Summary
Marine heterotrophic prokaryotes (HP) play a key role in organic matter processing in the ocean; however, the view of HP as dissolved organic matter (DOM) sources remains underexplored. In ...this study, we quantified and optically characterized the DOM produced by two single marine bacterial strains. We then tested the availability of these DOM sources to in situ Mediterranean Sea HP communities. Two bacterial strains were used: Photobacterium angustum (a copiotrophic gammaproteobacterium) and Sphingopyxis alaskensis (an oligotrophic alphaproteobacterium). When cultivated on glucose as the sole carbon source, the two strains released from 7% to 23% of initial glucose as bacterial derived DOM (B‐DOM), the quality of which (as enrichment in humic or protein‐like substances) differed between strains. B‐DOM induced significant growth and carbon consumption of natural HP communities, suggesting that it was partly labile. However, B‐DOM consistently promoted lower prokaryotic growth efficiencies than in situ DOM. In addition, B‐DOM changed HP exoenzymatic activities, enhancing aminopeptidase activity when degrading P. angustum DOM, and alkaline phosphatase activity when using S. alaskensis DOM, and promoted differences in HP diversity and composition. DOM produced by HP affects in situ prokaryotic metabolism and diversity, thus changing the pathways for DOM cycling (e.g. respiration over biomass production) in the ocean.
Spatial increases and temporal shifts in outbreaks of gelatinous plankton have been observed over the past several decades in many estuarine and coastal ecosystems. The effects of these blooms on ...marine ecosystem functioning and particularly on the dynamics of the heterotrophic bacteria are still unclear. The response of the bacterial community from a Mediterranean coastal lagoon to the addition of dissolved organic matter (DOM) from the jellyfish
Aurelia aurita
, corresponding to an enrichment of dissolved organic carbon (DOC) by 1.4, was assessed for 22 days in microcosms (8 l). The high bioavailability of this material led to (i) a rapid mineralization of the DOC and dissolved organic nitrogen from the jellyfish and (ii) the accumulation of high concentrations of ammonium and orthophosphate in the water column. DOM from jellyfish greatly stimulated heterotrophic prokaryotic production and respiration rates during the first 2 days; then, these activities showed a continuous decay until reaching those measured in the control microcosms (lagoon water only) at the end of the experiment. Bacterial growth efficiency remained below 20 %, indicating that most of the DOM was respired and a minor part was channeled to biomass production. Changes in bacterial diversity were assessed by tag pyrosequencing of partial bacterial 16S rRNA genes, DNA fingerprints, and a cultivation approach. While bacterial diversity in control microcosms showed little changes during the experiment, the addition of DOM from the jellyfish induced a rapid growth of
Pseudoalteromonas
and
Vibrio
species that were isolated. After 9 days, the bacterial community was dominated by
Bacteroidetes
, which appeared more adapted to metabolize high-molecular-weight DOM. At the end of the experiment, the bacterial community shifted toward a higher proportion of
Alphaproteobacteria
. Resilience of the bacterial community after the addition of DOM from the jellyfish was higher for metabolic functions than diversity, suggesting that jellyfish blooms can induce durable changes in the bacterial community structure in coastal lagoons.
We used carbon and nitrogen stable isotope analysis to describe the food web of the Lapalme Lagoon, one of the best preserved coastal lagoons along the French Mediterranean coast. Three surveys, ...corresponding to contrasting situations both in terms of continental inputs and of connection between the lagoon and the open sea, were conducted in June and September 2004 and in February 2005. There were significant spatio-temporal changes in the isotopic ratios of both primary producers and consumers. Temporal changes were mostly linked to important
13C-depleted continental inputs caused by the long period of heavy rainfall before the June survey. Conversely, isotopic ratios were rather similar in September and February despite the opening of the connection of the lagoon with the sea between these two surveys. The interpretation of the results in terms of the structure of the trophic network differed between the June period and the two other ones. In September 2004 and February 2005, the food web was mostly based on SOM and lagoon POM pools with only a few consumers departing from this general trend. In June 2004, a significant proportion of consumers were conversely
13C-depleted probably due to the assimilation of significant amounts of continental inputs by grazers and filter-feeders. This stresses the necessity of carrying out seasonal surveys to get a sound idea of the structure of the food web in highly variable ecosystems such as coastal lagoons. Spatial changes in isotopic ratios were likely linked to: (1) the salinity/confinement gradient with a trend toward lower
δ
13C and
δ
15N values (i.e., higher continental influence) in the inner part of the main lagoon; and (2) the high
δ
15N values of primary producers and discretely motile consumers in a semi-confined site located downstream a vineyard receiving large amounts of fertilizers. We hypothesised that these inputs are quickly
15N-enriched through denitrification and ammonia volatilisation processes and then contribute to the
15N-enrichment of salt marsh plants and seagrass at this site. This interpretation is supported by the fact that such a
15N-enrichment of primary producers only occurred in February (i.e., after the dispersion of the fertilizers). Conversely, discretely motile consumers were
15N-enriched all year round, which suggests that they were mostly exploiting the detritic pool derived from
15N-enriched salt marsh plants.
