Differing growth responses of major phylogenetic groups of marine bacteria to natural phytoplankton blooms in the western North Pacific Ocean

Growth and productivity of phytoplankton substantially change organic matter characteristics, which affect bacterial abundance, productivity, and community structure in aquatic ecosystems. We analyzed bacterial community structures and measured activities inside and outside phytoplankton blooms in the western North Pacific Ocean by using bromodeoxyuridine immunocytochemistry and fluorescence in situ hybridization (BIC-FISH). Roseobacter/Rhodobacter, SAR11, Betaproteobacteria, Alteromonas, SAR86, and Bacteroidetes responded differently to changes in organic matter supply. Roseobacter/Rhodobacter bacteria remained widespread, active, and proliferating despite large fluctuations in organic matter and chlorophyll a (Chl-a) concentrations. The relative contribution of Bacteroidetes to total bacterial production was consistently high. Furthermore, we documented the unexpectedly large contribution of Alteromonas to total bacterial production in the bloom. Bacterial abundance, productivity, and growth potential (the proportion of growing cells in a population) were significantly correlated with Chl-a and particulate organic carbon concentrations. Canonical correspondence analysis showed that organic matter supply was critical for determining bacterial community structures. The growth potential of each bacterial group as a function of Chl-a concentration showed a bell-shaped distribution, indicating an optimal organic matter concentration to promote growth. The growth of Alteromonas and Betaproteobacteria was especially strongly correlated with organic matter supply. These data elucidate the distinctive ecological role of major bacterial taxa in organic matter cycling during open ocean phytoplankton blooms.     

Changes in bacterioplankton composition under different phytoplankton regimens

The results of empirical studies have revealed links between phytoplankton and bacterioplankton, such as the frequent correlation between chlorophyll a and bulk bacterial abundance and production. Nevertheless, little is known about possible links at the level of specific taxonomic groups. To investigate this issue, seawater microcosm experiments were performed in the northwestern Mediterranean Sea. Turbulence was used as a noninvasive means to induce phytoplankton blooms dominated by different algae. Microcosms exposed to turbulence became dominated by diatoms, while small phytoflagellates gained importance under still conditions. Denaturing gradient gel electrophoresis (DGGE) of 16S rRNA gene fragments showed that changes in phytoplankton community composition were followed by shifts in bacterioplankton community composition, both as changes in the presence or absence of distinct bacterial phylotypes and as differences in the relative abundance of ubiquitous phylotypes. Sequencing of DGGE bands showed that four Roseobacter phylotypes were present in all microcosms. The microcosms with a higher proportion of phytoflagellates were characterized by four phylotypes of the Bacteroidetes phylum: two affiliated with the family Cryomorphaceae and two with the family Flavobacteriaceae. Two other Flavobacteriaceae phylotypes were characteristic of the diatom-dominated microcosms, together with one Alphaproteobacteria phylotype (Roseobacter) and one Gammaproteobacteria phylotype (Methylophaga). Phylogenetic analyses of published Bacteroidetes 16S rRNA gene sequences confirmed that members of the Flavobacteriaceae are remarkably responsive to phytoplankton blooms, indicating these bacteria could be particularly important in the processing of organic matter during such events. Our data suggest that quantitative and qualitative differences in phytoplankton species composition may lead to pronounced differences in bacterioplankton species composition.     

Dynamics of the hydrocarbon-degrading Cycloclasticus bacteria during mesocosm-simulated oil spills

We used catalysed reported deposition - fluorescence in situ hybridization (CARD-FISH) to analyse changes in the abundance of the bacterial groups Alphaproteobacteria, Gammaproteobacteria and Bacteroidetes, and of hydrocarbon-degrading Cycloclasticus bacteria in mesocosms that had received polycyclic aromatic hydrocarbons (PAHs) additions. The effects of PAHs were assessed under four contrasting hydrographic conditions in the coastal upwelling system of the Rías Baixas: winter mixing, spring bloom, summer stratification and autumn upwelling. We used realistic additions of water soluble PAHs (approximately 20-30 microg l(-1) equivalent of chrysene), but during the winter period we also investigated the effect of higher PAHs concentrations (10-80 microg l(-1) chrysene) on the bacterial community using microcosms. The most significant change observed was a significant reduction (68 +/- 5%) in the relative abundance of Alphaproteobacteria. The magnitude of the response of Cycloclasticus bacteria (positive with probe CYPU829) to PAHs additions varied depending on the initial environmental conditions, and on the initial concentration of added PAHs. Our results clearly show that bacteria of the Cycloclasticus group play a major role in low molecular weight PAHs biodegradation in this planktonic ecosystem. Their response was stronger in colder waters, when their background abundance was also higher. During the warm periods, the response of Cycloclasticus was limited, possibly due to both, a lower bioavailability of PAHs caused by abiotic factors (solar radiation, temperature), and by inorganic nutrient limitation of bacterial growth.     

