Spatial and temporal patterns in bacterial abundance, production and viral infection in a temporarily open/closed southern African estuary

The spatial and temporal patterns in bacterial abundance, biomass, production, nanoflagellate abundance and the loss of bacterial production due to viral lysis were investigated in a temporarily open/closed estuary along the eastern seaboard of southern Africa over the period May 2006 to April 2007. Bacterial abundance, biomass and production ranged between 1.00 × 10⁹ and 4.93 × 10⁹ cells l⁻¹, 32.43 and 108.59 μg C l⁻¹ and 0.01 and 1.99 μg C l⁻¹ h⁻¹, respectively. With a few exceptions there were no significant spatial patterns in the values (P > 0.05). Bacterial abundance, biomass and production, however, demonstrated a distinct temporal pattern with the lowest values consistently recorded during the winter months. Bacterial dynamics showed no effect of mouth opening events. Nanoflagellate and bacterial abundances were significantly correlated to one another (P < 0.05) suggesting a strong predator-prey relationship. The frequency of visibly infected bacterial cells and the number of virus particles within each bacterial cell during the study demonstrated no significant temporal or spatial pattern (P > 0.05) and ranged from 0.5 to 6.1% and 12.0 to 37.5 virus particles per bacterium, respectively. Viral infection and lysis was thus a constant source of bacterial mortality throughout the year. The estimated percentage of bacterial production removed by viral lysis ranged between 7.8 and 88.9% (mean = 30.3%) of the total which suggests that viral lysis represents a very important source of bacterial mortality during the study.     

Covariation of viral parameters with bacterial assemblage richness and diversity in the water column and sediments

Viruses are hypothesized to maintain diversity in microbial assemblages by regulating the abundance of dominant competitors and thereby allowing less-dominant competitors to persist in assemblages; however, there have been few empirical data sets to support this idea. In this study, we examined the relationship between the ratio of viral abundance to bacterial abundance, viral production, and the relative richness and diversity of bacterial assemblage fingerprints, in samples taken from geographically widespread locations (North Pacific gyre, the Amazon River plume and adjacent North Atlantic gyre, Gulf of Mexico, Southern California Bight and Arafura—Coral Seas) which are oligo- to mesotrophic. Bacterial assemblage richness and diversity as measured by automated rRNA intergenic spacer (ARISA) fingerprinting were significantly and positively correlated with the ratio of virus abundance to bacteria abundance (VBR) and to the rate of virus production only in the oligotrophic North Pacific gyre. ARISA fingerprint richness/diversity were not significantly correlated to viral parameters when assessed across all samples in surface waters, suggesting there is not a singular global quantitative relationship between viral pressure and host diversity within well evolved host/virus systems in different geographic locations in plankton. In sediments off Southern California, viral parameters significantly and negatively correlated with ARISA diversity, suggesting strong viral interactions in this habitat. To examine covariation of viral parameters and the relative abundance and diversity of rarer bacterial taxa (i.e., less-dominant competitor), the richness and diversity of diazotroph communities was measured using terminal restriction fragment length polymorphism (TRFLP) of a portion (nifH) of the nitrogenase gene. The richness and diversity of diazotrophic communities were significantly and negatively correlated with viral parameters across all locations. Since diazotrophs include many opportunistic taxa (e.g. Vibrionaceae), and because these bacteria may be more susceptible to viral attack due to enhanced resource uptake abilities and potentially rapid localized growth, it is possible that this negative effect was due to enhanced viral lysis. Consequently, virus infection may have positive effects upon bacterioplankton diversity in the oligotrophic ocean, by regulating the abundance of dominant competitors, and allowing rarer taxa to coexist; however, some rarer taxa (such as diazotrophs) may be more susceptible to viral attack due to opportunistic lifestyles.     