•Optical properties of dissolved organic matter were used for identifying different CDOM pools in coastal areas of contrasting stratification and freshwater input conditions.•Marked differences were ...found for a325, SCDOM, and SR within and among each of the three zones, although surface DOC concentrations remained constant at all sites.•In stratified systems, freshwater inputs determine the distribution of surface CDOM while in well-mixed conditions, the in situ production determine the general CDOM concentration.
Chromophoric dissolved organic matter (CDOM) is a biologically active component of dissolved organic matter that influences the optical properties of aquatic environments, thereby playing an important role in the photochemical and photobiology processes occurring in the euphotic layer. Three cruises were carried out at three contrasting oceanographic areas off central and southern Chile (36.8°S, 44.6°S, 54.9°S) between November 2013 and July 2017 in order to assess the variability of the CDOM pool's optical properties (a325, SCDOM, SR). Marked differences, including vertical variability, were found for a325, SCDOM, and SR within and among each of the three zones, although surface DOC concentrations remained constant at all sites (87.59 ± 26.04 µmol L−1). At the coastal area close to the Biobio River (36.8°S), a325 values varied between 0.26–1.93 m−1 with maximum measured in surface waters. For the Puyuhuapi fjord in northern Patagonia (44.6°S), a325 ranged between 0.69 and 3.86 m−1, while in the Beagle Channel (54.9°S) it oscillated between 0.59 and 1.73 m−1. We identified the terrestrial organic matter input to surface waters as the main factor influencing surface CDOM concentrations in central Chile. In the Puyuhuapi fjord, terrestrial influence was heterogeneous because of its seasonal variability of the Channel but it was overall less significant than in situ produced CDOM. We also observed the lowest contribution of terrestrial CDOM inputs compared to the in situ production in southern Patagonia (Beagle Channel). We observed that the distribution of CDOM is potentially dominated by biological in situ production below the surface freshwater-influenced layer in the southern area while highly influenced by terrestrial input in the coastal upwelling area at 36°S.
Overall, this study shows that the optical properties of dissolved organic matter (mainly SR) can be suitable for identifying different CDOM pools in coastal areas of contrasting oceanographic conditions. In stratified systems, freshwater inputs determine the distribution of surface CDOM while in well-mixed conditions, the in situ production determine the general CDOM concentration.
Natural iron fertilization of high-nutrient low-chlorophyll (HNLC) waters induces annually occurring spring phytoplankton blooms off the Kerguelen Islands (Southern Ocean). To examine the origin and ...fate of particulate and dissolved organic matter (POM and DOM), D- and L-amino acids (AA) were quantified at bloom and HNLC stations. Total hydrolyzable AA accounted for 21–25% of surface particulate organic carbon (%POCAA) at the bloom sites, but for 10% at the HNLC site. A marked decrease in %POCAA with depth was observed at the most productive stations leading to values between 3 and 5% below 300 m depth. AA contributed to only 0.9–4.4% of dissolved organic carbon (%DOCAA) at all stations. The only consistent vertical trend was observed at the most productive station (A3-2) where %DOCAA decreased from ~ 2% in the surface waters to 0.9% near 300 m. These AA yields revealed that POM and DOM were more rapidly altered or mineralized at the bloom sites compared to the HNLC site. Alteration state was also assessed by trends in C / N ratio, %D-AA and degradation index. Different molecular markers indicated that POM mostly originated from diatoms and bacteria. The estimated average proportion of POM from intact phytoplankton cells in surface waters was 45% at the bloom station A3-2, but 14% at the HNLC site. Estimates based on D-AA yields indicated that ~ 15% of POM and ~ 30% of DOM was of bacterial origin (cells and cell fragments) at all stations. Surprisingly, the DOM in HNLC waters appeared less altered than the DOM from the bloom, had slightly higher dissolved AA concentrations, and showed no sign of alteration within the water column. Unfavorable conditions for bacterial degradation in HNLC regions can explain these findings. In contrast, large inputs of labile organic molecules and iron likely stimulate the degradation of organic matter (priming effect) and the production of more recalcitrant DOM (microbial carbon pump) during iron-fertilized blooms.
