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.     

Effect of Natural Sunlight on Bacterial Activity and Differential Sensitivity of Natural Bacterioplankton Groups in Northwestern Mediterranean Coastal Waters

We studied the effects of natural sunlight on heterotrophic marine bacterioplankton in short-term experiments. We used a single-cell level approach involving flow cytometry combined with physiological probes and microautoradiography to determine sunlight effects on the activity and integrity of the cells. After 4 h of sunlight exposure, most bacterial cells maintained membrane integrity and viability as assessed by the simultaneous staining with propidium iodide and SYBR green I. In contrast, a significant inhibition of heterotrophic bacterial activity was detected, measured by 5-cyano-2,3 ditolyl tetrazolium chloride reduction and leucine incorporation. We applied microautoradiography combined with catalyzed reporter deposition-fluorescence in situ hybridization to test the sensitivity of the different bacterial groups naturally occurring in the Northwestern Mediterranean to sunlight. Members of the Gammaproteobacteria and Bacteroidetes groups appeared to be highly resistant to solar radiation, with small changes in activity after exposure. On the contrary, Alphaproteobacteria bacteria were more sensitive to radiation as measured by the cell-specific incorporation of labeled amino acids, leucine, and ATP. Within Alphaproteobacteria, bacteria belonging to the Roseobacter group showed higher resistance than members of the SAR11 cluster. The activity of Roseobacter was stimulated by exposure to photosynthetic available radiation compared to the dark treatment. Our results suggest that UV radiation can significantly affect the in situ single-cell activity of bacterioplankton and that naturally dominating phylogenetic bacterial groups have different sensitivity to natural levels of incident solar radiation.     

Consequences of protist-stimulated bacterial production for estimating protist growth efficiencies

The trophic link between bacteria and bacterivorous protists is a complex interaction that involves feedback of inorganic nutrients and growth substrates that are immeadiately available for prey growth. These interactions were examined in the laboratory and in incubations of concentrated natural assemblages of bacterioplankton. Growth dynamics of estuarine and marine bacterivorous protists were determined in laboratory culture using Vibrio natriegens as prey and were compared to growth of protists on bacterioplankton assemblages concentrated by tangential flow filtration from four northwest Florida Estuaries. Biomass transfers from bacteria to protists were monitored by tracing elemental carbon and nitrogen in particulate fractions of protist added and grazer free controls. Gross growth efficiencies of the protists on naturally occurring bacteria were within the range determined in lab estimates of growth efficiency on cultured bacteria (50%). However, bacterial response to protist excretion products was different in the lab and field incubations, and bacterial growth contributed to the biomass available to protists in the field incubations. As determined by radioisotope-labeled substrate incorporation, a time lag in bacterial reponse to protist excretion products was observed for laboratory batch cultures, allowing accurate estimation of growth efficiency. In incubations with concentrated natural bacterial assemblages, bacterial growth response coincided with protist growth and excretion. The additional bacterial production on protist excretion products reached a maximum of 2–3-fold higher than protist-free controls. In addition, ammonium concentrations increased with protist grazing and growth in lab cultures, but ammonium excreted by protists in concentrates did not accumulate. The C:N values for the bacterial concentrates suggests that these bacteria were nitrogen limited. It is speculated that dissolved organic carbon, concentrated by tangential flow filtration (> 100,000 MW membrane) with the bacterioplankton, was utilized by bacteria when nitrogen was supplied as ammonium and amino acids from protist excretion. Thus, estimates of protist growth efficiency on naturally occurring bacterioplankton, corrected for protist-stimulated bacterial production, were in the range of 13–21%.     

Relationship between Bacterioplankton Richness, Respiration, and Production in the Southern North Sea

We investigated the relationship between bacterioplankton production (BP), respiration (BR), and community composition measured by terminal restriction fragment length polymorphism in the southern North Sea over a seasonal cycle. Major changes in bacterioplankton richness were apparent from April to December. While cell-specific BP decreased highly significantly with increasing bacterioplankton richness, cell-specific BR was found to be variable along the richness gradient, suggesting that bacterioplankton respiration is rather independent from shifts in the bacterial community composition. As a consequence, the bacterial growth efficiency [BGE = BP/(BP + BR)] was negatively related to bacterioplankton richness, explaining ~43% of the variation in BGE. Our results indicate that despite the observed shifts in the community composition, the main function of the bacterioplankton, the remineralization of dissolved organic carbon to CO₂, is rather stable.     

