Heterotrophic Bacterial Activity Research Articles

Potential role of oxygen and inorganic nutrients on microbial carbon turnover in the Baltic Sea

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 83:95-108 (2019) - DOI: https://doi.org/10.3354/ame01902 Potential role of oxygen and inorganic nutrients on microbial carbon turnover in the Baltic Sea Marie Maßmig1,*, Judith Piontek1,2, Frédéric A. C. Le Moigne1,3, Carolina Cisternas-Novoa1, Anja Engel1 1GEOMAR Helmholtz Centre for Ocean Research, 24105 Kiel, Germany 2Present address: Leibniz Institute for Baltic Sea Research Warnemünde, 18119 Rostock, Germany 3Present address: Mediterranean Institute of Oceanography, UM 110, Aix Marseille Univ., Université 6 de Toulon, CNRS, IRD, 13288, Marseille, France *Corresponding author: mmassmig@geomar.de ABSTRACT: Oxygen (O2) deficiency and nutrient concentrations in marine systems are impacting organisms from microbes to higher trophic levels. In coastal and enclosed seas, O2 deficiency is often related to eutrophication and high degradation rates of organic matter. To investigate the impact of O2 concentration on bacterial growth and the turnover of organic matter, we conducted multifactorial batch experiments with natural microbial communities of the central Baltic Sea. Water was collected from suboxic (<5 µmol l-1) depths in the Gotland Basin during June 2015. Samples were kept for 4 d under fully oxygenated and low O2 conditions (mean: 34 µmol l-1 O2), with or without nutrient (ammonium, phosphate and nitrate) and labile carbon (glucose) amendments. We measured bacterial abundance, bacterial heterotrophic production, extracellular enzyme rates (leucine-aminopeptidase) and changes in dissolved and particulate organic carbon concentrations. Our results show that the bacterial turnover of organic matter was limited by nutrients under both oxic and low O2 conditions. In nutrient- and glucose-replete treatments, low O2 concentrations significantly reduced the net uptake of dissolved organic carbon and led to greater accumulation of more labile dissolved organic matter. Our results therewith suggest that the combined effects of eutrophication and deoxygenation on heterotrophic bacterial activity might favor the accumulation of dissolved organic carbon in the Baltic Sea. KEY WORDS: Oxygen · Heterotrophic bacteria · Baltic Sea · Organic matter · Bacterial heterotrophic production · Extracellular enzymes · Leucine-aminopeptidase Full text in pdf format Supplementary material PreviousCite this article as: Maßmig M, Piontek J, Le Moigne FAC, Cisternas-Novoa C, Engel A (2019) Potential role of oxygen and inorganic nutrients on microbial carbon turnover in the Baltic Sea. Aquat Microb Ecol 83:95-108. https://doi.org/10.3354/ame01902 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 83, No. 1. Online publication date: June 06, 2019 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2019 Inter-Research.

Differential responses of bacteria to diatom-derived dissolved organic matter in the Arctic Ocean