象山港网箱养殖区与非养殖区的细菌群落分布

近海集约化养殖是导致我国近海污染的主要来源之一。浮游细菌在近海生态系统的物质循环过程中发挥重要作用,研究海洋浮游细菌群落对养殖活动的响应,对于指示和评价海水养殖生态系统健康具有重要意义。采集了象山港网箱养殖区与非养殖区3个深度,包括表层(0.5 m)、中层(2.5 m)、底层(8.0 m)的水样,利用焦磷酸测序技术测定16S rRNA基因,研究浮游细菌的群落结构和多样性。结果表明:网箱养殖活动不仅使得附近区域水体理化性质发生改变,如化学需氧量浓度显著高于非养殖区域,而且显著地(P0.05)改变了浮游细菌的群落结构,但不同深度间群落结构和多样性的差异不显著。网箱养殖区和非养殖区中主要浮游细菌类群为α-变形菌(Alphaproteobacteria)、γ-变形菌(Gammaproteobacteria)、拟杆菌(Bacteroidetes)、放线菌(Actinobacteria)、β-变形菌(Betaproteobacteria)、ε-变形菌(Epsilonproteobacteria)和其它变形菌(Unclassified Proteobacteria),占细菌总序列数的98.64%。有些细菌类群的平均相对丰度从网箱区到非养殖区差异显著,如拟杆菌(P0.01)和放线菌(P0.05)显著降低,而γ-变形菌(P0.05)显著增加。相似度分析表明γ-变形菌、α-变形菌和拟杆菌是造成网箱区和非养殖区群落差异的主要类群,对群落差异总贡献率达到45.02%。偏冗余分析表明,影响细菌群落分布的主要环境因子有化学需氧量、磷酸盐、铵盐和总有机碳,共解释38.18%的群落变异,空间距离单独解释10.66%的群落变异。实验结果表明,养殖活动导致浮游细菌群落的改变,其中拟杆菌、放线菌和γ-变形菌的丰度显著变化,可能用于评价养殖水体的水质状况。     

Characterization of aerobic heterotrophic bacteria in cold and nutrient-poor freshwater ecosystems

Aerobic heterotrophic bacteria present in the surface water of three cold and nutrient-poor lakes in the Chilean Patagonia (Alto Reino, Las Dos Torres and Venus) were analysed for genetic similarity and metabolic diversity using 16S ribosomal DNA and the Biolog EcoPlate^TM system, respectively. Bacterial fingerprints of water samples in enriched and non-enriched nutrient broth demonstrated a >50% fingerprinting similarity between the lakes. Metabolic activity was also similar. However, the Biolog EcoPlate^TM system carbon substrates revealed functional diversity. Lake Las Dos Torres showed the most fingerprinting similarity between enriched and non-enriched cold water samples. The amounts of living and viable bacteria were also higher in this lake’s water sample, suggesting a predominance of facultative oligotrophic groups. DNA sequencing analysis demonstrated the presence of phylum Bacteroidetes in Lake Alto Reino; phyla Bacteroidetes and Gammaproteobacteria in Lake Las Dos Torres; and phyla Bacteroidetes, Alphaproteobacteria, and Gammaproteobacteria in Lake Venus. Although each lake had a unique bacterial community structure, the different bacterial groups may be performing similar metabolic functions, given the similarity in extreme environmental conditions.     