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。     

Effect of natural sunlight on bacterial activity and differential sensitivity of natural bacterioplankton groups in northwestern Mediterranean coastal waters

We studied the effects of natural sunlight on heterotrophic marine bacterioplankton in short-term experiments. We used a single-cell level approach involving flow cytometry combined with physiological probes and microautoradiography to determine sunlight effects on the activity and integrity of the cells. After 4 h of sunlight exposure, most bacterial cells maintained membrane integrity and viability as assessed by the simultaneous staining with propidium iodide and SYBR green I. In contrast, a significant inhibition of heterotrophic bacterial activity was detected, measured by 5-cyano-2,3 ditolyl tetrazolium chloride reduction and leucine incorporation. We applied microautoradiography combined with catalyzed reporter deposition-fluorescence in situ hybridization to test the sensitivity of the different bacterial groups naturally occurring in the Northwestern Mediterranean to sunlight. Members of the Gammaproteobacteria and Bacteroidetes groups appeared to be highly resistant to solar radiation, with small changes in activity after exposure. On the contrary, Alphaproteobacteria bacteria were more sensitive to radiation as measured by the cell-specific incorporation of labeled amino acids, leucine, and ATP. Within Alphaproteobacteria, bacteria belonging to the Roseobacter group showed higher resistance than members of the SAR11 cluster. The activity of Roseobacter was stimulated by exposure to photosynthetic available radiation compared to the dark treatment. Our results suggest that UV radiation can significantly affect the in situ single-cell activity of bacterioplankton and that naturally dominating phylogenetic bacterial groups have different sensitivity to natural levels of incident solar radiation.     

Solar UV radiation modulates daily production and DNA damage of marine bacterioplankton from a productive upwelling zone (36°S), Chile

In upwelling ecosystems, such as the Humboldt Current system (HCS) off Concepción, the effects of solar radiation on bacterioplankton incorporation rates have been related to previous light acclimation and responses to irradiance. In this paper, we study the daily effect of Photosynthetic Active Radiation (PAR, 400-700 nm) and ultraviolet radiation UVR (280-400 nm) on bacterial secondary production (BSP). We also considered the DNA damage-repair response to solar radiation stress by the induction of cyclobutane pyrimidine dimers (CPDs). Experiments were conducted with natural bacterioplankton assemblages (0.2-0.7 μm) collected off Concepción (36°S), during the austral Spring, October-November, 2004. Surface (0.5 m) and subsurface (80 m) bacterioplankton samples were exposed to different solar radiation treatments for 5-20 h. BSP was estimated by ¹⁴C-leucine and ³H-thymidine incorporation at several time intervals, whereas CPDs accumulation was assessed using immunoassay techniques. During high irradiance periods, BSP was mainly affected by PAR in both surface and subsurface assemblages and, to a lesser, but significant (Tukey < 0.05) extent, by UV-A (320-400 nm) and UV-B (280-320 nm) radiation. Maximum inhibition of BSP in surface waters was 78%; growth rates (μ) and bacterial growth efficiency (BGE) were also low (78% and 66% respectively). Subsurface water assemblages, on the contrary, showed a ∼ 25 fold enhancement of BSP, μ, and BGE. Both types of assemblages had a rapid CPDs accumulation (maximum 60 CPDs Mb^− 1) during high irradiance periods. Recovery of BSP inhibition and DNA damage in surface bacteria was total after sunset and after the night incubation period, resembling pre-exposure levels. Despite subsurface BSP enhancement during day-night exposure, residual DNA damage was detected at the end of the experiment (20 CPDs Mb^− 1) suggesting a chronic DNA damage. Our results represent the worst case scenario (i.e., assemblages receiving surface irradiances as may occur in this upwelling zone) and indicate that surface and subsurface bacterial assemblages in the HCS are both highly sensitive to solar irradiance. However, they showed different responses, with surface bacteria having more effective photorepair mechanisms, and sustaining higher BSP than subsurface assemblages.     

Temporal Patterns in Glycolate-Utilizing Bacterial Community Composition Correlate with Phytoplankton Population Dynamics in Humic Lakes

