Heterotrophic Bacterioplankton Research Articles

Evolution of Divergent Life History Strategies in Marine Alphaproteobacteria

ABSTRACTMarine bacteria in the Roseobacter and SAR11 lineages successfully exploit the ocean habitat, together accounting for ~40% of bacteria in surface waters, yet have divergent life histories that exemplify patch-adapted versus free-living ecological roles. Here, we use a phylogenetic birth-and-death model to understand how genome content supporting different life history strategies evolved in these related alphaproteobacterial taxa, showing that the streamlined genomes of free-living SAR11 were gradually downsized from a common ancestral genome only slightly larger than the extant members (~2,000 genes), while the larger and variably sized genomes of roseobacters evolved along dynamic pathways from a sizeable common ancestor (~8,000 genes). Genome changes in the SAR11 lineage occurred gradually over ~800 million years, whereas Roseobacter genomes underwent more substantial modifications, including major periods of expansion, over ~260 million years. The timing of the first Roseobacter genome expansion was coincident with the predicted radiation of modern marine eukaryotic phytoplankton of sufficient size to create nutrient-enriched microzones and is consistent with present-day ecological associations between these microbial groups. We suggest that diversification of red-lineage phytoplankton is an important driver of divergent life history strategies among the heterotrophic bacterioplankton taxa that dominate the present-day ocean.

Heterotrophic organisms and viruses in the Oka River and Cheboksary Reservoir during the abnormally hot summer of 2010

In July 2010, abnormally high water temperature (25-29 degrees C), as well as increased biomass and phytoplankton production caused intensive development of heterotrophic bacteria and heterotrophic nanoflagellates. It was found that the abundance, biomass, and production of heterotrophic bacterioplankton, as well as the abundance and biomass of heterotrophic nanoflagellates, and the number of planktonic viruses, which were calculated on average for the reservoir under study, turned out to be higher in the years with lower water temperature (20-23 degrees C). The virus-induced mortality of bacterioplankton in the Oka River and the Cheboksary Reservoir averaged 25.4 +/- 3.4 and 22.4 +/- 2.7% of the daily bacterioplankton production.

Away from darkness: a review on the effects of solar radiation on heterotrophic bacterioplankton activity.

Heterotrophic bacterioplankton are main consumers of dissolved organic matter (OM) in aquatic ecosystems, including the sunlit upper layers of the ocean and freshwater bodies. Their well-known sensitivity to ultraviolet radiation (UVR), together with some recently discovered mechanisms bacteria have evolved to benefit from photosynthetically available radiation (PAR), suggest that natural sunlight plays a relevant, yet difficult to predict role in modulating bacterial biogeochemical functions in aquatic ecosystems. Three decades of experimental work assessing the effects of sunlight on natural bacterial heterotrophic activity reveal responses ranging from high stimulation to total inhibition. In this review, we compile the existing studies on the topic and discuss the potential causes underlying these contrasting results, with special emphasis on the largely overlooked influences of the community composition and the previous light exposure conditions, as well as the different temporal and spatial scales at which exposure to solar radiation fluctuates. These intricate sunlight-bacteria interactions have implications for our understanding of carbon fluxes in aquatic systems, yet further research is necessary before we can accurately evaluate or predict the consequences of increasing surface UVR levels associated with global change.

Biogeochemistry of organic carbon, CO2, CH4, and trace elements in thermokarst water bodies in discontinuous permafrost zones of Western Siberia

