Variations In Bacterial Abundance Research Articles

Metabolic dysfunction-associated steatotic liver disease exhibits sex-specific microbial heterogeneity within intestinal compartments.

Evidence suggests that the gastrointestinal microbiome plays a significant role in the biology of metabolic dysfunction-associated steatotic liver disease (MASLD). However, it remains unclear whether disparities in the gut microbiome across intestinal tissular compartments between the sexes lead to MASLD pathogenesis. Sex-specific analyses of microbiome composition in two anatomically distinct regions of the gut, the small intestine and colon, were performed using an experimental model of MASLD. The study involved male and female spontaneously hypertensive rats and the Wistar-Kyoto control rat strain, which were fed either a standard chow diet or a high-fat diet for 12 weeks to induce MASLD (12 rats per group). High-throughput 16S sequencing was used for microbiome analysis. There were significant differences in the overall microbiome composition of male and female rats with MASLD, including variations in topographical gut regions. The beta diversity of the jejunal and colon microbiomes was higher in female rats than in male rats (PERMANOVA p-value=0.001). Sex-specific analysis and discriminant features using LEfSe showed considerable variation in bacterial abundance, along with distinct functional properties, in the jejunum and colon of animals with MASLD. Significantly elevated levels of lipopolysaccharide and protein expression of Toll-like receptor 4 were observed in the livers of male rats with MASLD compared with their female counterparts. This study uncovered sexual dimorphism in the gut microbiome of MASLD and identified microbial heterogeneity within intestinal compartments. Insights into sex-specific variations in gut microbiome composition could facilitate customised treatment strategies.

Multi-factor analysis of the quality of cellar mud of Luzhou-flavor liquor in Yibin production area.

The aim of this study was to conduct a thorough scientific investigation into the similarities and differences in the quality of the cellars of different Luzhou-flavor liquor wineries in Yibin production area and the reasons for them. This study analyzed cellar mud samples from five wineries in Yibin production area. The analysis of volatile flavor compounds was carried out using headspace solid-phase microextraction and gas chromatography-mass spectrometry. The bacterial and archaeal community structures and their correlations were analyzed by high-throughput sequencing. The study indicates that the Distillery A had the highest levels of ammonium nitrogen and effective phosphorus, Distillery F had the highest humus levels, and Distillery I had the highest pH levels. The community structure of the principal bacterial and archaeal communities in the five subterranean clays exhibited similarity, and all samples were dominated by Firmicutes as the primary bacterial group. However, there was variation in bacterial abundance. The cellar mud also has obvious regional differences, and there are three genera of differentially dominant archaea in the archaea. In summary, significant differences were observed in the physicochemical indexes of bacterial and archaeal abundance across all five samples. These differences led to variations in both the content and composition of volatile constituents.

Diversity and Dynamics of Bacterial Communities in the Digestive and Excretory Systems across the Life Cycle of Leafhopper, Recilia dorsalis.

Recilia dorsalis is a notorious rice pest that harbors numerous symbiotic microorganisms. However, the structure and dynamics of bacterial communities in various tissues of R. dorsalis throughout its life cycle remain unclear. In this study, we used high-throughput sequencing technology to analyze the bacterial communities in the digestive, excretory, and reproductive systems of R. dorsalis at different developmental stages. The results showed that the initial microbiota in R. dorsalis mostly originated from vertical transmission via the ovaries. After the second-instar nymphs, the diversity of bacterial communities in the salivary gland and Malpighian tubules gradually decreased, while the midgut remained stable. Principal coordinate analysis revealed that the structure of bacterial communities in R. dorsalis was primarily influenced by the developmental stage, with minimal variation in bacterial species among different tissues but significant variation in bacterial abundance. Tistrella was the most abundant bacterial genus in most developmental stages, followed by Pantoea. The core bacterial community in R. dorsalis continuously enriched throughout development and contributed primarily to food digestion and nutrient supply. Overall, our study enriches our knowledge of the bacterial community associated with R. dorsalis and provides clues for developing potential biological control technologies against this rice pest.