Previous observations of correlated community dynamics between phytoplankton and bacteria in lakes indicate that phytoplankton populations may influence bacterial community structure. To investigate the possibility that bacterial use of phytoplankton exudates contributes to observed patterns of community change, we characterized the diversity and dynamics of heterotrophic bacterioplankton with genetic potential to use glycolate, a photorespiration-specific exudate, in five lakes over a 15-week period. Culture-independent approaches were used to track different bacterial phylotypes represented by DNA sequence variation in the functional gene glycolate oxidase subunit D (glcD). glcD gene sequences from freshwater bacteria exhibited broad phylogenetic diversity, including sequences representing the Alpha-, Beta-, and Gammaproteobacteria, Actinobacteria, Bacteroidetes, Firmicutes, and Verrucomicrobia. The majority of glcD gene sequences were betaproteobacterial, with 48% of the sequences clustering with the glcD gene from the cosmopolitan freshwater species Polynucleobacter necessarius. Terminal restriction fragment length polymorphism fingerprinting of the glcD gene revealed changes in glycolate-utilizing assemblages over time. An average of 39% of within-lake temporal variation in glycolate-utilizing assemblages across five lakes was explained by phytoplankton community composition and dynamics. The interaction between phytoplankton populations and the environment explained an additional 17% of variation on average. These observations offer new insight into the diversity and temporal dynamics of freshwater bacteria with genetic potential to use glycolate and support the hypothesis that algal exudates influence the structure of bacterial communities.     

Relationship between bacterioplankton richness, respiration, and production in the Southern North Sea

We investigated the relationship between bacterioplankton production (BP), respiration (BR), and community composition measured by terminal restriction fragment length polymorphism in the southern North Sea over a seasonal cycle. Major changes in bacterioplankton richness were apparent from April to December. While cell-specific BP decreased highly significantly with increasing bacterioplankton richness, cell-specific BR was found to be variable along the richness gradient, suggesting that bacterioplankton respiration is rather independent from shifts in the bacterial community composition. As a consequence, the bacterial growth efficiency [BGE = BP/(BP + BR)] was negatively related to bacterioplankton richness, explaining approximately 43% of the variation in BGE. Our results indicate that despite the observed shifts in the community composition, the main function of the bacterioplankton, the remineralization of dissolved organic carbon to CO(2), is rather stable.     

Spatial differences in bacterioplankton composition along the Catalan coast (NW Mediterranean) assessed by molecular fingerprinting

Denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rRNA gene fragments was used to compare surface bacterioplankton assemblages along the Catalan coast (NW Mediterranean). Samples from three coastal stations were compared with samples taken inside the Barcelona harbour and open sea samples taken during a cruise. The bacterial assemblage of each sample showed a characteristic and reproducible DGGE fingerprint. Between 17 and 35 bands were detected in each sample, and about 40% of the bands accounted for more than 80% of the band intensity in each sample. The presence of bands as well as their relative intensity was used to compare bacterial assemblages. Clear differences between the harbour samples and the coastal samples were evident during all periods. Marked temporal changes in the bacterial assemblages were detectable for the coastal sites, suggesting seasonal succession of coastal bacterioplankton. During each season, two stations presented a very similar bacterial composition (Barcelona and Masnou) whereas bacterial assemblages in Blanes were slightly different. These differences were consistent with the different hydrography of the area. Diversity indices calculated from DGGE fingerprints were relatively similar for all samples analysed, even though harbour samples were expected to present lower diversity values.     

Diel Fluctuations in the Abundance and Community Diversity of Coastal Bacterioplankton Assemblages over a Tidal Cycle

The diel change in abundance and community diversity of the bacterioplankton assemblages within the Pacific Ocean at a fixed location in Monterey Bay, California (USA) were examined with several culture-independent (i.e., nucleic acid staining, fluorescence in situ hybridization {FISH}, and 16S ribosomal RNA gene libraries) approaches over a tidal cycle. FISH analyses revealed the quantitative predominance of bacterial members belonging to the Cytophaga-Flavobacterium cluster as well as two Proteobacteria (α- and γ-) subclasses within the bacterioplankton assemblages, especially during high tide (HT) and outgoing tide (OT) than the other tidal events. While the clone libraries showed that majority of the sequences were similar to the 16S rRNA gene sequences of unknown bacteria (32% to 73%), however, the operational taxonomic units from members of the α-Proteobacteria, Bacteroidetes, Firmicutes, and Cyanobacteria were also well represented during the four tidal events examined. Comparatively, sequence diversity was highest in OT, lowest in low tide, and very similar between HT and incoming tide. The results indicate that the dynamics of bacterial occurrence and diversity appeared to be more pronounced during HT and OT, further indicative of the ecological importance of several environmental variables including temperature, light intensity, and nutrient availability that are also concurrently fluctuating during these tidal events in marine systems.     