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 82:59-72 (2018) - DOI: https://doi.org/10.3354/ame01883 Differential responses of bacteria to diatom-derived dissolved organic matter in the Arctic Ocean Laetitia Dadaglio1, Julie Dinasquet1,2, Ingrid Obernosterer1, Fabien Joux1,* 1Sorbonne Université, CNRS, Laboratoire d’Océanographie Microbienne (LOMIC), Observatoire Océanologique de Banyuls, 66650 Banyuls/mer, France 2Present address: Marine Biological Research Division, Scripps Institution of Oceanography, UCSD, La Jolla, CA 92037, USA *Corresponding author: joux@obs-banyuls.fr ABSTRACT: The Arctic sea ice cover is undergoing an unprecedented decline due to climate change. This loss may result in the earlier start of ice algae blooms and more intense phytoplankton blooms, leading to higher concentrations of dissolved organic matter (DOM) derived from primary production. We investigated the response of early summer Arctic bacterial communities to the addition of Arctic diatom-derived DOM through biodegradation experiments in Baffin Bay. DOM produced by the planktonic diatom Chaetoceros neogracilis and the sea ice diatom Fragilariopsis cylindrus was added to seawater from 3 stations with different ice cover (2 ice zones and 1 open water zone) for 12 d. At the 3 stations, the addition of inorganic nutrients (PO4 and NO3) was not sufficient to stimulate bacterial growth compared to the controls, suggesting that bacteria were mainly limited by organic carbon. The addition of DOM from C. neogracilis stimulated bacterial abundance and production, with a more pronounced response in the ice zone compared to the open water zone. The enhanced bacterial metabolism was accompanied by changes in the bacterial community composition determined by 16S rRNA sequencing, driven by operational taxonomic units (OTUs) related to Pseudoalteromonas and Polaribacter that increased in relative abundance with DOM addition. Moreover, in the ice zone, DOM from C. neogracilis induced a priming effect on the bacterial utilization of ambient DOM. Our findings suggest that phytoplankton blooms, through the production of labile organic matter, will strongly affect bacterial heterotrophic activity, composition and dissolved organic carbon cycling in the Arctic Ocean. KEY WORDS: Arctic Ocean · Sea ice · Diatom-derived organic matter · Biodegradation · Bacterial growth · Bacterial diversity · 16S rRNA sequencing Full text in pdf format Supplementary material PreviousNextCite this article as: Dadaglio L, Dinasquet J, Obernosterer I, Joux F (2018) Differential responses of bacteria to diatom-derived dissolved organic matter in the Arctic Ocean. Aquat Microb Ecol 82:59-72. https://doi.org/10.3354/ame01883 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 82, No. 1. Online publication date: September 20, 2018 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2018 Inter-Research.

Evaluating the excess sludge reduction in activated sludge system with ultrasonic treatment.

Ultrasonic treatment for enhancing biological processes has recently attracted considerable attention in wastewater treatment. In this study, we systematically investigated the mixed liquor properties of activated sludge under ultrasonic treatment. The sludge samples were collected from the aerobic tank of a full-scale membrane bioreactor (MBR) treating municipal wastewater, and the volatile suspended solids (VSS) concentration was approximately 6.0 g/L. The results showed that ultrasonic treatment induced floc disintegration, organics release, temperature increase, microbial activity and pH variation. The maximum mg soluble chemical oxygen demand (COD) per mg VSS released was estimated to be 0.147 using the Monod equation. The exponential increase in the concentration of dissolved organic matter is related to the loss of relative heterotrophic bacterial activity. A sonolysis-cryptic growth model was demonstrated to be capable of describing ultrasonic sludge reduction, which would support the further development of ultrasonic treatment technology in activated sludge systems.

Mechanism of Action of Activated Sludge Properties in Nitrogen Removal by Endogenous Denitrification Through an Intelligent Aeration-controlled A/O Process

In order to evaluate the mechanism of action of activated sludge properties in nitrogen removal by endogenous denitrification (henceforth EDNR), a new kind of automatic oxygen supply device(AOSD), was applied to the A/O process. The domestication effect of the aeration mode on the activated sludge properties and microbial communities was investigated under the intelligent aeration-controlled A/O process (I-A/O)and the continuous aeration A/O process (C-A/O). The results demonstrated that the effluent NH4+-N and NO2--N components showed obvious accumulation efficiencies and activated sludge generated conspicuous limited bulking in the I-A/O process. Domesticated sludge in the I-A/O process was able to enrich more SCOD to transfer into the polymeric substances as Gly, under a rich exogenous carbon supply state, and stimulated nitrogen removal by endogenous denitrifying under a scarce exogenous carbon supply state. The EDNR rate went up to 0.83 mg·(L·h)-1 in the I-A/O process, which was more than that achieved by the C-A/O process. The microbial communities in the two processes were evaluated by the Illumina HiSeq high-throughput sequencing technology. The results showed that there was no obvious difference in the sludge microbial community diversity between the two processes, but the Candidate division TM7 proliferated in the I-A/O process, and become the abundant taxa to prompt limited filamentous sludge bulking and Gly storage capability enhancement. The oxygen supply mode of AOSD made the activated sludge properties and microbial communities to be screened selectively in the new environment, aerobic heterotrophic bacterial activity to decline, and endogenous denitrifying action to strengthen, which made the I-A/O process implement a kind of dynamic balanced state that limited the DO demand.