Distinctive copepod community of the estuarine turbidity maximum: comparative observations in three macrotidal estuaries (Chikugo, Midori, and Kuma Rivers), southwestern Japan

Copepods are considered to be a vital component connecting the unique macrotidal environment to the high productivity and high biodiversity of the Ariake Sea. To examine the spatiotemporal succession of copepod communities, we conducted monthly sampling (vertical hauls of a 100-μm mesh plankton net) in three neighboring macrotidal estuaries between 2005 and 2006. Irrespective of the season, three copepod communities were recognized in relation to the relatively long gradients of salinity and turbidity along the Chikugo and Midori River estuaries. The oligohaline community (salinity 1–10) was observed at higher turbidities (>100 NTU), whereas the freshwater (salinity <1) and meso/polyhaline (salinity >10) communities were associated with lower turbidities (<100 NTU). The oligohaline calanoid Sinocalanus sinensis occurred only in the Chikugo River estuary, maintaining a large biomass (dry weight >10 mg m^?3) in or close to the well-developed estuarine turbidity maximum (ETM) throughout the year. In the Midori River estuary, the oligohaline community lacked S. sinensis and showed a minimum biomass during winter (<10 mg m^?3). In both estuaries, the freshwater community always remained at a small biomass (<1 mg m^?3), whereas the meso/polyhaline community showed marked seasonal changes in biomass (0.1–657 mg m^?3). The prevalence of higher salinities allowed only the meso/polyhaline community to occur in the Kuma River estuary. In summary, S. sinensis characterized the copepod community distinctive of the well-developed ETM, potentially serving as an important link to higher trophic levels during winter when copepods are scarce in other areas.     

镜泊湖春、夏两季浮游生物群落结构及其与环境因子的关系

本研究于2015年在黑龙江省镜泊湖选取7个典型采样点,对镜泊湖春、夏两季间浮游生物群落结构的演替及其与环境因子关系进行分析。在调查期间共鉴定浮游植物152个分类单位,浮游动物38个分类单位,种类组成上浮游植物主要以绿藻门和硅藻门藻类为主,浮游动物以轮虫和原生动物为主。研究期间浮游生物优势种主要包括:河生集星藻(Actinastrumfluviatile(Schroed.)Fott)、梅尼小环藻(Cyclotella meneghiniana Kütz.)、谷皮菱形藻(Nitzschia palea (Kütz.) W.Smith)、前节晶囊轮虫(Asplanchna priodonta)、螺形龟甲轮虫(Keratella cochlearis)和瓶砂壳虫(Difflugia urceolata)。在研究期间水温、pH、电导率、和总氮在春、夏两季之间变化明显(P0.05或P0.01)。春、夏两季的浮游生物丰度呈现明显下降趋势(P0.05)。Spearman相关分析揭示镜泊湖水体浮游生物丰度与水温和电导率极其显著相关(P0.01)。冗余分析(RDA)表明,水温和电导率是驱动镜泊湖水体浮游生物群落生态分布的主要环境因子。与近10余年间镜泊湖相关研究对比发现,镜泊湖水体浮游植物群落结构较为稳定,浮游动物优势类群略有改变。综合营养状态指数评价表明镜泊湖水体为中营养状态,与前期研究结果相似,说明水体目前无进一步恶化的趋势。     

热带北太平洋西部固氮速率及固氮微生物组成

In the present study, we report N_2 fixation rate(~(15)N isotope tracer assay) and the diazotroph community structure(using the molecular method) in the western tropical North Pacific Ocean(WTNP)(13°–20°N, 120°–160°E). Our independent evidence on the basis of both in situ N_2 fixation activity and diazotroph community structure showed the dominance of unicellular N_2 fixation over majority of the WTNP surface waters during the sampling periods.Moreover, a shift in the diazotrophic composition from unicellular cyanobacteria group B-dominated to Trichodesmium spp.-dominated toward the western boundary current(Kuroshio) was also observed in 2013. We hypothesize that nutrient availability may have played a major role in regulating the biogeography of N_2 fixation.In surface waters, volumetric N_2 fixation rate(calculated by nitrogen) ranged between 0.6 and 2.6 nmol/(L·d) and averaged(1.2±0.5) nmol/(L·d), with 10 μm size fraction contributed predominantly(88%±6%) to the total rate between 135°E and 160°E. Depth-integrated N_2 fixation rate over the upper 200 m ranged between 150 μmol/(m~2·d)and 480 μmol/(m~2·d)(average(225±105) μmol/(m~2·d). N_2 fixation can account for 6.2%±3.7% of the depthintegrated primary production, suggesting that N_2 fixation is a significant N source sustaining new and export production in the WTNP. The role of N_2 fixation in biogeochemical cycling in this climate change-vulnerable region calls for further investigations.     