Variability in the Pre-Analytical Stages Influences Microbiome Laboratory Analyses.

Introduction: There are numerous confounding variables in the pre-analytical steps in the analysis of gut microbial composition that affect data consistency and reproducibility. This study compared two DNA extraction methods from the same faecal samples to analyse differences in microbial composition. Methods: DNA was extracted from 20 faecal samples using either (A) chemical/enzymatic heat lysis (lysis buffer, proteinase K, 95 °C + 70 °C) or (B) mechanical and chemical/enzymatic heat lysis (bead-beating, lysis buffer, proteinase K, 65 °C). Gut microbiota was mapped through the 16S rRNA gene (V3–V9) using a set of pre-selected DNA probes targeting >300 bacteria on different taxonomic levels. Apart from the pre-analytical DNA extraction technique, all other parameters including microbial analysis remained the same. Bacterial abundance and deviations in the microbiome were compared between the two methods. Results: Significant variation in bacterial abundance was seen between the different DNA extraction techniques, with a higher yield of species noted in the combined mechanical and heat lysis technique (B). The five predominant bacteria seen in both (A) and (B) were Bacteroidota spp. and Prevotella spp. (p = NS), followed by Bacillota (p = 0.005), Lachhnospiraceae (p = 0.0001), Veillonella spp. (p < 0.0001) and Clostridioides (p < 0.0001). Conclusion: As microbial testing becomes more easily and commercially accessible, a unified international consensus for optimal sampling and DNA isolation procedures must be implemented for robustness and reproducibility of the results.

Estimating soil bacterial abundance and diversity in the Southeast Qinghai-Tibet Plateau

Soil bacteria play important functional roles in ecosystems but are challenging to investigate because of time-consuming and costly laboratory analyses. Digital soil mapping (DSM) technology is an emerging and efficient tool for regionalization of soil bacteria. It has been introduced as a viable complementary approach to the traditional methods for expression of the spatial distribution of soil properties due to being fast and cost-effective. This study was conducted to develop a strategy for mapping the relative abundance of the dominant phyla and community diversities of bacteria to have a better understanding of their biogeography in the highly heterogeneous area of Southeast Tibet, China. Here, we developed state-factor models using predictor variables that were already mapped and publicly available soil and environmental proxies for edaphic, climatic, biotic and topographic factors. We evaluated seven statistical and machine learning algorithms; namely, partial least squares regression (PLSR), random forest (RF), Cubist, support vector machines (SVM), Gaussian process regression (GPR), XG-boost (XGB) and convolutional neural networks (CNNs). Ten-fold cross-validation with all observations revealed that the CNNs outperformed other algorithms and could explain between 48% and 72% of the variation in bacterial abundance and diversity. Estimates of the relative abundance of Actinobacteria and Proteobacteria produced the largest R2 values (≥0.70), while estimates of Acidobacteria, Gemmatimonadetes, Chloroflexi, and Planctomycetes produced values of 0.6 ≤ R2 < 0.7, and estimates of Verrucomicrobia, Bacteroidetes, Nitrospirae, OTUs, and Shannon diversity produced values of 0.5 ≤ R2 < 0.6. The estimates for Firmicutes were poorest, with R2 values < 0.5. The estimated phyla abundances and diversities clearly exhibited regional patterns and local characteristics. Soil total nitrogen (TN), carbon to nitrogen ratio (C/N), pH, clay content, and temperature were prominent controls that regulated bacterial community distribution.

Spatio-temporal Variations in Bacterial Abundance with an Emphasis on Fecal Indicator Bacteria and Vibrio spp. in and around Visakhapatnam Port, East Coast of India

The ecological health of port environments, which are dynamic and hotspots of anthropogenic activities, can be analyzed using specific pathogenic bacteria as they provide definite evidence and source of pollution. The influence of seasons and environmental settings on total bacterial count (TBC), distribution of fecal indicators, and Vibrio spp. was explored in Visakhapatnam port, located along the east coast of India. South-west monsoon had a significant influence on TBC, fecal indicators, and Vibrio spp., and the abundance was influenced by the eutrophic environment in the inner harbour. Fecal indicators were one order higher in sub-surface water when compared with sediment, indicating their inoculation due to turbulent conditions in south-west monsoon. The abundance of V. cholerae was influenced by salinity, temperature, and SPM, and was positively correlated to plankton; relating their distribution with disease dynamics and ecosystem functioning is a step ahead. Such an assessment is important from the perspective of human health and marine bioinvasion.