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

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

Response of Heterotrophic Planktonic Bacteria to the Zebra Mussel Invasion of the Tidal Freshwater Hudson River

Abstract Invasions of aquatic ecosystems by exotic bivalves are known to cause dramatic changes in phytoplankton and some other groups, but their effect on the microbial component is unknown. The invasion of the tidal freshwater Hudson River by the exotic zebra mussel (Dreissena polymorpha) has caused large changes in several components of the Hudson's food web. Planktonic bacteria in the tidal freshwater Hudson are a major part of the food web, and mediate important processes in the carbon budget. We used a long-term data set, spanning four years prior to the zebra mussel (ZM) invasion and four years post-invasion, to describe ZM effects on planktonic bacteria. Small and meso-scale experiments were conducted to specifically examine direct consumption of bacteria by ZM, as well as effects on protozoans. Bacterial abundances in the Hudson have increased roughly 2× since the ZM arrived, making it clear that direct consumption by Dreissena is a minor process. Experiments show that ZM do not remove bacteria from Hudson River water, but are very effective at clearing flagellated protozoans, the major predator of bacteria. The observed changes in bacterial abundance have not been accompanied by equally large changes in bacterial productivity, suggesting growth is primarily limited by carbon supply. Bacterial production has not declined despite a dramatic decline of phytoplankton, confirming previous suggestions that bacteria and phytoplankton are not strongly linked in the Hudson. As a result of the increase in bacterial abundance and removal of phytoplankton, the absolute and relative contributions of bacterial carbon to living particulate organic carbon (POC) standing stocks have increased dramatically. The maintenance of the bacterial component of the Hudson River's food web may be one mechanism whereby consumers are ``insulated' from effects of zebra mussel consumption of phytoplankton carbon. Received: 24 October 1997; Accepted: 9 February 1998     

Quantitative role of shrimp fecal bacteria in organic matter fluxes in a recirculating shrimp aquaculture system

Microorganisms play integral roles in the cycling of carbon (C) and nitrogen (N) in recirculating aquaculture systems (RAS) for fish and shellfish production. We quantified the pathways of shrimp fecal bacterial activities and their role in C- and N-flux partitioning relevant to culturing Pacific white shrimp, Penaeus (Litopenaeus) vannamei, in RAS. Freshly produced feces from P. vannamei contained 0.6-7 × 10(10) bacteria g(-1) dry wt belonging to Bacteroidetes (7%), Alphaproteobacteria (4%), and, within the Gammaproteobacteria, almost exclusively to the genus Vibrio (61%). Because of partial disintegration of the feces (up to 27% within 12 h), the experimental seawater became inoculated with fecal bacteria. Bacteria grew rapidly in the feces and in the seawater, and exhibited high levels of aminopeptidase, chitinase, chitobiase, alkaline phosphatase, α- and β-glucosidase, and lipase activities. Moreover, fecal bacteria enriched the protein content of the feces within 12 h, potentially enriching the feces for the coprophagous shrimp. The bacterial turnover time was much faster in feces (1-10 h) than in mature RAS water (350 h). Thus, shrimp fecal bacteria not only inoculate RAS water but also contribute to bacterial abundance and productivity, and regulate system processes important for shrimp health.     

Structure and activity of bacterial community inhabiting rice roots and the rhizosphere

Root-derived carbon provides a major source for microbial production and emission of CH4 from rice field soils. Therefore, we characterized the structure and activity of the bacterial community inhabiting rice roots and the rhizosphere. In the first experiment, DNA retrieved from rice roots was analysed for bacterial 16S rRNA genes using cloning, sequencing and in situ hybridization. In the second experiment, rice plants were pulse-labelled with 13CO2 (99% of atom 13C) for 7 days, and the bacterial RNA was isolated from rhizosphere soil and subjected to density gradient centrifugation. RNA samples from density fractions were analysed by terminal restriction fragment length polymorphism fingerprinting, cloning and sequencing. The experiments showed that the dominant bacteria inhabiting rice roots and the rhizosphere particularly belonged to the Alphaproteobacteria, Betaproteobacteria and Firmicutes. The RNA stable isotope probing revealed that the bacteria actively assimilating C derived from the pulse-labelled rice plants were Azospirillum spp. (Alphaproteobacteria) and members of Burkholderiaceae (Betaproteobacteria). Both anaerobic (e.g. Clostridia) and aerobic (e.g. Comamonas) degraders were present at high abundance, indicating that root environments and degradation processes were highly heterogeneous. The relative importance of iron and sulfate reducers suggested that cycling of iron and sulfur is active in the rhizosphere.     