Bacterial community structure in a latitudinal gradient of lakes: the roles of spatial versus environmental factors

1. We analysed the latitudinal variation of bacterioplankton in 45 freshwater environments (lakes, shallow lakes and ponds) across a transect of more than 2100 km stretching from Argentinean Patagonia (45°S) to Maritime Antarctica (63°S), to determine the factors that mainly determine bacterioplankton community structure. 2. Bacterioplankton community composition (BCC) was assessed by a fingerprinting method (denaturing gradient gel electrophoresis) followed by band sequencing, whereas the abundances of total bacteria and picocyanobacteria were estimated by epifluorescence microscopy. 3. Bacterioplankton community composition was controlled by a combination of spatial (latitude and longitude) and environmental [e.g. phosphate, light diffuse attenuation coefficient (K_d) and dissolved organic carbon] factors. Total bacterioplankton abundance declined with latitude. A multiple regression analysis showed that phosphate, K_d and latitude had significant effects on total bacterioplankton abundance. 4. Of 76 operational taxonomic units identified in the studied lakes, 45 were shared between Patagonian and Antarctic water bodies, 28 were present only in Patagonian lakes and three were restricted to the Antarctic lakes. Significant differences were found in BCC between Patagonia and Antarctica. Among the sequences, 54% were similar (>97% sequence similarity) to others reported from cold habitats elsewhere on the planet (glaciers, high mountain lakes, Arctic). 5. Our results provide new evidence that supports the hypotheses of biogeographic patterns of bacterial assemblages and suggest that both spatial and environmental factors control bacterioplankton community structure.     

Microbial Biogeography along an Estuarine Salinity Gradient: Combined Influences of Bacterial Growth and Residence Time

Shifts in bacterioplankton community composition along the salinity gradient of the Parker River estuary and Plum Island Sound, in northeastern Massachusetts, were related to residence time and bacterial community doubling time in spring, summer, and fall seasons. Bacterial community composition was characterized with denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S ribosomal DNA. Average community doubling time was calculated from bacterial production ([¹⁴C]leucine incorporation) and bacterial abundance (direct counts). Freshwater and marine populations advected into the estuary represented a large fraction of the bacterioplankton community in all seasons. However, a unique estuarine community formed at intermediate salinities in summer and fall, when average doubling time was much shorter than water residence time, but not in spring, when doubling time was similar to residence time. Sequencing of DNA in DGGE bands demonstrated that most bands represented single phylotypes and that matching bands from different samples represented identical phylotypes. Most river and coastal ocean bacterioplankton were members of common freshwater and marine phylogenetic clusters within the phyla Proteobacteria, Bacteroidetes, and Actinobacteria. Estuarine bacterioplankton also belonged to these phyla but were related to clones and isolates from several different environments, including marine water columns, freshwater sediments, and soil.     

Characterization of Lysogens in Bacterioplankton Assemblages of the Southern California Borderland

Viruses cause significant mortality of marine microorganisms; however, their role in shaping the composition of microbial assemblages has not been fully elucidated. Because viruses may form lysogenic relationships with their hosts, temperate viruses may influence bacterial assemblage structures through direct lysis of hosts when induced by environmental stimuli or by homoimmunity (i.e., immunity to closely related viruses). We investigated the components of bacterioplankton assemblages that bore prophage using the lysogenic induction agent mitomycin C. Seawater was collected at two locations (the San Pedro Ocean Time Series Station and in the Santa Barbara Channel) in the Southern California Borderland and amended with mitomycin C. After 24-h incubation, the community structure of bacterioplankton was compared with unamended controls using automated rRNA intergenic spacer analysis. The addition of mitomycin C to seawater had effects on the community structure of bacterioplankton, stimulating detectable overall diversity and richness of fingerprints and causing the assemblages within incubations to become different to control assemblages. Most negatively impacted operational taxonomic units (OTU) in mitomycin C-amended incubations individually comprised a large fraction of total amplified DNA in initial seawater (5.3-23.3% of amplified DNA fluorescence) fingerprints, and data suggest that these include organisms putatively classified as members of the gamma-Proteobacteria, SAR11 cluster, and Synechococcus groups. The stimulation of assemblage richness by induction of lysogens, and the reduction in the contribution to total DNA of common OTU (and concomitant increase in rare OTU), suggests that temperate phage have the potential to strongly influence the diversity of bacterioplankton assemblages. Because lysogenic OTU may also be resistant to closely related lytic (i.e., free-living) viruses, the impact of lytic virioplankton on assemblages may only be pronounced transiently or when conditions causing lysogenic induction arise.     