Bacterial transcriptome remodeling during sequential co-culture with a marine dinoflagellate and diatom.

In their role as primary producers, marine phytoplankton modulate heterotrophic bacterial activities through differences in the types and amounts of organic matter they release. This study investigates the transcriptional response of bacterium Ruegeria pomeroyi, a member of the Roseobacter clade known to affiliate with diverse phytoplankton groups in the ocean, during a shift in phytoplankton taxonomy. The bacterium was initially introduced into a culture of the dinoflagellate Alexandrium tamarense, and then experienced a change in phytoplankton community composition as the diatom Thalassiosira pseudonana gradually outcompeted the dinoflagellate. Samples were taken throughout the 30-day experiment to track shifts in bacterial gene expression informative of metabolic and ecological interactions. Transcriptome data indicate fundamental differences in the exometabolites released by the two phytoplankton. During growth with the dinoflagellate, gene expression patterns indicated that the main sources of carbon and energy for R. pomeroyi were dimethysulfoniopropionate (DMSP), taurine, methylated amines, and polyamines. During growth with the diatom, dihydroxypropanesulfonate (DHPS), xylose, ectoine, and glycolate instead appeared to fuel the bulk of bacterial metabolism. Expression patterns of genes for quorum sensing, gene transfer agent, and motility suggest that bacterial processes related to cell communication and signaling differed depending on which phytoplankton species dominated the co-culture. A remodeling of the R. pomeroyi transcriptome implicating more than a quarter of the genome occurred through the change in phytoplankton regime.

Heterotrophic bacterial production, respiration, and growth efficiency associated with upwelling intensity in the Ulleung Basin, East Sea

We investigated bacterial production (BP) and respiration (BR), as well as the physico-chemical properties of the water column, to elucidate the effect of upwelling on heterotrophic bacterial metabolic activities and growth efficiency (BGE) in July 2012 and May 2013 in the Ulleung Basin (UB), East/Japan Sea. The upwelled conditions were characterized by higher chlorophyll-a (Chl-a) concentrations resulting from the upward shift of the nitracline compared to that of the non-upwelled condition. Analyses of the size fractions of Chl-a and pigment composition revealed that large size phytoplankton (> 20µm), mainly consisting of diatoms, appeared to be the major phytoplankton component. BP and BR were significantly correlated with Chl-a (P < 0.001), but the correlations with temperature were not significant (P > 0.05). These results suggest that bacterial metabolic activities are stimulated by the availability of organic resources enhanced by upwelling in the UB. Further statistical analysis showed that the difference in BP and BGE with variations in upwelling intensity were significant (P = 0.018 for BP, P = 0.035 for BGE), but the difference in BR was not significant (P = 0.321). These results suggest that metabolic energy is partitioned more for BP under a strong upwelling condition, i.e. high nutrient and Chl-a conditions. In contrast, the energy generated via respiration was partitioned more for maintaining metabolism rather than for biomass production under weakly or non-upwelled conditions, i.e. stratified and low Chl-a conditions. Overall, our results suggest that any changes in upwelling intensity would significantly affect the carbon cycle associated with the fate of primary production, and the role of the microbial loop in the UB where changes in the intensity and frequency of upwelling associated with climatic changes are in progress.