Active processes of permafrost thaw in Western Siberia increase the number of soil subsidencies, thermokarst lakes and thaw ponds. In continuous permafrost zones, this process promotes soil carbon mobilisation to water reservoirs, as well as organic matter (OM) biodegradation, which produces a permanent flux of carbon dioxide (CO2) to the atmosphere. At the same time, the biogeochemical evolution of aquatic ecosystems situated in the transition zone between continuous permafrost and permafrost-free terrain remains poorly known. In order to better understand the biogeochemical processes that occur in thaw ponds and lakes located in discontinuous permafrost zones, we studied ~30 small (1–100,000 m2) shallow (<1 m depth) lakes and ponds formed as a result of permafrost subsidence and thaw of the palsa bog located in the transition zone between the tundra and forest-tundra (central part of Western Siberia). There is a significant increase in dissolved CO2 and methane (CH4) concentration with decreasing water body surface area, with the largest supersaturation with respect to atmospheric CO2 and CH4 in small (<100 m2) permafrost depressions filled with thaw water. Dissolved organic carbon (DOC), conductivity, and metal concentrations also progressively increase from large lakes to thaw ponds and depressions. As such, small water bodies with surface areas of 1–100 m2 that are not accounted for in the existing lake and pond databases may significantly contribute to CO2 and CH4 fluxes to the atmosphere, as well as to the stocks of dissolved trace elements and organic carbon. In situ lake water incubation experiments yielded negligible primary productivity but significant oxygen consumption linked to the mineralisation rate of dissolved OM by heterotrophic bacterioplankton, which produce a net CO2 flux to the atmosphere of 5 ± 2.5 mol C m2 year−1. The most significant result of this study, which has long-term consequences on our prediction of aquatic ecosystem development in the course of permafrost degradation is CO2, CH4, and DOC concentrations increase with decreasing lake age and size. As a consequence, upon future permafrost thaw, the increase in the number of small water bodies, accompanied by the drainage of large thermokarst lakes to the hydrological network, will likely favour (i) the increase of DOC and colloidal metal stocks in surface aquatic systems, and (ii) the enhancement of CO2 and CH4 fluxes from the water surface to the atmosphere. According to a conservative estimation that considers that the total area occupied by water bodies in Western Siberia will not change, this increase in stocks and fluxes could be as high as a factor of ten.

Expression patterns reveal niche diversification in a marine microbial assemblage

Resolving the ecological niches of coexisting marine microbial taxa is challenging due to the high species richness of microbial communities and the apparent functional redundancy in bacterial genomes and metagenomes. Here, we generated over 11 million Illumina reads of protein-encoding transcripts collected from well-mixed southeastern US coastal waters to characterize gene expression patterns distinguishing the ecological roles of hundreds of microbial taxa sharing the same environment. The taxa with highest in situ growth rates (based on relative abundance of ribosomal protein transcripts) were typically not the greatest contributors to community transcription, suggesting strong top-down ecological control, and their diverse transcriptomes indicated roles as metabolic generalists. The taxa with low in situ growth rates typically had low diversity transcriptomes dominated by specialized metabolisms. By identifying protein-encoding genes with atypically high expression for their level of conservation, unique functional roles of community members emerged related to substrate use (such as complex carbohydrates, fatty acids, methanesulfonate, taurine, tartrate, ectoine), alternative energy-conservation strategies (proteorhodopsin, AAnP, V-type pyrophosphatases, sulfur oxidation, hydrogen oxidation) and mechanisms for negotiating a heterogeneous environment (flagellar motility, gliding motility, adhesion strategies). On average, the heterotrophic bacterioplankton dedicated 7% of their transcriptomes to obtaining energy by non-heterotrophic means. This deep sequencing of a coastal bacterioplankton transcriptome provides the most highly resolved view of bacterioplankton niche dimensions yet available, uncovering a spectrum of unrecognized ecological strategies.