Plankton community and bacterial metabolism in Arctic sea ice leads during summer 2010

Microbial plankton metabolism was examined during summer 2010 in sea ice-influenced waters of the Fram Strait, eastern Arctic Ocean. Rates of gross primary production and community respiration were tightly coupled over a wide range of values (33±3–143±6 and 20±3–126±6 mmol O₂ m^{−2  −1}, respectively) leading to a prevalence of positive net community production. The high variability in community respiration, similar to that of gross primary production, suggests that heterotrophic metabolism may exhibit a significant response to environmental change. Bacterial respiration was assessed at similar time scales to bacterial production measurements, by determining the in vivo INT reduction capacity without pre-filtering the community. Bacteria seem to play a major role in total community respiration, contributing between 5% and 61% of total community respiration, indicating that a high fraction of the organic carbon in Arctic planktonic food webs could flow through these microbes.     

Viruses,bacteria, and heterotrophic nanoflagellates in Laptev Sea plankton

The paper considers the concentrations and functional characteristics of viruses, bacteria, and heterotrophic nanoflagellates determined for the first time in the Laptev Sea in August-September, 2014. The abundance of bacteria, viruses, and heterotrophic nanoflagellates varied from 110.1 × 10³ to 828.4 × 10³ cells/mL, from 384.2 × 10³ to 2932.8 × 10³ particles/mL, and from 108 to 651 cells/mL, respectively. The daily bacterioplankton production varied from 4.2 × 10³ to 381.7 × 10³ cells/mL, with an average of 117.6 × 10³ cells/mL. Electron transmission microscopy has for the first time shown that the frequency of visibly infected bacterial cells varied from 0.2 to 2.0% (0.8% on average) of N_B. The average virus-induced mortality of bacteria was 6.3% of bacterioplankton production, with variations ranging from 1.4 to 16.9%. Grazing on bacteria by heterotrophic nanoflagellates contributed more to bacteria mortality than virus-induced bacterial lysis. By grazing on bacteria, heterotrophic nanoflagellates consumed large quantities of viruses located on the surface and inside bacterial cells.     

影响北欧海和楚科奇海夏季细菌丰度和生产力的因素

Abundance and production of bacterioplankton were measured in the Nordic seas and Chukchi Sea during the5 th Chinese Arctic Research Expedition in summer 2012.The results showed that average bacterial abundances ranged from 3.31×10~(11) cells/m~3 to 2.25× 10~(11)cells/m~3,and average bacterial productions(calculated by carbon)were 0.46 mg/(m~3·d) and 0.54 mg/(m~3·d) in the Nordic seas and Chukchi Sea,respectively.T-test result showed that bacterial abundances were significantly different between the Nordic seas and Chukchi Sea,however,no significant difference was observed regarding bacterial productions.Based on the slope of lg bacterial biomass versus lg bacterial production,bacterial communities in the Nordic seas and Chukchi Sea were moderately dominated by bottom-up control.Both Pearson correlation analysis and multivariable linear regression indicated that temperature had significant positive correlation with bacterial abundance in the Chukchi Sea,while no correlations with productions in both areas.Meanwhile,Chl a had positive correlations with both bacterial abundance and production in these two regions.As the temperature and Chl a keep changing in the future,we suggest that both bacterial abundance and production been hanced in the Chukchi Sea but weaken in the Nordic seas,though the enhancement will not be dramatic as a result of higher pressure of predation and viral lysis.     

Production and degradation of fluorescent dissolved organic matter derived from bacteria

Dynamics of fluorescent dissolved organic matter (FDOM) in ocean environments has received attention over the past few decades. Although it has appeared that in situ production of oceanic FDOM is mainly due to bacteria, the production and bio- and photodegradation processes of bacterial FDOM have not been elucidated. In this study, a culture experiment with bacteria was carried out to assess the production and biodegradation processes of bacterial FDOM. Photodegradation of bacterial FDOM and dissolved organic carbon (DOC) was also examined by exposure to a solar simulator. Bacterial FDOM consists of six components which were determined by parallel factor analysis (PARAFAC). Fluorescence intensities of protein-like FDOM increased with the bacterial biomass, but the increases of humic-like FDOM lagged behind the protein-like FDOM by 5–10 days. Exposure to simulated sunlight caused significant decreases in fluorescence intensities of all components; 52–94% of the initial intensities were lost during 24 h. While, the DOC concentration exhibited a small decrease through the experiment (1.9–11.1%). These results showed that photodegradability of bacteria derived DOC was much less than the fluorescence, indicating that the lifetime of bacteria-derived DOC is much longer than the length estimated by the fluorescence. The role of photobleached FDOM derived from bacteria may be significant in the biogeochemical cycle at the surface layer.     

