round for this site, but whether changes in the activities of the different bacterial groups also occurred on the seasonal scale was unknown. Microautoradiography combined with catalyzed reporter deposition fluorescence in situ hybridization was used to analyze the patterns of glucose, amino acid, and ATP uptake by different bacterial groups. Gammaproteobacteria and Bacteroidetes were not very active in the uptake of glucose at any time of the year (<10% of cells were active) compared to Alphaproteobacteria (generally >20% of cells were active). Dissolved free amino acids were taken up considerably by Alphaproteobacteria and Gammaproteobacteria but not by Bacteroidetes. Relatively high percentages of cells of the three broad phylogenetic groups actively took up ATP, which could be related to the important phosphorous limitation of bacterial production during most of the year in Blanes Bay. The contribution of SAR11 to the uptake of the monomers was variable year-round, generally with fewer than 30% of the cells being active. By contrast, Roseobacter were highly overrepresented in the uptake of all the substrates throughout all the year, with more than 50% of cells being active in all the samples and for all substrates. Our results suggest that substantial changes in the activity of some phylogenetic groups of bacteria occur throughout the year.     

象山港国华宁海电厂附近海域小型底栖动物的群落结构

于2008年夏季(6月)和冬季(12月),在象山港国华宁海电厂沿排水口右侧、从近到远相隔500m设立三个断面A、B、C,并对小型底栖动物丰度及其群落结构变化进行调查研究,结果表明:共鉴定出10个小型底栖动物类群,平均丰度达9407.9inds/10cm².从类群上看,自由生活海洋线虫占总丰度的62.3%,介形类占第二位,为19.3%.ANOSIM分析结果显示小型底栖动物季节间的丰度存在明显差异(p<0.05),夏季平均丰度(8055.3±1282.9inds/10cm²)比冬季(2141.1±614.2inds/10cm²)约高出3倍.冬季不同断面间的丰度无显著差异,但夏季断面间差异显著(p<0.05),尤其在排水口附近的A断面丰度最低,为1002.8inds/10cm².SIMPER分析结果显示海洋线虫、介形类、腹足类、涡虫、桡足类是各断面之间非相似的关键类群,这些类群的迁移指示电厂温排水已导致排水口附近海域小型底栖群落结构不稳定,不仅水平分布有差异,垂直分布也明显不同.     

A phylogenetic analysis of the evolution of moult strategies in Western Palearctic warblers (Aves: Sylviidae)

Adult birds replace their flight feathers (moult) at least once per year, either in summer after termination of breeding or (in the case of some long-distance migratory species) in the winter quarters. We reconstructed the evolutionary pathways leading to summer and winter moult using recently published molecular phylogenetic information on the relationships of the Western Palearctic warblers (Aves: Sylviidae). Our phylogenetic analysis indicates that summer moult is the ancestral pattern and that winter moult has evolved 7–10 times in this clade. As taxa increased their migratory distance and colonized northern breeding areas, summer moult disappeared and winter moult evolved. Our data also allows us to trace the historical origins of unusual moult patterns such as the split-moult and biannual moult strategies: the most parsimonous explanations for their origins is that they evolved from ancestral states of summer moult. We briefly discuss our results in the light of recent criticisms against phylogenetic comparative methods and the utility of historical versus functional definitions of adapation.     

Seasonal Variations in the Contributions of Different Bacterial Groups to the Uptake of Low-Molecular-Weight Compounds in Northwestern Mediterranean Coastal Waters

We analyzed the contributions of different heterotrophic bacterial groups to the uptake of several low-molecular weight compounds during a seasonal cycle on the northwestern Mediterranean coast (Blanes Bay Microbial Observatory). The bacterial assemblage structure had been shown to change substantially year-round for this site, but whether changes in the activities of the different bacterial groups also occurred on the seasonal scale was unknown. Microautoradiography combined with catalyzed reporter deposition fluorescence in situ hybridization was used to analyze the patterns of glucose, amino acid, and ATP uptake by different bacterial groups. Gammaproteobacteria and Bacteroidetes were not very active in the uptake of glucose at any time of the year (<10% of cells were active) compared to Alphaproteobacteria (generally >20% of cells were active). Dissolved free amino acids were taken up considerably by Alphaproteobacteria and Gammaproteobacteria but not by Bacteroidetes. Relatively high percentages of cells of the three broad phylogenetic groups actively took up ATP, which could be related to the important phosphorous limitation of bacterial production during most of the year in Blanes Bay. The contribution of SAR11 to the uptake of the monomers was variable year-round, generally with fewer than 30% of the cells being active. By contrast, Roseobacter were highly overrepresented in the uptake of all the substrates throughout all the year, with more than 50% of cells being active in all the samples and for all substrates. Our results suggest that substantial changes in the activity of some phylogenetic groups of bacteria occur throughout the year.     

A single-cell analysis of virioplankton adsorption, infection, and intracellular abundance in different bacterioplankton physiologic categories

Culture studies of phage–host systems have shown that phage proliferation strongly depends on the physiological state of the host, but it is still unclear to what extent this holds true within aquatic ecosystems. We used a combination of flow sorting and electron microscopy to explore how the frequency of bacterial cells with attached viruses (FCAV), of visibly infected cells, and the number of intracellular viruses are distributed within five physiologic categories: cells with high (HNA) and low (LNA) nucleic acid content, with a compromised membrane, in division, and with an intact-looking morphology. FCAV was not different between the cellular physiologic categories, suggesting low influence of host physiology on viral adsorption. Infected cells were found within all the physiologic categories, besides the dividing cells, but showed different levels of new virion production, with the abundance of intracellular viruses ranked as follows: HNA > intact-looking cells > LNA > compromised membrane cells. These results favor the physiological control hypothesis of viral progeny production. The calculation of viral production rate of the HNA and LNA cells show that viral infection of HNA cells likely accounts for the majority of viral production. It also show that cells considered as less active can still act as resources for phages, although they contain much less intracellular phage particles.