Sunlight effects on the DMSP-sulfur and leucine assimilation activities of polar heterotrophic bacterioplankton

The influence of solar ultraviolet radiation and photosynthetically active radiation (PAR) on summertime marine bacterial uptake and assimilation of sulfur from radiolabeled dimethlysulfoniopropionate (35S-DMSP) was studied at four Arctic and two Antarctic stations. Incubations with 3H-leucine were also conducted for comparative purposes as a measurement of bacterial activity. Arctic waters were characterized by large numbers of colonial Phaeocystis pouchetii and higher DMSP concentrations than in the two diatom-dominated Antarctic samples. Exposure to full sunlight radiation (280–700 nm), and to a lesser extent to PAR + UVA (320–700 nm), generally decreased the bacterial assimilation of 3H-leucine with respect to darkness, and caused variable effects on 35S-DMSP assimilation. By using a single-cell approach involving microautoradiography we found high percentages of sulfur assimilating cells within the bacterial groups Gammaproteobacteria, Bacteroidetes, SAR11 and Roseobacter despite the varying DMSP concentrations between Arctic and Antarctic samples. The dominant SAR11 clade contributed 50–70% of the cells assimilating both substrates in the Arctic stations, whereas either Gammaproteobacteria or SAR11 were the largest contributors to 3H-leucine uptake in samples from the two Antarctic stations. Only one station was analyzed for single-cell 35S-DMSP assimilation in Antarctica, and Gammaproteobacteria were major contributors to its uptake, providing the first evidence for Antarctic bacteria actively taking up 35S-DMSP. PAR + UVA repeatedly increased the number of SAR11 cells assimilating 3H-leucine. This pattern also occurred with other 35S-DMSP assimilating groups, though not so consistently. Our results support a widespread capability of polar bacteria to assimilate DMSP-sulfur during the season of maximum DMSP concentrations, and show for the first time that all major polar taxa can be highly active at this assimilation under the appropriate circumstances. Our findings further confirm the role of sunlight as a modulator of heterotrophic carbon and sulfur fluxes in the surface ocean.

Do human activities affect the picoplankton structure of the Ahe atoll lagoon (Tuamotu Archipelago, French Polynesia)?

The spatial variations of the picoplankton (photoautotrophic and heterotrophic microorganisms) in the Ahe atoll lagoon were studied in May and October 2008 to assess whether they were affected by human activities along the atoll. Spatial patterns were studied using 10 sampling stations chosen according to the location of the anthropogenic activities (pearl farming, harbor). Experiments were also carried out to determine whether bacterial growth, with or without predators, was limited by inorganic (N and P) substrates. The results showed that heterotrophic bacterioplankton abundance was superior to the photoautotrophic organisms, especially in May. Significant increases in bacterial abundance were observed in May after 24h incubation with +P and +N (but not in October). All samples complied with the quality levels for fecal indicator bacteria (FIB) defined by the European Union and there was no evidence that human sewage had any impact on picoplankton over the whole atoll.