Heterotrophic bacteria of the Dapeng Bay in the South China Sea: community structure, abundance, and the relationships of culturablity with environmental factors

The community composition,abundance and culturablity of heterotrophic bacterioplankton in the Dapeng Bay of the South China Sea were investigated by morphological method,bacterial cultivation and biochemical methods over an annual cycle.Aeromonas and Pseudomonas were the predominant genera among the Gram-negative bacteria.The bacterioplankton abundance fluctuated from 2.07×10 8 to 2.44×10 9 cells/dm 3 ,and culturable bacteria abundance ranged between 0.1×10 6 and 16.07×10 6 CFU/dm 3 .The Culturablity of heterotrophic bacteria ranged between 0.01% and 1.31%.The relationship between bacterial culturablity and the environmental factors was also studied in different sites of the Dapeng Bay.Throughout the bay,a significantly positive relationship was evidenced between bacterial culturability and temperature,meanswhile bacterial culturability was significantly inversely related with salinity.The positive relationship was shown at the central bay while a negative relationship was observed at the bay mouth and there was no significant correlation at top bay between bacterial culturablity and chlorophyll a of sea water.A positive relationship was found between culturability and DIN all the bay,but DIP showed a significantly negative relationship with bacterial culturability only at the mouth of the bay.     

施肥对盐沼沉积物微生物生物量和细菌群落组成的影响

The effects of nitrogen(N) addition on microbial biomass, bacterial abundance, and community composition in sediment colonized by Suaeda heteroptera were examined by chloroform fumigation extraction method, real-time quantitative polymerase chain reaction, and denaturing gradient gel electrophoresis(DGGE) in a salt marsh located in Shuangtai Estuary, China. The sediment samples were collected from plots treated with different amounts of a single N fertilizer(urea supplied at 0.1, 0.2, 0.4 and 0.8 g/kg(nitrogen content in sediment) and different forms of N fertilizers(urea,(NH_4)_2SO_4, and NH_4NO_3, each supplied at 0.2 g/kg(calculated by nitrogen).The fertilizers were applied 1–4 times during the plant-growing season in May, July, August, and September of 2013. Untreated plots were included as a control. The results showed that both the amount and form of N positively influenced microbial biomass carbon, microbial biomass nitrogen, and bacterial abundance. The DGGE profiles revealed that the bacterial community composition was also affected by the amount and form of N.Thus, our findings indicate that short-term N amendment increases microbial biomass and bacterial abundance,and alters the structure of bacterial community.     

乳山湾外海溶氧低值区细菌多样性初步研究

采用化学裂解法从乳山湾外海溶氧低值区不同溶解氧质量浓度(3.7～7.0mg.L-1)的6个站位海水样品中提取了环境DNA样品。以试剂盒纯化后的DNA样品为模板扩增其16SrRNA基因V3区,通过变性梯度凝胶电泳、分子文库构建及DNA测序对溶氧低值区海水中的细菌群落结构进行研究。结果表明,乳山湾外海溶氧低值区不同溶解氧质量浓度的6个站位底层海水样品中的细菌群落结构是相似的,它们均由隶属于Alteromonas(交替单胞菌属)、Salegentibacter(需盐杆菌属)等10个属的18种细菌组成。系统发育分析发现这些细菌分别属于α变形菌纲(2种)、γ变形菌纲(12种)和黄杆菌纲(3种)三个大类。在乳山湾外海溶氧低值区的海水样品中细菌多样性最高的类群是γ变形菌纲。     