Family‐based study reveals decreased abundance of sputum Granulicatella in asthmatics

It has been documented that the airway microbiome differs between asthmatics and healthy individuals. However, the microbial signature associated with asthma remains unclear. One of the major limitations is that the case-control study design used in existing studies may not adequately control for baseline variation in bacterial abundance between individuals. In order to further identify asthma-associated airway microbes, we performed a family-based pilot study. In this study, we investigated the microbiome of induced sputum samples from 14 participants from three two-generational pedigrees. Each pedigree has both asthmatic and non-asthmatic offspring. We compared the differences in the relative abundance of bacteria between asthmatic and non-asthmatic siblings within each family and identified bacterial genus with the same trend across the three families. Our results, for the first time, linked asthma with decreased abundance of sputum Granulicatella and verified the previous finding that Veillonella is increased in the airway of asthmatic subjects. This demonstrates the promise of a family-based study design in the search for a bacterial signature for asthma.

Temporal and Spatial Impact of Human Cadaver Decomposition on Soil Bacterial and Arthropod Community Structure and Function.

As vertebrate carrion decomposes, there is a release of nutrient-rich fluids into the underlying soil, which can impact associated biological community structure and function. How these changes alter soil biogeochemical cycles is relatively unknown and may prove useful in the identification of carrion decomposition islands that have long lasting, focal ecological effects. This study investigated the spatial (0, 1, and 5 m) and temporal (3–732 days) dynamics of human cadaver decomposition on soil bacterial and arthropod community structure and microbial function. We observed strong evidence of a predictable response to cadaver decomposition that varies over space for soil bacterial and arthropod community structure, carbon (C) mineralization and microbial substrate utilization patterns. In the presence of a cadaver (i.e., 0 m samples), the relative abundance of Bacteroidetes and Firmicutes was greater, while the relative abundance of Acidobacteria, Chloroflexi, Gemmatimonadetes, and Verrucomicrobia was lower when compared to samples at 1 and 5 m. Micro-arthropods were more abundant (15 to 17-fold) in soils collected at 0 m compared to either 1 or 5 m, but overall, micro-arthropod community composition was unrelated to either bacterial community composition or function. Bacterial community structure and microbial function also exhibited temporal relationships, whereas arthropod community structure did not. Cumulative precipitation was more effective in predicting temporal variations in bacterial abundance and microbial activity than accumulated degree days. In the presence of the cadaver (i.e., 0 m samples), the relative abundance of Actinobacteria increased significantly with cumulative precipitation. Furthermore, soil bacterial communities and C mineralization were sensitive to the introduction of human cadavers as they diverged from baseline levels and did not recover completely in approximately 2 years. These data are valuable for understanding ecosystem function surrounding carrion decomposition islands and can be applicable to environmental bio-monitoring and forensic sciences.

Investigation of bacterial effects of Asian dust events through comparison with seasonal variability in outdoor airborne bacterial community.

Atmospheric bacterial dispersion with aeolian dust has been reported to have a potential impact on public health and ecosystems. Asian dust is a major aeolian event that results in an estimated 4 million tons of Asian dust particles falling in Japan annually, 3,000–5,000 km away from their source regions. However, most studies have only investigated the effects of Asian dust during dust seasons. Therefore, in this study, outdoor bacterial abundance and community composition were determined by 16S rRNA quantitative PCR and amplicon sequencing, respectively, and compared on Asian and non-Asian dust days (2013–2015; 44 samples over four seasons). Seasonal variations in bacterial abundance of non-Asian dust days were not observed. Bacterial abundance of individual samples collected on non-Asian dust days changed dynamically relative to Asian dust days, with bacterial abundance occasionally reaching those of Asian dust days. The bacterial community composition on non-Asian dust days was rather stable seasonally, and did not differ from that on Asian dust days. These results indicate that bacteria in Asian dust does not immediately influence indigenous bacterial communities at the phylum/class level in distant downwind areas; accordingly, further studies of bacterial communities in downwind areas closer to the dust source are warranted.