全球CO2水平升高对浮游植物生理和生态影响的研究进展

工业革命以来由于化石燃料的大量燃烧,大气CO₂水平不断增加,预计在21世纪末将增至现有水平的两倍,达到750 μL/L。作为全球初级生产力的重要贡献者,浮游植物应对CO₂水平升高的生理生态响应必然会对水生生态系统和碳、氮等元素的生物地球化学循环产生重要影响。全球CO₂水平的升高将显著改变水体的碳化学环境,淡水生态系统(湖泊和河流)由于容量小变化比海洋更为显著。水体碳化学环境的改变首先会影响浮游植物个体,在高CO₂水平下,浮游植物的细胞会有变小的趋势,并且细胞的光合作用强度也会有不同程度的增加,其中细胞较小或者不具有碳浓缩机制(CCM)的浮游植物增加较多,此外浮游植物细胞的化学元素计量值也将显著改变。随后浮游植物个体水平上的变化会进一步影响水生生态系统,例如水体初级生产力水平的提高,浮游植物、浮游动物群落结构组成以及水体微食物网结构的变化等。此外浮游植物对CO₂水平升高的生理生态响应程度还与水体的营养水平有关。总结了大气CO₂水平升高对浮游植物生理生态影响的研究方法,展望了未来可能的研究方向。;With the increase of global carbon emissions since the industrial revolution, atmospheric CO₂ level is expected to be twice higher than the present level at the end of this century, reaching to 750 μL/L. As atmospheric CO₂ is the most important source of soluble CO₂ in water due to the deliberate CO₂ sequestration, global CO₂ elevation would significantly alter carbonate chemical environments of aquatic ecosystems. When atmospheric CO₂ concentration doubles at the end of century, it is predicted that pH value of surface oceans would drop approximately 0.3 units, the concentration of bicarbonate ion would increase by 6%, and carbonate ion concentration would decrease by 50%. Considering the smaller buffering capacity of freshwater ecosystems than that of oceans, carbonate chemical environments of freshwater lakes and rivers may change more notably after the increase of CO₂ level. The shift of carbonate chemistry would in turn have dramatic effects on aquatic ecosystems. Phytoplankton has been used widely as an indicator of changes of aquatic ecosystems because this relatively short-lived organism responds rapidly to subtle changes. Meanwhile, it contributes approximately 50% of the total global primary productivity, which plays a vital role in global carbon cycling. Therefore, the physiological and ecological responses of phytoplankton to global CO₂ elevation would have great significance on water ecosystems and biogeochemical cycle. Elevated carbon resource could enhance the photosynthetic activity of phytoplankton, particularly small phytoplankton or non-CCM (non-carbon concentration mechanism) phytoplankton. When CO₂ level increases, the cell size of phytoplankton would potentially decrease, and the elemental ratio of carbon to nitrogen in phytoplankton cell would increase up to more than 10%. Then, those changes in the individual phytoplankton cell would cause certain variations in the ecological level. Primary productivity would rise significantly as a result of enhanced photosynthetic activity of phytoplankton. The shift in assemblage composition of phytoplankton, and the variation of elemental ratio could affect the edibility of phytoplankton, which would alter the abundances and the community structure of zooplankton. Furthermore, the accumulating concentration of organic carbon in water would promote the growth and reproduction of heterotrophic bacterioplankton, which would strengthen the competition between bacterioplankton and phytoplankton for nutrients, such as nitrogen and phosphorus. More importantly, elevated CO₂ effects differ between oligotrophic and eutrophic water body. It is expected that effects on algal productivity in eutrophic aquatic ecosystems could be potentially larger, since in eutrophic water the input of nitrogen and phosphorus to the global biochemical cycle exceeds that of the carbon input by several orders of magnitude. Finally, we summarized the development of the methodology for studying the effect of enhanced CO₂ level on water ecosystems. Long-term in situ simulating experiment would be the best approach for studying the effect of enhanced CO₂ on natural phytoplankton community. In future research, picophytoplankton will attract more attention, since they are sensitive to the change of CO₂ level compared with large ones because of their high surface-area-to-volume ratio. In addition, more efforts should be exerted for freshwater ecosystems with respect to elevated CO₂ effects, as they are far less investigated than marine ecosystems even though they are closely linked to the survival of human being.

Systematic removal of neutral sugars within dissolved organic matter across ocean basins

Received 23 June 2011; revised 2 August 2011; accepted 3 August 2011; published 7 September 2011. [1] Dissolved combined neutral sugars (DCNS) support heterotrophic bacterioplankton metabolism throughout the ocean, which affects ocean carbon cycling and biogeochemistry. Variability in DCNS composition also provides information about the diagenetic state of oceanic dissolved organic matter (DOM). Here, we present results of the DCNS composition in ∼600 discrete samples from ocean basin‐scale sections within the North Atlantic and South Pacific Oceans; and at the Bermuda Atlantic Time‐series Study site in the Sargasso Sea. As DCNS concentrations decline with water mass age the mole percentages of glucose, mannose + xylose, and galactose change in a ratio of +2.10:−1.10:−1.00 enriching the DOM pool in glucose relative to mannose + xylose, and galactose. A new proxy is presented based on the relative change in these major sugars, diagenetic distance, which allows for comparison of the diagenetic state of DOM over broad regions of the global ocean while simultaneously quantifying progress along this pathway. In all, this inter‐ basin comparison suggests that there is a common diagenetic pathway for oceanic DOM. Citation: Goldberg, S. J., C. A. Carlson, M. Brzezinski, N. B. Nelson, and D. A. Siegel (2011), Systematic removal of neutral sugars within dissolved organic matter across ocean basins, Geophys. Res. Lett., 38, L17606,