Bacterioplankton communities of Crater Lake,OR: dynamic changes with euphotic zone food web structure and stable deep water populations

The distribution of bacterial and archaeal species in Crater Lake plankton varies dramatically over depth and with time, as assessed by hybridization of group-specific oligonucleotides to RNA extracted from lakewater. Nonmetric, multidimensional scaling (MDS) analysis of relative bacterial phylotype densities revealed complex relationships among assemblages sampled from depth profiles in July, August and September of 1997 through 1999. CL500-11 green nonsulfur bacteria (Phylum Chloroflexi) and marine Group I crenarchaeota are consistently dominant groups in the oxygenated deep waters at 300 and 500 m. Other phylotypes found in the deep waters are similar to surface and mid-depth populations and vary with time. Euphotic zone assemblages are dominated either by β-proteobacteria or CL120-10 verrucomicrobia, and ACK4 actinomycetes. MDS analyses of euphotic zone populations in relation to environmental variables and phytoplankton and zooplankton population structures reveal apparent links between Daphnia pulicaria zooplankton population densities and microbial community structure. These patterns may reflect food web interactions that link kokanee salmon population densities to community structure of the bacterioplankton, via fish predation on Daphnia with cascading consequences to Daphnia bacterivory and predation on bacterivorous protists. These results demonstrate a stable bottom-water microbial community. They also extend previous observations of food web-driven changes in euphotic zone bacterioplankton community structure to an oligotrophic setting.     

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.     

The Effect of Mineral Particulate Matter on the Productive Characteristics of Bacterioplankton and the Degradation of Labile Organic Material

The effect of mineral particulate matter on the population of bacterioplankton, its aggregation, and productive characteristics was studied in model experiments with different concentrations of particulate kaolin and the same concentration of organic substance (sodium humate). It was found that the presence of mineral particulate matter stimulated the aggregation of bacterioplankton, improved bacterial production, and extended the productive period of bacterioplankton. The integral specific production of aggregated bacterioplankton was higher than that of free-swimming bacterioplankton. The energy metabolic coefficient K ₂ of bacterioplankton in the presence of mineral particulate matter was higher than in its absence.     

Biogeography and Host Fidelity of Bacterial Communities in Ircinia spp. from the Bahamas

Research on sponge microbial assemblages has revealed different trends in the geographic variability and specificity of bacterial symbionts. Here, we combined replicated terminal-restriction fragment length polymorphism (T-RFLP) and clone library analyses of 16S rRNA gene sequences to investigate the biogeographic and host-specific structure of bacterial communities in two congeneric and sympatric sponges: Ircinia strobilina, two color morphs of Ircinia felix and ambient seawater. Samples were collected from five islands of the Bahamas separated by 80 to 400 km. T-RFLP profiles revealed significant differences in bacterial community structure among sponge hosts and ambient bacterioplankton. Pairwise statistical comparisons of clone libraries confirmed the specificity of the bacterial assemblages to each host species and differentiated symbiont communities between color morphs of I. felix. Overall, differences in bacterial communities within each host species and morph were unrelated to location. Our results show a high degree of symbiont fidelity to host sponge across a spatial scale of up to 400 km, suggesting that host-specific rather than biogeographic factors play a primary role in structuring and maintaining sponge–bacteria relationships in Ircinia species from the Bahamas.     

塔里木河源流和干流胡杨年轮生长的差异性

借助树木年轮学的方法和技术手段,通过对阿克苏河、叶尔羌河和塔里木河干流不同河段胡杨年轮样本采集,分析了塔里木河两源流与干流不同河段胡杨年轮生长特征;采用单因素方差分析方法和Kendall协同系数(W)研究了塔里木河两源流和干流不同河段胡杨年轮指数的差异性;利用突变检验和小波分析方法探讨了塔河两源流、塔河干流不同河段胡杨径向生长的周期和突变性。研究结果表明:(1)1980—2010年期间,塔里木河两源流和干流不同河段年轮宽度和年轮指数均呈下降趋势。在阿克苏河、肖夹克、沙子河口3个断面胡杨年轮宽度和年轮指数下降趋势明显,而叶尔羌河和新渠满断面的胡杨年轮宽度和年轮指数下降趋势不明显。(2)塔里木河两源流和干流不同河段的胡杨年轮生长在空间上不具有一致性(W0.5),阿克苏河断面与肖夹克、沙子河口断面,叶尔羌河与肖夹克、沙子河口断面,肖夹克与新渠满断面,新渠满与沙子河口断面之间胡杨年轮指数均具有显著的差异性(P0.05)。(3)塔里木河两源流胡杨年轮指数的突变时间与河道径流量突变时间相同或者滞后,这可能与地表水转化为地下水过程有关。(4)塔里木河两源流和干流不同河段的胡杨年轮指数变化的周期均小于其地表径流变化的周期,这可能与胡杨自身的生理特点和当地气温、水分对胡杨生长综合影响有关。本研究有助于深入了解干旱区内陆河流域胡杨生长特征变化规律,同时可为该流域胡杨资源生态保育提供参考。