Transformation of organo-ferric peat colloids by a heterotrophic bacterium

Bacterial mineralization of allochthonous (soil) dissolved organic matter (DOM) in boreal waters governs the CO2 flux from the lakes and rivers to the atmosphere, which is one of the main factor of carbon balance in high latitudes. However, the fate of colloidal trace element (TE) during bacterial processing of DOM remains poorly constrained. We separated monoculture of Pseudomonas saponiphila from a boreal creek and allowed it to react with boreal Fe-rich peat leachate of approximate colloidal (3kDa–0.45µm) composition C1000Fe12Al3.3Mg2Ca3.7P1.2Mn0.1Ba0.5 in nutrient-free media. The total net decrease of Dissolved Organic Carbon (DOC) concentration over 4day of exposure was within 5% of the initial value, whereas the low molecular weight fraction of Corg (LMW<3kDa) yielded a 16%-decrease due to long-term bio-uptake or coagulation. There was a relative depletion in Fe over Corg of 0.45µm, colloidal and LMW fraction in the course of peat leachate interaction with P. saponiphila. Al, Mn, Ni, Cu, Ga, REEs, Y, U were mostly affected by bacterial presence and exhibited essentially the adsorption at the cell surface over first hours of reaction, in contrast to Fe, Ti, Zr, and Nb that showed both short-term adsorption and long-term removal by physical coagulation/coprecipitation with Fe hydroxide. The low molecular weight fraction (LMW<3kDa) of most TE was a factor of 2–5 less affected by microbial presence via adsorption or removal than the high molecular weight (HMW) colloidal fractions (<0.45µm and <50kDa). The climate change-induced acceleration of heterotrophic bacterial activity in boreal and subarctic waters may lead to preferential removal of Fe over DOC from conventionally dissolved fraction and the decrease of the proportion of LMW<3kDa fraction and the increase of HMW colloids. Enhanced heterotrophic mineralization of organo-ferric colloids under climate warming scenario may compensate for on-going “browning” of surface waters.

Bioavailable dissolved organic matter and biological hot spots during austral winter in Antarctic waters

AbstractPrimary production and heterotrophic bacterial activity in the Antarctic Ocean are generally low during the austral winter. Organic carbon is considered to be a major factor limiting bacterial metabolism, but few studies have investigated the bioavailability of organic matter during winter. Herein, the chemical composition and bioavailability of dissolved organic matter (DOM) were investigated in surface (5–100 m) and mesopelagic (200–750 m) waters off the northwestern Antarctic Peninsula during August 2012. Concentrations of dissolved organic carbon (DOC) were low (42 ± 4 µmol L−1) and showed no apparent spatial patterns. By contrast, the composition of DOM exhibited significant spatial trends that reflected varying ecosystem productivity and water masses. Surface distributions of chlorophyll‐a and particulate organic carbon depicted a southward decline in primary productivity from open waters (60.0°S–61.5°S) to ice‐covered regions (61.5°S–62.5°S). This trend was evident from concentrations and DOC‐normalized yields of dissolved amino acids in the surface waters, indicating decreasing DOM bioavailability with increasing latitude. A different pattern of DOM bioavailability was observed in the mesopelagic water masses, where amino acids indicated highly altered DOM in the Circumpolar Deep Water and bioavailable DOM in the Transitional Weddell Water. Depth distributions of amino acid yields and compositions revealed hot spots of elevated bioavailable DOM at ∼75 m relative to surrounding waters at most ice‐free stations. Relatively low mole percentages of bacterially derived d‐amino acids in hot spots were consistent with an algal source of bioavailable DOM. Overall, these results reveal the occurrence and spatial heterogeneity of bioavailable substrates in Antarctic waters during winter.

Contribution of mono and polysaccharides to heterotrophic N2 fixation at the eastern Mediterranean coastline.