The utilization of polysaccharides by heterotrophic bacterioplankton in the Bay of Biscay (North Atlantic Ocean)

This study investigates the turnover of polysaccharides by heterotrophic bacterioplankton in the northern Bay of Biscay, a productive marine system on the continental margin of the temperate Atlantic Ocean. Bacterial biomass production (BBP) near the surface ranged from 0.5 to 25.7 nmol C L−1 h−1 during small phytoplankton blooms in May and June that occurred after the main spring bloom. A direct relationship between BBP and total polysaccharides strongly suggests the dependence of bacterial growth on the availability of semi-labile organic matter. Concentrations of combined glucose as well as rate constants of extracellular glucosidase activity and glucose uptake were determined to estimate the actual carbon fluxes from bacterial polysaccharide turnover. Results reveal that the degradation of polysaccharides in the upper 100 m of the water column sustained a glucose flux of 15.2–32.3 mg C m−2 d−1 that was available for bacterial consumption. The mean turnover time for polysaccharides was 170 and 165 days for α- and β-glycosidic linked polymers, respectively. Incorporation of free glucose supported 0.4–19.6% of BBP. The availability of nitrate plus nitrite (NOx) was identified as one factor increasing bacterial incorporation of glucose in most samples. Our results demonstrate that the bacterial recycling of polysaccharides generated a significant flux of organic carbon in microbial food-webs and biogeochemical processes.

Ocean time-series reveals recurring seasonal patterns of virioplankton dynamics in the northwestern Sargasso Sea

There are an estimated 10(30) virioplankton in the world oceans, the majority of which are phages (viruses that infect bacteria). Marine phages encompass enormous genetic diversity, affect biogeochemical cycling of elements, and partially control aspects of prokaryotic production and diversity. Despite their importance, there is a paucity of data describing virioplankton distributions over time and depth in oceanic systems. A decade of high-resolution time-series data collected from the upper 300 m in the northwestern Sargasso Sea revealed recurring temporal and vertical patterns of virioplankton abundance in unprecedented detail. An annual virioplankton maximum developed between 60 and 100 m during periods of summer stratification and eroded during winter convective mixing. The timing and vertical positioning of this seasonal pattern was related to variability in water column stability and the dynamics of specific picophytoplankton and heterotrophic bacterioplankton lineages. Between 60 and 100 m, virioplankton abundance was negatively correlated to the dominant heterotrophic bacterioplankton lineage SAR11, as well as the less abundant picophytoplankton, Synechococcus. In contrast, virioplankton abundance was positively correlated to the dominant picophytoplankton lineage Prochlorococcus, and the less abundant alpha-proteobacteria, Rhodobacteraceae. Seasonally, virioplankton abundances were highly synchronous with Prochlorococcus distributions and the virioplankton to Prochlorococcus ratio remained remarkably constant during periods of water column stratification. The data suggest that a significant fraction of viruses in the mid-euphotic zone of the subtropical gyres may be cyanophages and patterns in their abundance are largely determined by Prochlorococcus dynamics in response to water column stability. This high-resolution, decadal survey of virioplankton abundance provides insight into the possible controls of virioplankton dynamics in the open ocean.