Vertical stratification of bacterial communities driven by multiple environmental factors in the waters (0–5000 m) off the Galician coast (NW Iberian margin)

The processes mediated by microbial planktonic communities occur along the entire water column, yet the microbial activity and composition have been studied mainly in surface waters. This research examined the vertical variation in bacterial abundance, activity and community composition and structure from surface down to 5000m depth following a longitudinal transect off the Galician coast (NW Iberian margin, from 43°N, 9°W to 43°N, 15°W). Community activity and composition changed with depth. The leucine incorporation rates decreased from the euphotic layer to the bathypelagic waters by three orders of magnitude, whereas prokaryotic abundance decreased only by one order of magnitude. The relative abundance of SAR11 and Alteromonas, determined by catalyzed reported deposition fluorescence in situ hybridization (CARD-FISH), decreased with depth. Meanwhile, the contribution of SAR 202 and SAR324 was significantly higher in the deeper layers (i.e. NEADW, North East Atlantic Deep Water and LDW, Lower Deep Water) than in the euphotic zone. Bacterial community structure, assessed by Automated Ribosomal Intergenic Spacer Analysis (ARISA), was depth-specific. A distance based linear model (DistLM) revealed that the variability found in bacterial community structure was mainly explained by temperature nitrate, phosphate, dissolved organic matter (DOM) fluorescence, prokaryotic abundance, leucine incorporation and to a lesser extent salinity, oxygen, CDOM absorbance and dissolved organic carbon concentration. Our results displayed a bacterial community structure shaped not only by depth-related physicochemical features but also by DOM quality, indicating that different prokaryotic taxa have the potential to metabolize particular DOM sources.