N2 fixation should be a critical process in the nitrogen-poor surface water of the eastern Mediterranean Sea. Despite favorable conditions, diazotroph abundance and N2 fixation rates remains low for reasons yet explained. The main goal of this study was to investigate the limiting nutrients for diazotrophy in this oligotrophic environment. Hence, we conducted dedicated bottle-microcosms with eastern Mediterranean Sea water that were supplemented with mono and polysaccharides as well as inorganic nitrogen and phosphorous. Our results indicate that the diazotrophic community expressing nifH was primarily represented by heterotrophic Proteobacteria. N2 fixation and heterotrophic bacterial activity increased up-to tenfold following two days of dark incubations, once seawater was supplemented with organic carbon substrate in the form of glucose (monosaccharides) or gum-xanthan (polysaccharide surrogate). Furthermore, our results point that carbon-rich polysaccharides, such as transparent exopolymer particles, enhance heterotrophic N2 fixation, by forming microenvironments of intense metabolic activity, high carbon: nitrogen ratio, and possibly low O2 levels. The conclusions of this study indicate that diazotrophs in the eastern Mediterranean coast are primarily limited by organic carbon substrates, as possibly in many other marine regions.

Nutrient pulses driven by internal solitary waves enhance heterotrophic bacterial growth in the South China Sea.

This study demonstrated the potential effects of internal waves (IWs) on heterotrophic bacterial activities for the first time. Nine anchored studies were conducted from 2009-2012 in the South China Sea areas with different physical conditions, i.e. areas subjected to elevation IWs, to depression IWs, and to weak/no IWs. The latter two areas were treated as the Control sites. Field survey results indicated that within the euphotic zone, the minima of the depth-averaged bacterial production (IBP; ∼1.0 mgC m-3 d-1 ) and growth rate (IBμ; ∼0.1 d-1 ) at all sites were similar. Except for one case, the maxima of IBP (6-12 mgC m-3 d-1 ) and IBμ (0.55-1.13 d-1 ) of the elevation IWs areas were ∼fivefolds higher than those of the Control sites (IBP 1.7-2.1 mgC m-3 d-1 ; IBμ 0.13-0.24 d-1 ). Replicate surveys conducted at the north-western area of the Dongsha atoll during spring-to-neap (NW1 survey) and neap-to-spring (NW2 survey) tide periods showed a great contrast to each other. Low variation and averages of IBμ in NW1 survey were similar to those of the Control sites, while those in NW2 were similar to the other elevation IWs sites with larger variation and higher averages of IBμ. This finding suggests that bacterial activities may be a function of the lunar fortnightly (14-day) cycle. Enrichment experiments suggested more directly that the limiting inorganic nutrients introduced by the elevation waves (EIWs) may contribute a higher IBμ within the euphotic zone.

Enhanced particle fluxes and heterotrophic bacterial activities in Gulf of Mexico bottom waters following storm-induced sediment resuspension

Enhanced particle fluxes and heterotrophic bacterial activities in Gulf of Mexico bottom waters following storm-induced sediment resuspension

Enhanced particle fluxes and heterotrophic bacterial activities in Gulf of Mexico bottom waters following storm-induced sediment resuspension

Bottom nepheloid layers (BNLs) in the deep sea transport and remobilize considerable amounts of particulate matter, enhancing microbial cycling of organic matter in cold, deep water environments. We measured bacterial abundance, bacterial protein production, and activities of hydrolytic enzymes within and above a BNL that formed in the deep Mississippi Canyon, northern Gulf of Mexico, shortly after Hurricane Isaac had passed over the study area in late August 2012. The BNL was detected via beam attenuation in CTD casts over an area of at least 3.5km2, extending up to 200m above the seafloor at a water depth of ~1500m. A large fraction of the suspended matter in the BNL consisted of resuspended sediments, as indicated by high levels of lithogenic material collected in near-bottom sediment traps shortly before the start of our sampling campaign. Observations of suspended particle abundance and sizes throughout the water column, using a combined camera-CTD system (marine snow camera, MSC), revealed the presence of macroaggregates (>1mm in diameter) within the BNL, indicating resuspension of canyon sediments. A distinct bacterial response to enhanced particle concentrations within the BNL was evident from the observation that the highest enzymatic activities (peptidase, β-glucosidase) and protein production (3H-leucine incorporation) were found within the most particle rich sections of the BNL. To investigate the effects of enhanced particle concentrations on bacterial activities in deep BNLs more directly, we conducted laboratory experiments with roller bottles filled with bottom water and amended with experimentally resuspended sediments from the study area. Macroaggregates formed within 1 day from resuspended sediments; by day 4 of the incubation bacterial cell numbers in treatments with resuspended sediments were more than twice as high as in those lacking sediment suspensions. Cell-specific enzymatic activities were also generally higher in the sediment-amended compared to the unamended treatments. The broader range and higher activities of polysaccharide hydrolases in the presence of resuspended sediments compared to the unamended water reflected enzymatic capabilities typical for benthic bacteria. Our data suggest that the formation of BNLs in the deep Gulf of Mexico can lead to transport of sedimentary organic matter into bottom waters, stimulating bacterial food web interactions. Such storm-induced resuspension may represent a possible mechanism for the redistribution of sedimented oil-fallout from the Deepwater Horizon spill in 2010.