Bacterial diversity in the sediment from polymetallic nodule fields of the Clarion-Clipperton Fracture Zone

The Clarion-Clipperton Fracture Zone (CCFZ) is located in the northeastern equatorial Pacific and contains abundant polymetallic nodules. To investigate its bacterial diversity, four libraries of 16S rRNA genes were constructed from sediments of four stations in different areas of the CCFZ. In total, 313 clones sequenced from the 4 libraries were assigned into 14 phylogenetic groups and 1 group of 28 unclassified bacteria. High bacterial diversity was predicted by the rarefaction analysis. The most dominant group overall was Proteobacteria, but there was variation in each library: Gammaproteobacteria was the most dominant group in two libraries, E2005-01 and ES0502, while Alphaproteobacteria and Deltaproteobacteria were the most dominant groups in libraries EP2005-03 and WS0505, respectively. Seven groups, including Alphaproteobacteria, Gammaproteobacteria, Deltaproteobacteria, Betaproteobacteria, Acidobacteria, Actinobacteria, and Bacteroidetes, were common to all four libraries. The remaining minor groups were distributed in libraries with different patterns. Most clones sequenced in this study were clustered with uncultured bacteria obtained from the environment, such as the ocean crust and marine sediment, but only distantly related to isolates. Bacteria involved in the cycling of metals, sulfur and nitrogen were detected, and their relationship with their habitat was discussed. This study sheds light on the bacterial communities associated with polymetallic nodules in the CCFZ and provides primary data on the bacterial diversity of this area.     

Pelagic Bacteria–Particle Interactions and Community-Specific Growth Rates in Four Lakes along a Trophic Gradient

Abstract The relationships between bacterial concentration, bacterial production, and cell-specific activity of both free and attached bacteria and environmental factors such as suspended solids, nutrients, and temperature were examined in four lakes, two in New Zealand and two in Switzerland. Estimates of cell-specific production were obtained by microautoradiographic counts of [³H]thymidine-labeled cells. Bacteria attached to particles accounted for only 1.3 to 11.6% of the total bacterial abundance, but showed overall 20-fold higher specific growth rates and were relatively more active than their free counterparts. On average, 80 to 100% of epibacteria were attached to organic particles. The abundance and production of free and attached bacteria were positively correlated; however, relationships between these fractions and some environmental variables differed. Cell-specific activities of active bacteria were not equivalent to mean cellular activities of the entire bacterial community and differed in their relationship to trophic state. [³H]Thymidine-positive bacteria were more tightly linked to chlorophyll a than were total bacteria. Our findings indicate that production by attached bacteria, fueled by phytoplankton carbon, supplies ``new' free bacteria to the bacterial community. Our results support the idea that particulate organic matter acts as a source of dissolved nutrients to free bacteria. Bottom-up control of bacterial biomass, as shown by regressions of biomass vs production, appeared to be stronger in two ultraoligotrophic lakes than in two more eutrophic ones. Received: 17 April 1998; Accepted: 24 August 1998     

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.     

Host-Specificity and Dynamics in Bacterial Communities Associated with Bloom-Forming Freshwater Phytoplankton

Many freshwater phytoplankton species have the potential to form transient nuisance blooms that affect water quality and other aquatic biota. Heterotrophic bacteria can influence such blooms via nutrient regeneration but also via antagonism and other biotic interactions. We studied the composition of bacterial communities associated with three bloom-forming freshwater phytoplankton species, the diatom Aulacoseira granulata and the cyanobacteria Microcystis aeruginosa and Cylindrospermopsis raciborskii. Experimental cultures incubated with and without lake bacteria were sampled in three different growth phases and bacterial community composition was assessed by 454-Pyrosequencing of 16S rRNA gene amplicons. Betaproteobacteria were dominant in all cultures inoculated with lake bacteria, but decreased during the experiment. In contrast, Alphaproteobacteria, which made up the second most abundant class of bacteria, increased overall during the course of the experiment. Other bacterial classes responded in contrasting ways to the experimental incubations causing significantly different bacterial communities to develop in response to host phytoplankton species, growth phase and between attached and free-living fractions. Differences in bacterial community composition between cyanobacteria and diatom cultures were greater than between the two cyanobacteria. Despite the significance, major differences between phytoplankton cultures were in the proportion of the OTUs rather than in the absence or presence of specific taxa. Different phytoplankton species favoring different bacterial communities may have important consequences for the fate of organic matter in systems where these bloom forming species occur. The dynamics and development of transient blooms may also be affected as bacterial communities seem to influence phytoplankton species growth in contrasting ways.     