两种混养方式对异枝江蓠(Gracilaria bailinae)生长性能、表面附生细菌群落及抗生素抗性基因的影响

为揭示"鱼-藻"和"鱼-虾-藻"混养对异枝江蓠(Gracilaria bailinae)生长性能、表面附生细菌群落和抗生素抗性基因(antibiotic resistance genes,ARGs)的影响,阐明异枝江蓠表面附生细菌群落与生长性能、ARGs之间的关系,利用16S rDNA高通量测序技术和Real-time qPCR技术分析了异枝江蓠表面附生细菌群落和ARGs的组成与差异,冗余分析(RDA)探讨细菌群落与生长性能、ARGs之间的关联。结果表明:(1)"鱼-虾-藻"混养会促进异枝江蓠的生长性能,增加表面附生细菌群落的多样性。(2)异枝江蓠表面附生细菌群落主要属于变形菌门、蓝藻门、浮霉菌门和拟杆菌门,不同混养方式中优势菌属组成不同,"鱼-虾-藻"混养优势菌属多样性较高。(3)"鱼-虾-藻"混养的异枝江蓠ARGs/MGEs的相对丰度大多高于"鱼-藻"混养。(4)RDA分析表明,生长性能主要与Ralstonia、Blastopirellula等显著相关,ARGs/MGEs主要与Nitrosomonas、Alteromonas、Pleurocapsa;CC-7319等显著相关。"鱼-虾-藻"混养能够增强异枝江蓠的生长性能,提高异枝江蓠表面附生细菌群落的多样性。但"鱼-虾-藻"混养能够增加异枝江蓠ARGs/MGEs的相对丰度,存在一定的生态风险。因此,在注重经济效益的同时也要关注可能存在的对人类健康的危害。研究结果将有助于海水养殖环境的优化,为大型海藻在海水养殖业中的应用与推广提供理论基础。     

Bacterial community succession in response to dissolved organic matter released from live jellyfish

Jellyfish blooms have increased worldwide, and the outbreaks of jellyfish population not only affect the food web structures via voracious predation but also play an important role in the dynamics of nutrients and oxygen in planktonic food webs. However, it remains unclear whether specific carbon compounds released through jellyfish metabolic processes have the potential to shape bacterial community composition. Therefore, in this study, we aimed to investigate the compositional succession of the bacterioplankton community in response to the dissolved organic matter (DOM) released by the live Scyphomedusae Cyanea lamarckii and Chrysaora hysoscella collected from Helgoland Roads of the North Sea. The bacterial community was significantly stimulated by the DOM released form live jellyfish and different dominant phylotypes were observed for these two Scyphomedusae species. Furthermore, the bacterial community structures in the different DOM sources, jellyfish-incubated media, Kabeltonne seawater, and artificial seawater (DOM-free) were significantly different, as revealed by automated ribosomal intergenic spacer analysis fingerprints. Catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH) revealed a rapid species-specific shift in bacterial community composition. Gammaproteobacteria dominated the community instead of the Bacteroidetes community for C. lamarckii, whereas Gammaproteobacteria and Bacteroidetes dominated the community for C. hysoscella. The significant differences in the bacterial community composition and succession indicate that the components of the DOM released by jellyfish might differ with jellyfish species.     

Total organic carbon and nitrogen contents in surface sediments of Harima Nada,eastern Seto Inland Sea,Japan: a comparison under two different trophic states

Phytoplankton abundance in the surface waters of Harima Nada, eastern Seto Inland Sea of Japan, decreased after around the late 1970s following a reduction of terrestrial phosphorous inputs, and relatively low levels persisted thereafter. This symptom of de-eutrophication in recent decades might have reduced organic matter storage in surficial sediments. To test this possibility, we examined total organic carbon (TOC) and nitrogen (TN) contents of Harima Nada sediments in 2011, and compared the results to past data from more eutrophic states. In 2011, the TOC and TN varied horizontally within the basin, in relation to physicochemical properties (water and mud content) and phytoplankton remnants (pheo-pigments) of the sediments. These horizontal patterns were fundamentally similar to those of the early 1980s (1982–1983), although discrepancies between the periods were observed in some areas. Differences of mean TOC and TN in the 0–2 cm layer at 63 stations from 1982–1983 to 2011 were consequently <10 %. Hence, the recent de-eutrophication in Harima Nada was associated with little overall change of TOC and TN storage in the sediment. In Harima Nada, changes in water clarity and particle size composition of the sediment seem to have occurred during recent decades. The aforementioned paradoxical phenomenon may therefore be explained by factors that are affected by these environmental changes. Such factors include the ratio of export flux to the seafloor to phytoplankton production, phytoplankton biomass and production in the entire euphotic zone, and accumulation rate of surface sediment.     