Impact of heterotrophic bacterium Pseudomonas aureofaciens on the release of major and trace elements from podzol soil into aqueous solution

The release of major and trace elements (TEs) from mineral and organic-rich podzol soil horizons has been studied in batch reactors with live and dead soil heterotrophic Pseudomonas aureofaciens bacteria and in bacteria-free systems. Dissolved organic carbon (DOC) concentrations decreased and dissolved inorganic carbon concentrations increased over the course of experiments with live bacteria due to on-going DOC mineralization processes. Several families of major and trace elements could be distinguished, depending on their release patterns in bacteria-free systems and live bacteria-bearing systems. Live bacteria enhanced the release of Mg, Rb, Cd, Pb, Al, Fe and V from the mineral soil horizon and the release of Rb, Ni, Pb, As, Fe, V and La from the organic soil horizon relative to bacteria-free soil or dead bacteria experiments. Unexpectedly, K, Ca, Sr, Cu, Ti, Mn, Zn and As release from the mineral horizon and Mg, K, Ca, Sr, Ba, Cr, Ti, Mn and Zn release from the organic horizon decreased in the presence of live P. aureofaciens compared to bacteria-free and dead bacteria systems. Finally, live bacteria exhibited no effect on the release of Si, Al, Cu and Mo from the humic horizon and Ni, Mo, Cr, Ba, Si and La from the mineral horizon relative to the bacteria-free system. These results can be interpreted via a combination of several simultaneous processes, which occur in the soil–bacteria suspension and lead to changes in TE speciation and affinities to mineral surfaces and bacteria and include the following: 1) a slight decrease in the pH due to exometabolite production; 2) degradation of DOC and TE organic complexes by heterotrophic bacteria; 3) element adsorption at cell surfaces and biological uptake; and 4) element release from the soil mineral and organic particles.Compared to abiotic systems, the observed decreases in the concentrations of major elements and many heavy metals that leach from the soil in the presence of bacteria have important consequences regarding our understanding of the role of bacteria in element mobilizations from soil to rivers. It follows that chemical weathering in both organic and mineral horizons of podzol soil may not be strongly affected by heterotrophic bacterial activity; rather, the solution pH and DOC levels may control the intensity of element mobilization. The aqueous concentrations of many TEs (Fe, Al, La, Cr, Ni, Cd, Pb, Cu, and Rb) at the end of live-bacteria experiments were comparable with reported compositions of interstitial podzol soil solutions.

Microbial metabolism of transparent exopolymer particles during the summer months along a eutrophic estuary system.

This study explores the role of transparent exopolymer particles (TEP) as an additional carbon source for heterotrophic microbial activity in the eutrophic Qishon estuary. From the coastal station and upstream the estuary; TEP concentrations, β-glucosidase activity, bacterial production and abundance have gradually increased. TEP were often found as bio-aggregates, scaffolding algae, detritus matter and bacteria that likely formed “hotspots” for enhance microbial activity. To further demonstrate the link between TEP and heterotrophic bacterial activity, confined incubations with ambient and polysaccharide-enriched estuary water were carried out. Following polysaccharide addition, elevated (~50%) β-glucosidase activity rates were observed, leading to TEP hydrolysis. This newly formed bioavailable carbon resulted in significantly higher growth rates, with up to a 5-fold increase in heterotrophic bacterial biomass, comprising mostly high nucleic acid content bacteria. Taking together the findings from this research, we conclude that even in highly eutrophic environments heterotrophic bacteria may still be carbon limited. Further, TEP as a polysaccharide matrix can act as a metabolic surrogate, adding fresh bioavailable carbon through tight associations with bacteria in eutrophic ecosystems such as the Qishon estuary.