Bacterial communities in the sediments of Dianchi Lake, a partitioned eutrophic waterbody in China

Bacteria play an important role in the decomposition and cycling of a variety of compounds in freshwater aquatic environments, particularly nutrient-rich eutrophic lakes. A unique Chinese eutrophic lake--Dianchi--was selected for study because it has two separate and distinct basins, Caohai with higher organic carbon levels and Waihai with lower organic carbon levels. Sediment bacterial communities were studied in the two basins using samples collected in each season from June 2010 to March 2011. Barcoded pyrosequencing based on the 16 S rRNA gene found that certain common phyla, Proteobacteria, Bacteroidetes, Firmicutes and Chloroflexi, were dominant in the sediments from both basins. However, from the class to genus level, the dominant bacterial groups found in the sediments were distinct between the two basins. Correlation analysis revealed that, among the environmental parameters examined, total organic carbon (TOC) accounted for the greatest proportion of variability in bacterial community. Interestingly, study results suggest that increasing allochthonous organic carbon could enhance bacterial diversity and biomass in the sediment. In addition, analysis of function genes (amoA and nosZ) demonstrated that ammonia-oxidizing bacteria (AOB) were dominant in sediments, with 99% belonging to Nitrosomonas. Denitrifying bacteria were comparatively diverse and were associated with some cultivatable bacteria.     

微生物降解芘过程中的关键细菌

【目的】探究在渤海沉积物中参与降解芘的关键细菌及他们之间潜在的相互关系。【方法】构建以芘为唯一碳源的微宇宙培养体系驯化来自渤海的表层沉积物,借助Illumina Hiseq 2500获取驯化过程中的细菌群落组成,基于CCLasso算法及相对丰度数据预测细菌之间的相互作用关系以构建微生物生态网络。【结果】30 d后芘的降解率为(67.07±2.37)%,细菌群落结构也发生了明显改变:Alphaproteobacteria、Flavobacteriia、Planctomycetia等的相对丰度明显增加,而Deltaproteobacteria、Anaerolineae及Spirochaetes等则明显减少。本研究获得一个由29个点143条边构成的微生物生态网络。分类已知的属中,Erythrobacter及Planctomyces等拥有较高的点度中心度。较强的互作关系发生在Erythrobacter与Flavobacteriaceae、Alphaproteobacteria中的未知属之间。【结论】在芘的微生物降解过程中,关键细菌之间存在紧密互作。Erythrobacter为关键细菌的代表属。     

Springtime dynamics,productivity and activity of prokaryotes in two Arctic fjords

In the Kongsfjorden–Krossfjorden system (Spitsbergen), increasing temperatures enhance glacier melting and concomitant intrusion of freshwater. These altered conditions affect the timing, intensity, and composition of the phytoplankton spring bloom in Kongsfjorden; yet, the effects on prokaryotes (bacteria and archaea) are not well understood. The aim of this study was to examine springtime prokaryote communities in both fjords as a function of hydrographic and phytoplankton variability. Prokaryote community composition was studied in two consecutive years by molecular fingerprinting of the 16S rRNA gene. In addition, we measured bacterial abundance, productivity (³H-Leucine uptake), and single-cell activity using catalyzed reporter deposition fluorescence in situ hybridization combined with microautoradiography. Differences in bacterial and archaeal communities were found between Kongsfjorden and Krossfjorden. Furthermore, an increase in productivity, abundance, and proportion of active bacterial cells was observed during the course of spring. Bacteroidetes were the most abundant bacterial group among the assessed taxa in both Kongsfjorden and Krossfjorden. Multivariate analysis of the microbial community fingerprints revealed a strong temporal shaping of both the bacterial and archaeal communities in addition to a spatial separation between the two fjords. A significant part of the observed bacterial variation could be explained by cyanobacterial biomass, as deduced from pigment analysis, and by phosphate concentration. Archaea were mainly controlled by abiotic factors. We speculate that the bacterial response to hydrographic changes and glacier meltwater is mediated through shifts in phytoplankton abundance and composition, whereas archaea are directly influenced by abiotic environmental variables.     