Heterotrophic bacterial communities in marine environments are exposed to a heterogeneous mixture of dissolved organic compounds with different bioreactivity that may control both their activity and ...composition. The coastal environment is an example of a mixing area where recalcitrant allochthonous organic matter from rivers can encounter labile organic matter from marine phytoplanktonic blooms. The objective of this study was to explore the effects of mixed qualities of dissolved organic matter (DOM) on bacterial community activity (BCA) and bacterial community composition (BCC) and to test for a priming effect when DOM sources are added in combination. Coastal marine bacterial communities were incubated separately with a mixture of amino acids and with natural riverine DOM or with both sources together for 42 days. Addition of amino acids alone or in combination with riverine DOM led to a similar stimulation of BCA compared to control condition, whereas addition of riverine DOM alone did not modify BCA compared to the control. On the contrary, BCC analyzed by 16S rRNA gene pyrosequencing was not affected by the addition of amino acids alone, but changed dramatically with riverine DOM alone or in combination with amino acids. Our results show that changes in BCA and BCC can be driven by different types of DOM, but that these changes are not necessarily coupled. Moreover, the addition of labile DOM did not modify the microbial decomposition of riverine DOM, nor the BCC, suggesting that a priming effect did not occur under these experimental conditions.
Summary
Open‐ocean convection is a fundamental process for thermohaline circulation and biogeochemical cycles that causes spectacular mixing of the water column. Here, we tested how much the ...depth‐stratified prokaryotic communities were influenced by such an event, and also by the following re‐stratification. The deep convection event (0–1500 m) that occurred in winter 2010–2011 in the NW Mediterranean Sea resulted in a homogenization of the prokaryotic communities over the entire convective cell, resulting in the predominance of typical surface Bacteria, such as Oceanospirillale and Flavobacteriales. Statistical analysis together with numerical simulation of vertical homogenization evidenced that physical turbulence only was not enough to explain the new distribution of the communities, but acted in synergy with other parameters such as exported particulate and dissolved organic matters. The convection also stimulated prokaryotic abundance (+21%) and heterotrophic production (+43%) over the 0–1500 m convective cell, and resulted in a decline of cell‐specific extracellular enzymatic activities (−67%), thus suggesting an intensification of the labile organic matter turnover during the event. The rapid re‐stratification of the prokaryotic diversity and activities in the intermediate layer 5 days after the intense mixing indicated a marked resilience of the communities, apart from the residual deep mixed water patch.
We investigated the abundance and activity of SAR11 on a monthly time scale between January 2008 and October 2008 in the oligotrophic NW Mediterranean Sea. Applying MICRO-CARD-FISH, we observed that ...SAR11 had a large contribution to bulk abundance (37 ± 6% of DAPI-stained cells) and to bulk bacterial heterotrophic production (BHP), as estimated from leucine incorporation (55 ± 15% of DAPI-cells assimilating leucine) in surface waters (5 m) throughout the study period. SAR11 contributed also substantially to the assimilation of glucose, ATP, and a combination of amino acids (44 ± 17%, 37 ± 14%, and 43 ± 12% of DAPI cells assimilating these compounds, respectively), organic compounds that provide either single or combined sources of C, P, and N. Temporal changes in the abundance of SAR11 cells that assimilated leucine, glucose, amino acids, and ATP revealed a pattern consistent with that of substrate-active DAPI cells, suggesting that the activity of SAR11 can explain to a large extent the variability in total cells contributing to the utilization of these compounds. Short-term nutrient enrichment experiments performed on each sampling date revealed a strong co-limitation of at least two of the three elements analyzed (C, N, P), in particular, during summer and early autumn. The in situ abundance of SAR11 cells assimilating leucine appeared to increase with P limitation as determined in the nutrient enrichment experiments (r = 0.81, p = 0.015). Our results demonstrate that SAR11 is an important component of the active bacterial community in the NW Mediterranean Sea. Our observations further indicate that the activity of the bulk bacterial community is linked to the activity of SAR11, possibly due to its adaptation to nutrient limitation.