博斯腾湖细菌丰度时空分布及其与环境因子之间的关系

于2010年6月-2011年6月在新疆博斯腾湖共进行了9次采样，获得了140份样品，采用表面荧光显微镜直接计数法及广义可加模型（Generalized Additive Models，GAM）研究了表层水体中细菌丰度的时空分布规律及其与环境因子的关系。结果表明博斯腾湖细菌丰度平均值为（1.48±0.95）×10⁶ 个/mL；细菌丰度季节差异显著，夏季最高（2.05×10⁶ 个/mL），冬季最低（3.81×10⁵ 个/mL）。在大湖区，春季和冬季细菌丰度最大值均位于湖区中部，冬季细菌丰度由湖区中部向东南、西南逐渐减少。夏季和秋季细菌丰度分布与春季大致相反，湖区中部较低，西北部较高。GAM分析结果表明，温度、溶解性有机碳（DOC）、叶绿素a、电导率、浊度等5个环境因子对细菌丰度总偏差解释率为81.2%，其中温度贡献最大，贡献率为63.3%，DOC、叶绿素a和电导率，贡献率分别为12.5%、2.7%和1.7%；浊度对偏差的解释率仅为1.0%。在温度超过22℃时，影响博斯腾湖细菌丰度空间分布的主要因子是DOC。;In aquatic systems, bacteria are often closely linked to environmental conditions, which influence the intrasystem bacterial distribution and abundance. To investigate the response of bacterial abundance to environmental factors in lakes located in arid regions, we investigated Lake Bosten in Xinjiang Uygur Autonomous Region, China, which is the largest and most typical inland lake in this region. The inflow of the lake is supplied by melting ice, precipitation and groundwater from the mountainous regions of Tianshan, which formed a salinity gradient from freshwater to oligosaline, making it an ideal model to study the response of bacterial abundance to environmental factors in lakes. Surface water samples from 17 sites of the large-lake area (surface area ≈950 km², mean depth ≈7 m) and six sites of the small-lake area (surface area ≈300 km², depth ≈1-4 m) in Lake Bosten were collected from June 2010 to June 2011. Environmental variables of each sampling site were measured using standard methods. Additionally, the DAPI-combined epifluorescence direct counting method (DAPI-FDC) was used to investigate the bacterial abundance and their spatial and temporal distribution in the lake. The relationship between bacterial abundance and environmental parameters was then explored by the Generalized Additive Model (GAM), which is a flexible and effective technique for modelling nonlinear time-series in studies of effects of environmental factors on organisms. The average abundance of bacteria in Lake Bosten was (1.48±0.95)×10⁶ cells/mL. Generally, bacterial abundance varies from 10⁵-10⁷ cells/L from oligotrophic to hyper eutrophic lakes. Bacterial abundance in mesotrophic Bosten Lake was two times higher than the abundance in oligotrophic lakes during the same season such as Seven Rila Lake in Bulgaria, and in Shield Lake in Canada, while it was slightly lower than that in mesotrophic Lake Schhsee. In addition, the bacterial abundance in Bosten Lake was an order of magnitude lower than that of Taihu Lake and Wuliangsuhai Lake, which are eutrophic. The bacterial abundance also varied significantly with season, with the highest value being recorded in summer (2.05×10⁶ cells/mL) and the lowest in winter (3.81×10⁵ cells/mL). In the large-lake area, the maximum bacterial abundance appeared in the center during spring and winter and decreased from the center to southeast and southwest during winter; however, the opposite spatial tendency was observed during summer and autumn. The result of GAMs showed that temperature, dissolved organic carbon (DOC), chlorophyll <em>a</em> (Chl-a), conductivity and turbidity, explained about 81.2% of the total variation in bacterial abundance in Lake Bosten. Temperature provided the largest contribution to this variation (63.3%), followed by DOC (12.5%), Chl-a (2.7%) and conductivity (1.7%). Our data also showed that, when water temperature was ≥ 22℃, DOC was the most important variable affecting the spatial dynamics of bacterial abundance.

Seasonal variations of virus- and nanoflagellate-mediated mortality of heterotrophic bacteria in the coastal ecosystem of subtropical western Pacific

Abstract. Since viral lysis and nanoflagellate grazing differ in their impact on the aquatic food web, it is important to assess the relative importance of both bacterial mortality factors. In this study, an adapted version of the modified dilution method was applied to simultaneously estimate the impact of both virus and nanoflagellate grazing on the mortality of heterotrophic bacteria. A series of experiments was conducted monthly from April to December 2011 and April to October 2012. The growth rates of bacteria we measured ranged from 0.078 h−1 (April 2011) to 0.42 h−1 (September 2011), indicating that temperature can be important in controlling the seasonal variations of bacterial growth. Furthermore, it appeared that seasonal changes in nanoflagellate grazing and viral lysis could account for 34% to 68% and 13% to 138% of the daily removal of bacterial production, respectively. We suggest that nanoflagellate grazing might play a key role in controlling bacterial biomass and might exceed the impact of viral lysis during the summer period (July to August) because of the higher abundance of nanoflagellates at that time. Viral lysis, on the other hand, was identified as the main cause of bacterial mortality between September and December. Based on these findings in this study, the seasonal variations in bacterial abundance we observed can be explained by a scenario in which both growth rates and loss rates (grazing &amp;amp;plus; viral lysis) influence the dynamics of the bacteria community.