Wind forcing and short-term variability of phytoplankton and heterotrophic bacterioplankton in the coastal zone of the Concepción upwelling system (Central Chile)

Along South-Central Chile, upwelling-favorable winds do not blow steadily equatorward, and may remain calm or even reverse for periods of 2–8 days shifting the balance between water column stability and replenishment of inorganic nutrients to the photic zone. In this study, we focus on the short-term variability of phytoplankton and heterotrophic bacterioplankton associated with the temporal variability in upwelling at a single nearshore station at the entrance of Coliumo Bay, Central Chile. In situ sampling took place every other day between 24 January and 14 February 2007. Observed variability of wind, sea surface temperature, and surface chlorophyll concentrations during the preceding weeks and throughout our experiments indicated that nearshore productivity was tightly coupled to local upwelling conditions. Gross primary production remained relatively low (22.5 ± 6.1 μg C L −1 h −1) during the first 8 days (24 January–1 February), and increased six fold (142.4 ± 67.1 μg C L −1 h −1) during the second period (3–14 February). Average in situ chlorophyll concentrations increased from 2.0 ± 0.6 mg m −3 to 6.3 ± 3.8 mg m −3 over the same period. Bacterial Carbon Demand presented higher values (5–29.2 μg C L −1 h −1) during the first 6 days of sampling and lower values (<0.1 μg C L −1 h −1) for the rest of study period. Our results show that both biological productivity and the structure of the planktonic community can vary considerably due to short-term changes in wind conditions. Hence, previous productivity estimates for the Concepción upwelling ecosystem – based on observations gathered at lower frequencies – may not truly reflect its productivity potential.

Vertical and longitudinal gradients in HNA-LNA cell abundances and cytometric characteristics in the Mediterranean Sea

Abstract. Heterotrophic bacterioplankton abundance and production were investigated with depth (down to bathypelagic layers) and with longitude (from 4.9° E to 32.7° E) along a cruise track across the Mediterranean Sea in early summer 2008. Abundances and flow cytometric characteristics (green fluorescence and side scatter signals) of high nucleic acid (HNA) and low nucleic acid (LNA) bacterial cells were determined using flow cytometry. Contrary to what is generally observed, the relative importance of HNA cells, as a percent of total cells, (%HNA, range 30–69 %) was inversely related to bacterial production (range 0.15–44 ng C l−1 h−1) although the negative relation was weak (log–log regression r2=0.19). The %HNA as well as the mean side scatter of HNA group increased significantly with depth in the meso and bathypelagic layers. Vertical stratification played an important role in influencing the distribution and characteristics of bacterial cells especially with regard to layers located above, within or below the deep chlorophyll maximum. Within a given layer, the relationships between the flow cytometric characteristics and environmental variables such as chlorophyll-a, nutrients or bacterial production changed. Overall, the relationships between HNA and LNA cells and environmental parameters differed vertically more than longitudinally.

Virus impact on heterotrophic bacterioplankton of water reservoirs

The quantitative distribution of viruses and their impact on heterotrophic bacterioplankton were studied in mesotrophic and eutrophic reservoirs of the Volga and Volga-Baltic waterway. The abundance of planktonic virus particles ranged from 9.4 × 106 to 120 × 106 ml−1 and was from 2.5 to 9 times greater than the bacterial numbers. Production of virioplankton varied from 2.1 × 106 to 132 × 106 particles (ml day)−1 and the population turnover time values were between 0.3 and 11.6 days. The maximum values of numbers and production of virio- and bacterioplankton were observed in the eutrophic Ivan’kovo reservoir. Distribution of the viruses in the Volga reservoirs depended to a significant degree on the number and activity of heterotrophic bacterioplankton. The infected bacteria accounted for 5.5–33.5% of the total bacterial abundance. Phages were an important factor of bacterial mortality. During July to September virus-induced bacterial mortality varied between 6.1 and 40.6% (20.2% on average) of daily bacterioplankton production.