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.     

Utilization of Organic Nitrogen Sources by Two Phytoplankton Species and a Bacterial Isolate in Pure and Mixed Cultures

Algal production of dissolved organic carbon and the regeneration of nutrients from dissolved organic carbon by bacteria are important aspects of nutrient cycling in the sea, especially when inorganic nitrogen is limiting. Dissolved free amino acids are a major carbon source for bacteria and can be used by phytoplankton as a nitrogen source. We examined the interactions between the phytoplankton species Emiliania huxleyi and Thalassiosira pseudonana and a bacterial isolate from the North Sea. The organisms were cultured with eight different amino acids and a protein as the only nitrogen sources, in pure and mixed cultures. Of the two algae, only E. huxleyi was able to grow on amino acids. The bacterium MD1 used all substrates supplied, except serine. During growth of MD1 in pure culture, ammonium accumulated in the medium. Contrary to the expectation, the percentage of ammonium regenerated from the amino acids taken up showed no correlation with the substrate C/N ratio. In mixed culture, the algae grew well in those cultures in which the bacteria grew well. The bacterial yields (cell number) were also higher in mixed culture than in pure culture. In the cultures of MD1 and T. pseudonana, the increase in bacterial yield (number of cells) over that of the pure culture was comparable to the bacterial yield in mixed culture on a mineral medium. This result suggests that T. pseudonana excreted a more-or-less-constant amount of carbon. The bacterial yields in mixed cultures with E. huxleyi showed a smaller and less consistent difference than those of the pure cultures of MD1. It is possible that the ability of E. huxleyi to use amino acids influenced the bacterial yield. The results suggest that interactions between algae and bacteria influence the regeneration of nitrogen from organic carbon and that this influence differs from one species to another.     

Homogenisation of water and sediment bacterial communities in a shallow lake (lake Balihe,China)

1. Planktonic and benthic bacterial communities hold central roles in the functioning of freshwater ecosystems and mediate key ecosystem services such as primary production and nutrient remineralisation. Although it is clear that such communities vary in composition both within and between lakes, the environmental factors and processes shaping the diversity and composition of freshwater bacteria are still not fully understood.
2. In order to assess seasonal and spatial variability in lake bacterial communities and identify environmental factors underpinning biogeographical patterns, we performed a large-scale sampling campaign with paired water and sediment sample collection at 18 locations during four seasons in Lake Balihe, a subtropical shallow fish-farming lake in mid-eastern China.
3. Pelagic and benthic bacterial communities were distinctly different in terms of diversity, taxonomic composition and community structure, with Actinobacteria, Bacteroidetes, Cyanobacteria and Alphaproteobacteria dominating lake water, and Acidobacteria, Bacteroidetes, Chloroflexi, Gammaproteobacteria and Deltaproteobacteria dominating sediment. Nevertheless, these two communities had stronger spatial concordance and overlap in taxa during spring and autumn seasons. Together, the main drivers of both the spatial and temporal variations in Lake Balihe bacterial communities were identified as water temperature, turbidity, nitrogen and phosphorus availability, and thermal stratification controlled by wind-mixing and activity of the dense farmed fish populations. Notably, populations affiliated with Firmicutes, known to be abundant in fish gut microbiome, were especially abundant in the summer season and locations where high fish biomass was found, suggesting a potential link between fish gut microbiome and the pelagic bacterial communities.
4. Our findings demonstrated seasonal homogenisation of pelagic and benthic bacterial communities linked to marked shifts in a set of seasonally-driven environmental variables including water temperature and nutrient availability.