Diversity and physiology of culturable bacteria associated with a coastal Antarctic ice core

Microbiological studies of polar ice at different depths may provide important comparisons, as they preserve records of microbial cells and past climate. In this study, we examined bacterial abundance, diversity and glaciochemical composition from three depths of an ice core from coastal Dronning Maud Land, East Antarctica. Higher bacterial abundance corresponded with high in situ sea-salt Na+ and dust concentration, suggesting that bacteria might have been transported and deposited into ice along with dust particles and marine aerosols. Fourteen bacterial isolates belonging to the genera Methylobacterium, Brevundimonas, Paenibacillus, Bacillus and Micrococcus were retrieved. Frequent isolation of similar bacterial genera from different cold environments suggests that they possess features that enable survival and metabolism for extended periods of time at sub-zero temperatures. The highest number and diversity of recoverable bacteria was obtained from 49m depth corresponding to 1926 AD and consisted of bacteria from 4 different genera whereas at 11m (1989 AD) and 33m (1953 AD) samples only species belonging to the genera Bacillus was recovered. Among the Bacillus species, Bacillus aryabhattai which has been reported only from the upper stratosphere, was isolated and is the first record from the Earth's surface. Methylobacterium was the most dominant genera at 49m depth and its prevalence is attributable to a combination of high in situ methanesulfonate concentration, specialized metabolism and environmental hardiness of Methylobacterium. Some of the isolated bacteria were found to respire and grow using methanesulfonate, suggesting that they may utilize this substrate to sustain growth in ice. In addition, NO3− (2.93–3.69μM), NH4+ (1.45–3.90μM) and PO43− (0.01–0.75μM) present in the ice could be potential sources fueling bacterial metabolism in this environment. It could be deduced from the study that variation in bacterial abundance and diversity was probably associated with the prevailing in situ conditions in ice.

印旛沼におけるバクテリアの季節変動パターン及び変動要因の解明

印旛沼は日本で最も富栄養な湖のひとつである。水質と密接に関係のあるプランクトンの生態に関する研究はされていない。本研究ではプランクトンの中でもバクテリアについて，その季節変動パターンおよび変動要因を明らかにするため，2009年4月から10月にかけて週１回の調査を行った。バクテリアの密度に影響を与えるボトムアップ効果として窒素・リン・有機物（クロロフィルa，懸濁態リン），トップダウン効果として捕食者（鞭毛虫）を考慮に入れ，これら要因がバクテリアの密度に与える影響の相対的重要性について検討した。バクテリアの密度は約1.8～9.5×106cells mL-1の間で変動し4月下旬に最大となった。重回帰分析の結果，バクテリア数 は懸濁態リンとは正の相関，鞭毛虫とは負の相関がみられたが，水温，栄養塩濃度（窒素，リン），クロロフィル濃度との間には有意な関係は認められなかった。懸濁態リンとの関係はあったがクロロフィル量との関係がみられなかったことから，外来性の物も含め有機物量全体がバクテリアの変動を制限していると考えられた。栄養塩のうち，リン濃度は非常に低く，バクテリア密度との間には関係がみられなかったが，バクテリアの成長を制限しているのかもしれない。一方，鞭毛虫との間に負の相関関係があり，バクテリアの密度は鞭毛虫による捕食によって制限されていることが示唆された。以上の結果より，富栄養な印旛沼のバクテリア数はトップダウン効果（鞭毛虫による捕食）だけでなくボトムアップ効果（懸濁態リンの濃度）によっても制限されていることが示された。

Temporal dynamics of freshwater bacterio‐ and virioplankton along a littoral–pelagic gradient

Summary1. Viruses have been identified as an important component of plankton communities but their temporal dynamics remain poorly characterized, especially in littoral zones of lakes. Seasonal and diel dynamics of planktonic viruses and their main hosts (i.e. bacteria) were therefore assessed along four parallel transects of a lake, extending from within a littoral reed stand across the littoral shelf to the pelagic zone.2. Viral abundance ranged from 1.9 to 9.7 × 107 virus‐like particles (VLP) mL−1 with distinct seasonal peaks in spring at all locations. Seasonal variation in bacterial abundance varied more than 10‐fold (0.85–12 × 106 cells mL−1) and was synchronized with changes in viral abundance. As a result, the ratio of viruses to bacteria (average of 12.5) varied much less over the year.3. Bacterial production (0.11–0.98 μg C L−1 h−1) and growth rates (0.06–0.85 day−1) also varied seasonally but, in contrast to abundances, high production rates in spring were maintained in mid‐summer, particularly in the reed stand, where growth rates were highest.4. Variation among locations was small in most measured parameters, probably due to extensive exchange between pelagic and littoral water masses. Nevertheless, average bacterial growth and production rates in all seasons were consistently highest in the reed stand.5. In the August of an exceptionally warm summer, viral and bacterial abundance varied little over a diel cycle (1.23–1.75 × 108 VLP mL−1 and 0.78–1.90 × 107 cells mL−1, respectively) in spite of a trend towards higher values at night. Diel changes in bacterial production (0.23–0.69 μg C L−1 h−1) and growth rate (0.02–0.06 day−1) were more pronounced, with higher values also occurring during the night, possibly indicating that bacterio‐ and phytoplankton production were not tightly coupled during our diel study.6. Overall these results indicate that seasonal changes of virio‐ and bacterioplankton were mainly driven by high microbial activity in spring and were more pronounced than either diel changes on a hot summer day or spatial differences at a scale of 10–100 m.