Impact of viruses on heterotrophic bacterioplankton and picocyanobacteria in reservoirs

In aquatic ecosystems, viruses are considered to bean important factor controlling bacterio and phytoplankton abundance, production, and diversity andplay an important role in the biogeochemical cycles ofcarbon and other components [1]. Analysis of literature on ecology of aquatic viruses shows that, atpresent, viruses in freshwaters are less studied than inmarine bodies of water [2]. Despite numerous publications on virioplankton in lakes, only scanty data can befound on virioplankton in reservoirs [3–5]. Thisrestricts our knowledge of the functioning of this widespread group of artificial water objects. We have performed the first study on the quantitative distributionof planktonic viruses and their role in the mortality ofheterotrophic bacteria and picocyanobacteria in reservoirs of the Volga–Baltic Sea basin.The studies were carried out in July–August of theyears 2005 and 2007. Planktonic viral particles werecounted by epifluorescent microscopy using the SYBRGreen 1 fluorochrome and Al

Effect of permafrost thawing on organic carbon and trace element colloidal speciation in the thermokarst lakes of western Siberia

Abstract. To examine the mechanisms of carbon mobilization and biodegradation during permafrost thawing and to establish a link between organic carbon (OC) and other chemical and microbiological parameters in forming thermokarst (thaw) lakes, we studied the biogeochemistry of OC and trace elements (TEs) in a chronosequence of small lakes that are being formed due to permafrost thawing in the northern part of western Siberia. Twenty lakes and small ponds of various sizes and ages were sampled for dissolved and colloidal organic carbon, metals and culturable heterotrophic bacterial cell number. We observed a sequence of ecosystems from peat thawing and palsa degradation due to permafrost subsidence in small ponds to large, km-size lakes that are subject to drainage to, finally, the khasyrey (drained lake) formation. There is a systematic evolution of both total dissolved and colloidal concentration of OC and TEs in the lake water along with the chronosequence of lake development that may be directly linked to the microbial mineralization of dissolved organic matter and the liberation of the inorganic components (Fe, Al, and TEs) from the organo-mineral colloids. In this chronosequence of lake development, we observed an apparent decrease in the relative proportion of low molecular weight &lt;1 kDa (1 kDa ~ 1 nm) OC concentration along with a decrease in the concentration of total dissolved (&lt;0.45 μm) OC. This decrease was accompanied by an increase in the small size organic ligands (probably autochthonous exometabolites produced by the phytoplankton) and a simultaneous decrease in the proportion of large-size organic (humic) complexes of allochthonous (soil) origin. This evolution may be due to the activity of heterotrophic bacterioplankton that use allochthonous organic matter and dissolved nutrients originating from peat lixiviation. Most insoluble TEs demonstrate a systematic decrease in concentration during filtration (5 μm, 0.45 μm) exhibiting a similar pattern among different samples. At the same time, there is an increase in the relative proportion of large size particles over the &lt;1 kDa fraction for most insoluble elements along the chronosequence of lake evolution. TEs are likely to be bound to colloidal OC and coprecipitate with the mineral (Fe, Al) part of the colloids. Upon progressive consumption of dissolved OC by the heterotrophic bacteria, there is liberation of Fe, Al, and insoluble TEs in the water column that may be subjected to coagulation in the form of particles or large-size mineral colloids.

Bacterial biomass production in an estuarine system: high variability of leucine conversion factors and changes in bacterial community structure during incubation

Heterotrophic bacterioplankton play a key role in the transfer of organic carbon from the dissolved pool into planktonic trophic webs. The accurate measurement of bacterial biomass pro- duction (BBP) is crucial for the establishment of carbon budgets in aquatic systems. Estimation of BBP from the incorporation of radiolabelled leucine into bacterial proteins requires the use of empirical, semitheoretical or theoretical conversion factors (CFs). In this study, our estimate of the BBP for the estuarine system of Ria de Aveiro was higher when we used empirical and semitheoretical CFs than when we used theoretical ones. Empirical CFs ranged between 9.26 and 29.81 kg C mol -1 in the marine zone and between 4.25 and 16.88 kg C mol -1 in the brackish zone. Semitheoretical CFs ranged from 5.06 to 9.49 kg C mol -1 in the marine zone and from 5.28 to 9.34 kg C mol -1 in the brack- ish zone. During experiments to determine empirical CFs, the structure of the bacterial community was also analyzed. Sample preparation and the addition of leucine altered the structure of the bacte- rial community, and this probably affected the determination of the empirical CFs. The results show that, besides the effect of the use of an inadequate CF, the estimates of BBP can be biased in a sys- tem-specific way by the alteration of the structure of the bacterial community as a result of sample manipulation (i.e. filtration/dilution, incubation, and addition of non-radiolabelled and radiolabelled leucine).