For centuries, many Mediterranean catchments were covered with vineyards in which copper was widely applied to protect grapevines against fungus. In the Mediterranean-type flow regime, brief and ...intense flood events increase the stream water discharge by up to 10 times and cause soil leaching and storm runoff. Because vineyards are primarily cultivated on steep slopes, high Cu fluxes are discharged by surface water runoff into the rivers. The purpose of this work was to investigate the riverine behavior and transport of anthropogenic Cu by coupling a sequential chemical extraction (SCE) procedure, used to determine Cu partitioning between residual and non-residual fractions, with δ65Cu isotopic measurements in each fraction. In the Baillaury catchment, France, we sampled soils (cultivated and abandoned), river bed sediments (BS), suspended particulate matter (SPM), and river water during the flash flood event of February 2009. Copper partitioning using SCE show that most of Cu in abandoned vineyard soil was in the residual phase (>60%) whereas in cultivated soil, BS and SPM, Cu was mostly (>25%) in non-residual fractions, mainly adsorbed onto iron oxide fractions. A small fraction of Cu was associated with organic matter (5 to 10%). Calculated enrichment factors (EF) are higher than 2 and the anthropogenic contribution was estimated between 50 to 85%. Values for δ65Cu in bulk samples were similar to bedrock therefore; δ65Cu on SCE fractions of superficial soils and SPM allowed for discrimination between Cu origin and distribution. Copper in residual fractions was of natural mineral origin (δ65Cu close to local bedrock, +0.07‰). Copper in water soluble fraction of SPM (δ65Cu=+0.26‰) was similar to dissolved river Cu (δ65Cu=+0.31‰). Copper from fungicide treatment (δ65Cu=−0.35‰) was bound to organic matter (δ65Cu=−0.20‰) without or with slight isotopic fractioning. A preferential adsorption of 65Cu onto iron oxides (δ65Cu=+0.5‰) is shown.
•We investigated the fate of copper applied as a fungicide in a vineyard catchment.•Soils, river sediments and suspended matter, and local bedrock were sampled.•Copper concentrations and isotopes were measured in sequential extractions solutions.•Based on our work fungicide application significantly enriched all samples in copper.•Isotope data provides new information regarding Cu bonding and fractionation.
The aim of this study is to understand the biogeochemical cycles of the northwestern Mediterranean Sea (NW Med), where a recurrent spring bloom related to dense water formation occurs. We used a ...coupled physical‐biogeochemical model at high resolution to simulate realistic 1 year period and analyze the nitrogen (N) and phosphorus (P) cycles. First, the model was evaluated using cruises carried out in winter, spring, and summer and a Bio‐Argo float deployed in spring. Then, the annual cycle of meteorological and hydrodynamical forcing and nutrients stocks in the upper layer were analyzed. Third, the effect of biogeochemical and physical processes on N and P was quantified. Fourth, we quantified the effects of the physical and biological processes on the seasonal changes of the molar NO3:PO4 ratio, particularly high compared to the global ocean. The deep convection reduced the NO3:PO4 ratio of upper waters, but consumption by phytoplankton increased it. Finally, N and P budgets were estimated. At the annual scale, this area constituted a sink of inorganic and a source of organic N and P for the peripheral area. NO3 and PO4 were horizontally advected from the peripheral regions into the intermediate waters (130–800 m) of the deep convection area, while organic matter was exported throughout the whole water column toward the surrounding areas. The annual budget suggests that the NW Med deep convection constitutes a major source of nutrients for the photic zone of the Mediterranean Sea.
Key Points
Modeling of annual nitrogen and phosphorus cycles in the northwestern Mediterranean deep convection region for the period 2012–2013
The deep convection area was a sink of inorganic matter and a source of organic matter for the surrounding area over the period 2012–2013
The N:P ratio in the surface layer is submitted to drastic variations during deep convection and bloom transition periods
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