Viral dynamics in a coastal sediment: seasonal pattern, controlling factors and relations to the pelagic–benthic coupling

The present study demonstrates for the first time a clear seasonal variability in the abundance of virobenthos that largely followed the variations in bacterial abundance. Following the spring bloom settlement, the benthic metabolic activity intensified and viral and bacterial abundance in the surface sediment increased 4.0- and 2.6-fold while the increase in a deeper anoxic sediment layer (6–10 cm) was 5.0- and 2.5-fold, respectively. The data strongly indicated a relationship between the pelagic–benthic coupling of organic material, the benthic metabolic activity, and the abundance and activity of bacteria and viruses in marine sediments on a seasonal basis. These suggestions were supported by parallel measurements of viral production rates in short-term laboratory incubations of surface and deep sediments: in these, viral production was significantly enhanced by addition of organic carbon and increasing temperature, with maximum values obtained in organic carbon-enriched incubations in July (19.7×106 and 38.4×106 virus-like particles cm−3 h−1 in surface and deep incubations, respectively). However, the relative enhancement was most explicit during periods of low in situ sedimentation. Conservative estimates indicated that viral lysis caused bacterial mortality rates of 2.8–27.4×105 cells cm−3 h−1, corresponding to 2–61% day−1 of bacterial standing stock during incubations (average 12.3±13.9%, n=18). An estimation of virus-generated dissolved organic carbon production using theoretical values of bacterial cell carbon content (20–50 fg cell−1), showed that viral lysates could sustain 2–38% of the measured bacterial respiration. The data suggest that viruses may have considerable impact on benthic microbial mortality and carbon cycling.

Bacterioplankton heterotrophic activity in relation to the phytoplankton compartment in a recently formed reservoir

Seasonal and spatial variations in bacterial abundance, biomass and potential heterotrophic activity in a recently flooded reservoir were measured for two consecutive years, in conjunction with phytoplankton biomass (chlorophyll-a concentration) and activity (primary production). The mean value of primary production remained constant between the two study years, while those of the chlorophyll-a concentrations, bacterial abundance, bacterial biomass and bacterial heterotrophic activity decreased. The observed trends in the bacterial variables were linked to changes in the relative importance of allochthonous dissolved organic matter, in addition to selective grazing activity on bacterioplankton. Multivariate regression analyses identified bacterial abundance (29%) and temperature (17%) as dominant correlates of the bacterial potential heterotrophic activity. We concluded that, in a new reservoir, organic matter, other than that from phytoplankton, might be of great importance for bacterioplankton metabolism. Furthermore, grazing activity on bacteria by metazoa in a new reservoir represents, on occasion, an important trophic link between the top consumer and otherwise unavailable dissolved organic carbon sources. Finally, even if little energy is transferred to larger consumers, the microbial route is still important in re-mineralizing organic matter in Sep Reservoir.