Linking diel patterns in community respiration to bacterioplankton in an oligotrophic high‐elevation lake

We examined the role of heterotrophic bacterioplankton in structuring diel patterns in community respiration (CR) in an oligotrophic alpine lake. We used continuous measurements of dissolved oxygen from both free‐water and in situ incubation chambers to characterize a consistent diel pattern in oxygen drawdown with two distinct phases. CR rates averaged 15.4 times higher during the late afternoon to early night phase, which lasted for up to 5 h past dusk before rapidly transitioning to the lower rate phase, which lasted through dawn. Bacterioplankton always accounted for the majority of CR (73% seasonally averaged). We examined patterns in dissolved organic carbon (DOC) and bacterial growth to determine the degree to which bacterioplankton were affecting diel variability in respiration rates. There was a diel pattern in DOC accumulation, with the magnitude of overnight drawdown of DOC (2.0 ± 0.8 μmol L−1) matching our oxygen‐based estimates of the amount of DOC required to sustain higher CR rates (2.4 ± 1.3 μmol L−1). Bacterioplankton communities cultured on dusk water had growth‐rate constants two times higher than those cultured on dawn water and removed the excess DOC in dusk water. Our results indicate that higher daytime CR of bacterioplankton is directly coupled to daily production of DOC at a timescale of hours, likely through phytoplankton exudation. Ignoring diel variability in CR may result in substantial underestimation of daily CR and gross primary production.

Relationships Between Coastal Bacterioplankton Growth Rates and Biomass Production: Comparison of Leucine and Thymidine Uptake with Single-Cell Physiological Characteristics

Specific growth rates of heterotrophic bacterioplankton have been frequently estimated from in situ bacterial production (BP) to biomass (BB) ratios, using a series of assumptions that may result in serious discrepancies with values obtained from predator-free cultures. Here, we used both types of approaches together with a comprehensive assessment of single-cell physiological characteristics (membrane integrity, nucleic acid content, and active respiration) of coastal bacterioplankton during a complete annual cycle (February 2007-January 2008) in the southern Bay of Biscay off Xixón, Spain. Both leucine and thymidine incorporation rates were used in conjunction with empirical tracer to carbon or cells conversion factors (eCFs) to accurately derive BP. Leu and TdR incorporation rates covaried year-round, as did the corresponding eCFs at 0 and 50 m depth. eCFs peaked in autumn, with mean annual values close to the theoretical ones (3.4 kg C mol Leu(-1) and 2.0 × 10(18) cells mol TdR(-1)). Bacterial abundance (0.2-1.5 × 10(6) cells L(-1)) showed a bimodal distribution with maxima in May and October and minima in March. Live (membrane-intact) cells dominated year-round (79-97%), with high nucleic acid cells (42-88%) and actively respiring bacteria (CTC+, 1-16%) showing distinct surface maxima in April and July, respectively. BB (557-1,558 mg C m(-2)) and BP (7-139 mg C m(-2) day(-1)) presented two distinct peaks in spring and autumn, both of similar size due to a strong upwelling event observed in September. Specific growth rates (0.35-3.8 day(-1)) were one order of magnitude higher in predator-free incubations than bacterial turnover rates derived from integrated BP:BB ratios (0.01-0.16 and 0.01-0.09 day(-1), for Leu and TdR, respectively) and were not correlated, probably due to a significant contribution of low activity cells to total standing stocks. The Leu:TdR molar ratio averaged for the water column (6.6-25.5) decreased significantly with higher integrated BB, indicating that low standing stocks tend to present unbalanced growth. Discrepancies about the true magnitude of specific growth rates must be solved before fully appreciating the role of bacteria in the ocean carbon cycle.

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

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