Fe and C co-limitation of heterotrophic bacteria in the naturally fertilized region off the Kerguelen Islands

Abstract. It has been univocally shown that iron (Fe) is the primary limiting nutrient for phytoplankton metabolism in high-nutrient, low-chlorophyll (HNLC) waters, yet the question of how this trace metal affects heterotrophic microbial activity is far less understood. We investigated the role of Fe for bacterial heterotrophic production and growth at three contrasting sites in the naturally Fe-fertilized region east of the Kerguelen Islands and at one site in HNLC waters during the KEOPS2 (Kerguelen Ocean and Plateau Compared Study 2) cruise in spring 2011. We performed dark incubations of natural microbial communities amended either with iron (Fe, as FeCl3) or carbon (C, as trace-metal clean glucose), or a combination of both, and followed bacterial abundance and heterotrophic production for up to 7 days. Our results show that single and combined additions of Fe and C stimulated bulk and cell-specific bacterial production at the Fe-fertilized sites, while in HNLC waters only combined additions resulted in significant increases in these parameters. Bacterial abundance was enhanced in two out of the three experiments performed in Fe-fertilized waters but did not respond to Fe or C additions in HNLC waters. Our results provide evidence that both Fe and C are present at limiting concentrations for bacterial heterotrophic activity in the naturally fertilized region off the Kerguelen Islands in spring, while bacteria were co-limited by these elements in HNLC waters. These results shed new light on the role of Fe in bacterial heterotrophic metabolism in regions of the Southern Ocean that receive variable Fe inputs.

Characteristics of colored dissolved organic matter (CDOM) in the Western Arctic Ocean: Relationships with microbial activities

Colored dissolved organic matter (CDOM), a significant fraction of dissolved organic carbon (DOC), plays various roles in physical and biogeochemical processes in natural waters. In the Arctic Ocean, CDOM is abundant because of major input by large rivers. To better understand the processes that drive variations in CDOM, light absorption coefficients of CDOM [aCDOM(λ), m−1] were extensively documented together with temperature, salinity, chlorophyll a, nitrate concentrations, and bacterial production (BP) and abundance (BA) in the Western Arctic Ocean (WAO) from early to late summer as part of the MALINA and the ICESCAPE expeditions. The data set covered contrasting situations, from bloom to post-bloom conditions and from river-influenced to oceanic water masses. While CDOM photobleaching occurred in the surface layer (<20m), we observed significantly lower spectral slopes for CDOM absorption spectra (SCDOM) in addition to higher aCDOM(440) in the layer below (intermediate layer: 30.7<salinity<33.9). In particular, the low SCDOM values were found in the Chukchi Sea and the western part of the Beaufort Sea, which coincided with high BP and BA values. Considering the high primary production observed in these areas during our cruises (Arrigo et al., 2012. Science. 10.1126/science.1215065), we hypothesize that SCDOM variations reflect the degradation of phytoplankton that is associated with heterotrophic bacterial activity. In our datasets, a simple regression analysis showed that SCDOM was significantly correlated with BP and BA. A principal component analysis further supported this conclusion. From our field observations, it was shown that variations in aCDOM(440) and SCDOM result to a large extent from bacterial activity, at least in the WAO.

An annual survey of bacterial production, respiration and ectoenzyme activity in coastal NW Mediterranean waters: temperature and resource controls.