Field experiments on mitigation of harmful algal blooms using a Sophorolipid—Yellow clay mixture and effects on marine plankton

This study examined a new method of mitigating harmful algal blooms (HABs) by combining biosurfactant sophorolipid and yellow clay. To investigate the effects and practicability of this HAB mitigation method, field experiments were carried out during a Cochlodinium bloom near Miruk Island, South Korea, in August 2002. Field experiments examined the effects of sophorolipid and yellow clay on Cochlodinium bloom mitigation and on marine plankton such as bacteriaplankton, heterotrophic protists, and zooplankton. A mixture of 5 mg l −1 sophorolipid and 1 g l −1 yellow clay was sprayed directly on the sea surface and its effect was compared with that of 10 g l −1 of yellow clay applied under similar conditions. The sophorolipid–yellow clay mixture more efficiently mitigated the Cochlodinium bloom (95% removal efficiency after 30 min) than yellow clay alone (79% after 30 min). Further, no variation in bacterial abundance occurred 30 min after spraying the sophorolipid–yellow clay mixture. After 30 min, heterotrophic protist abundance at the surface decreased 21 and 41%, respectively, following the sophorolipid–yellow clay mixture and yellow clay treatments. Zooplankton decreased by 38% 15 min after spraying the mixture and 67% 30 min after spraying the yellow clay. These results indicate that the mixture of sophorolipid and yellow clay had a less adverse effect on bacteriaplankton, heterotrophic protists, and zooplankton than the yellow clay, suggesting that the sophorolipid–yellow clay mixture can mitigate HABs efficiently with fewer negative effects on the pelagic ecosystem.

Relationship between bacterial and primary production in a newly filled reservoir: temporal variability over 2 consecutive years

Seasonal and spatial variations in bacterial abundance, biomass and production in a recently flooded reservoir were followed for 2 consecutive years, in conjunction with phytoplankton biomass (chlorophyll a) and activity (primary production). Between the 2 years of the study, the mean value of primary production remained constant, while those of the chlorophyll a concentration, bacterial abundance (BA), bacterial biomass (BB) and bacterial production (BP) decreased. The observed trends of the bacterial variables were linked to changes in the relative importance of allochthonous dissolved organic matter. Moreover, this factor would explain discrepancies observed between the slope of the model II regression equations established from results of the present study and those of the predictive models from the literature, relating to bacterial and phytoplankton variables. An estimate of the carbon budget indicated that 22 and 5% of the ambient primary production in the Sep Reservoir might be channeled through the microbial loop via BP during the 1st and 2nd year of the study, respectively. We conclude that heterotrophic BP in the Sep Reservoir may, on occasion, represent a significant source of carbon for higher order consumers.

Meso-scale spatial variation in bacterial abundance and production associated with surface convergence and divergence in the NE equatorial Pacific

To elucidate the effect of the water column structure associated with surface water convergence and divergence on bacterial parameters, we investigated the meso-scale spatial variations in bacterial abundance and production in the northeast equatorial Pacific from 5 to 12° N (longitude 131° 30' W). The area is characterized by 3 major surface currents: the South Equatorial Current (SEC), North Equatorial Counter Current (NECC), and North Equatorial Current (NEC). Surface water convergence occurred at the boundary of the westward-flowing SEC and eastward- flowing NECC, and divergence occurred at the boundary of the NECC and westward NEC. A verti- cally well-mixed water column observed in the SEC/NECC area appeared to be high in nutrients and chlorophyll a (chl a), whereas the strongly stratified NEC area had a relatively low trophic status, with a surface layer depleted of nitrates and chl a. Accordingly, bacterial abundance, production, and mean biomass turnover rates, as well as grazing by heterotrophic nanoflagellates (HNF) in the well- mixed SEC/NECC area were consistently higher than in the highly stratified NEC area. The greater bacterial production and HNF grazing in the SEC/NECC region imply that heterotrophic bacteria are a significant trophic link between dissolved organic carbon and higher trophic levels in the microbial loop of the SEC/NECC area. Our results indicate that changes in the water column structure in rela- tion to convergence and divergence play a key role in enhancing the amount of nutrients and chl a, thereby stimulating bacterial abundance and production and intensifying the trophic link between heterotrophic bacteria and heterotrophic protists in the microbial food web in the study area.