Differences in the damage caused by hypoxia to the macrobenthic communities in source regions of hypoxic water and in regions with advected hypoxic water

The objective of this study was to determine the effects of transported hypoxic water on macrobenthic species composition in Isahaya Bay, Kyushu, Japan. To this end, we conducted a field survey of water quality, sediment quality, and the macrobenthic community across seasons. Hypoxic intensity, defined as the percentage of the time that a region exhibits hypoxic conditions [dissolved oxygen (DO) <?3.0 mg L^?1], was calculated for the summer period from July 1 to September 30, 2014 based on continuous monitoring data derived from six monitoring towers in the bay. Here, we discuss how the hypoxia-induced damage, defined as the change in the species compositions of the macrobenthic communities in the bay due to hypoxia, differs between the inner and middle regions of the bay (with muddy sediment) and the outer regions of the bay (with sandy sediment). A decrease in the density of macrobenthic species that was correlated to the period of lowest DO concentration (<?1.0 mg L^?1) was observed in the inner region of the bay. A large number of species disappeared from the macrobenthic community in the outer region after the DO concentration had dropped to its lowest level, even though this region presented the lowest hypoxic intensity (6.6%). The species that disappeared, including crustaceans and some polychaetes, were presumed to be sensitive to, and to have escaped from, hypoxia. Fewer species disappeared from the inner and intermediate regions of the bay, which had high hypoxic intensities of 14.0 and 26.7%, respectively, than from the outer region. The reason for this appeared to be a predominance of hypoxia-tolerant species in the inner and intermediate regions but not in the outer region. Our study suggests that estimating the effect of hypoxia in a spatially heterogeneous environment must be done with caution.     

Harmful algal blooms (HABs), dissolved organic matter (DOM), and planktonic microbial community dynamics at a near-shore and a harbour station influenced by upwelling (SW Iberian Peninsula)

The surface microalgal community, including harmful species, dissolved organic matter (DOM), and bacterial and viral populations were studied during an annual cycle (November 2007–October 2008) in a Near-shore (NS) and a Harbour (H) station located in an upwelling area (Sagres, SW Iberian Peninsula). The higher water residence time, water stability and shallowness of harbours in comparison with open waters likely contributed to the differences found between stations regarding chemical variables, statistical correlations and harmful algal proliferations. Also, several differences were noticed from a previous assessment (Loureiro et al., 2005) including higher SST, lower nitrate and chlorophyll a concentrations, along with a shift in the microplankton community structure from diatom to nanoflagellate predominance. These variations feasibly reflect the response of this dynamic system to regional environmental modifications contributing to the understanding of common patterns in environmental change trends. The division of the sampling period into (1) non-upwelling (Non-Uw), (2) “spin-up” of upwelling (SU-Uw), and (3) “spin-down” and relaxation-downwelling (SD-Rel) stages allowed the identification of natural groupings of microplankton samples by Multi Dimensional Scaling (MDS) analysis. Dissolved organic nitrogen (DON) and viruses were the most significant abiotic and biotic variables, respectively, contributing to the dissimilarities between these stages (SIMPER analysis) and, therefore, potentially affecting the microplankton community structure. Harmful algal species and a stable viral community appeared to be favoured by SD-Rel conditions. Data seem to indicate that both Gymnodinium catenatum and Heterosigma akashiwo, the most abundant potentially harmful species, have been imported into the sampling area. Also, the H location, together with potential retention sites developing around the Cabo de São Vicente upwelling centre, may contribute to the local accumulation of harmful species. The associations found between harmful algal species and bacterioplankton and virioplankton call for detailed studies in order to evaluate species-specific interactions.     