We simultaneously measured bacterial production (BP), bacterial respiration (BR), alkaline phosphatase activity (phos) and ectoaminopeptidase activity (prot) in relation to biogeochemical parameters, nutritive resources and in situ temperature over a 1-year survey at the long-term observatory the SOLEMIO station (Marseille bay, NW Mediterranean Sea). Despite its proximity to the coast, oligotrophic conditions prevailed at this station (yearly mean of Chl a = 0.43 μg dm(-3), NO3 = 0.55 μmol dm(-3) and PO4 = 0.04 μmol dm(-3)). Episodic meteorological events (dominant winds, inputs from the Rhone River) induced rapid oscillations (within 15 days) in temperature and sometimes salinity that resulted in rapid changes in phytoplankton succession and a high variability in C/P ratios within the particulate and dissolved organic matter. Throughout the year, BP ranged from 0.01 to 0.82 μg C dm-(3) h-(1) and bacterial growth efficiency varied from 1 to 39%, with higher values in summer. Enrichment experiments showed that BP was limited most of the year by phosphorus availability (except in winter). A significant positive correlation was found between in situ temperature, BP, BR and phos. Finally, we found that temperature and phosphate availability were the main factors driving heterotrophic bacterial activity and thus play a fundamental role in carbon fluxes within the marine ecosystem.

Spectral irradiance dependence of sunlight effects on plankton dimethylsulfide production

We investigated the short‐term effects of variable solar irradiance and spectrum on the gross biological production of dimethylsulfide (DMS), a trace gas with potential climatic effects, in eight experiments performed at different times of the year in a northwest Mediterranean coastal site. Experimentally determined net community DMS production, DMS photolysis, and dark microbial DMS consumption rates were used to calculate gross community DMS production by budgeting. In addition, the composition of the bacterioplankton and phytoplankton communities in the initial samples, and the photoinhibition of bulk bacterial heterotrophic activity and phytoplankton photosynthetic efficiency were monitored. Our results show that: (1) gross DMS production is irradiance dependent, with a maximum short‐term stimulation factor of 2‐ to 6‐fold compared to dark incubations; (2) its spectral shape is variable but generally similar to that of phytoplankton photoinhibition or photodamage, with more effective stimulation at shorter ultraviolet wavelengths; and (3) stronger stimulation occurs when samples are overexposed with respect to their prior exposure. Remarkably, the photoresponse of gross DMS production was in most cases strong enough to (at least) compensate the photochemical DMS loss at the water subsurface. Such response would prevent DMS depletion in stratified and highly irradiated waters. Since the initial microbial communities were representative of meso‐ to oligotrophic conditions, our observations should apply to a wide variety of oceanic regimes.

Response of bacterioplankton activity in an Arctic fjord system to elevated &amp;lt;i&amp;gt;p&amp;lt;/i&amp;gt;CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;: results from a mesocosm perturbation study

Abstract. The effect of elevated seawater carbon dioxide (CO2) on the activity of a natural bacterioplankton community in an Arctic fjord system was investigated by a mesocosm perturbation study in the frame of the European Project on Ocean Acidification (EPOCA). A pCO2 range of 175–1085 μatm was set up in nine mesocosms deployed in the Kongsfjorden (Svalbard). The activity of natural extracellular enzyme assemblages increased in response to acidification. Rates of β-glucosidase and leucine-aminopeptidase increased along the gradient of mesocosm pCO2. A decrease in seawater pH of 0.5 units almost doubled rates of both enzymes. Heterotrophic bacterial activity was closely coupled to phytoplankton productivity in this experiment. The bacterioplankton community responded to rising chlorophyll a concentrations after a lag phase of only a few days with increasing protein production and extracellular enzyme activity. Time-integrated primary production and bacterial protein production were positively correlated, strongly suggesting that higher amounts of phytoplankton-derived organic matter were assimilated by heterotrophic bacteria at increased primary production. Primary production increased under high pCO2 in this study, and it can be suggested that the efficient heterotrophic carbon utilisation had the potential to counteract the enhanced autotrophic CO2 fixation. However, our results also show that beneficial pCO2-related effects on bacterial activity can be mitigated by the top-down control of bacterial abundances in natural microbial communities.

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.