Metabolism and chemical composition of marine pelagic amphipods: synthesis toward a global bathymetric model

Respiration and ammonia excretion data and chemical composition data [water content, ash, carbon (C), nitrogen (N) and C:N ratios] of 18–32 amphipods (hyperiids and gammarids) from the epipelagic through bathypelagic zones of the world’s oceans were compiled. The independent variables including body mass, habitat temperature and mid-sampling depth were all significant predictors of respiration, accounting for 65–83 % of the variance in the data, while the former two variables were significant predictors of ammonia excretion, accounting for 64–77 % of the variance. Atomic O:N ratios (respiration:ammonia excretion) ranged from 11 to 74 (median 21.5). C composition was negatively correlated with habitat temperature, but water contents, ash, N, and the C:N ratio were uncorrelated with the three independent variables. As judged by C:N ratios, protein was considered to be the major organic component of most pelagic amphipods. However, some amphipods from >500 m depth exhibited high C:N ratios (>10) suggesting a large deposition of lipids in the body. Comparison of the present results with global bathymetric models of euphausiids and pelagic copepods revealed that respiration rates of the pelagic amphipods were near-equal to the rates of euphausiids but greater than the rates of pelagic copepods, reflecting taxon-specific body morphology and swimming behavior among the three taxa. As a marked feature of body chemical composition, the pelagic amphipods exhibited extremely high ash content (mean 25 % of DM) due to their possession of a robust exoskeleton.     

Temporal dynamics of dissolved organic carbon (DOC) produced in a microcosm with red tide forming algae <Emphasis Type="Italic">Chattonella marina</Emphasis> and its associated bacteria

Acute and severe hypoxia associated with harmful algal bloom has become one of the major causes for the environmental deterioration of coastal areas. Although it is generally thought that a large part of the dissolved oxygen consumption at a bloom site is initiated by heterotrophic bacteria that attack organic matter derived from dead or dying algal cells, precise microbial processes leading to the hypoxia are yet to be examined. Here we show temporal dynamics of extracellular dissolved organic carbon (DOC) of the red tide forming raphidophyte Chattonella marina and bacterial populations associating with the algae under laboratory conditions. During the growth of non-axenic strains of C. marina, we monitored abundance of algae, associated bacteria, and DOC in the culture media. Bacterial cell abundance increased in response to the increase in DOC both at the beginning and the late log phase of the algal growth. Flow cytometric analysis revealed that transient increase in the percentage of respiratory-active bacterial cells also coincided with the timing of the increase in bacterial abundance and DOC. These results strongly suggest that DOC released from growing C. marina fuels respiration and growth of planktonic bacteria surrounding the algae. This has implications for the role of DOC released from C. marina bloom before the collapse in mediating interactions between neighboring algae and bacterial assemblage which may eventually lead to algal bloom-associated hypoxia.     

The response of microbial communities to diverse organic matter sources in the Arctic Ocean

The response of Arctic microbial communities to a variety of natural organic matter substrates, including peat, ice algae and ice-rafted debris was examined using bacterial regrowth experiments and compared to unamended controls. Bacterial growth and production were followed together with the phylogenetic composition using length-heterogeneity polymerase chain reaction (LH-PCR), and 16S rRNA gene cloning and sequencing. Intact phospholipids (IPLs) and fatty acids evaluated the relationship between lipids and bacterial community structure and the impact of varied organic substrates on microbial lipid synthesis. Differential responses to organic matter sources were observed, with ice algae supporting both higher bacterial growth and production than terrestrial-derived peat. In spite of disparate growth kinetics, the community composition remained similar in all amended incubations as was confirmed by automated ribosomal intergenic spacer analysis (ARISA). Gammaproteobacteria dominated the initial incubations, whereas in extended incubations with terrestrial peat Alphaproteobacteria dominated; in particular Sulfitobacter phylotypes closely related (>99%) to an Arctic sea-ice-associated member of the Roseobacter clade (ARK10278). Arctic bacterioplankton preferentially synthesized two phospholipids, phosphatidylethanolamine (PE) and phosphatidylglygerol (PG), with 18:0n, 18:1Δ11, 16:0n and 16:1Δ9 as the primary fatty acids. Overall, results show that organic matter source plays an important role in structuring bacterioplankton community composition, with similar IPL and fatty acid lipid distributions observed among phylogenetically distinct bacteria.