Contrasted Saharan dust events in LNLC environments: impact on nutrient dynamics and primary production

The response of the phytoplanktonic community (primary production and algal biomass) to contrasted Saharan dust events (wet and dry deposition) was studied in the framework of the DUNE ("a DUst experiment in a low-Nutrient, low-chlorophyll Ecosystem") project. We simulated realistic dust deposition events (10 gm(-2)) into large mesocosms (52m(3)). Three distinct dust addition experiments were conducted in June 2008 (DUNE-1-P: simulation of a wet deposition; DUNE-1-Q: simulation of a dry deposition) and 2010 (DUNE-2-R1 and DUNE-2-R2: simulation of two successive wet depositions) in the northwestern oligotrophic Mediterranean Sea. No changes in primary production (PP) and chlorophyll a concentrations (Chl a) were observed after a dry deposition event, while a wet deposition event resulted in a rapid (24 h after dust addition), strong (up to 2.4-fold) and long (at least a week in duration) increase in PP and Chl a. We show that, in addition to being a source of dissolved inorganic phosphorus (DIP), simulated wet deposition events were also a significant source of nitrate (NO3-) (net increases up to +9.8 mu M NO3- at 0.1m in depth) to the nutrient-depleted surface waters, due to cloud processes and mixing with anthropogenic species such as HNO3. The dry deposition event was shown to be a negligible source of NO3-. By transiently increasing DIP and NO3- concentrations in N-P starved surface waters, wet deposition of Saharan dust was able to relieve the potential N or NP co-limitation of the phytoplanktonic activity. Due to the higher input of NO3- relative to DIP, and taking into account the stimulation of the biological activity, a wet deposition event resulted in a strong increase in the NO3-/DIP ratio, from initially less than 6, to over 150 at the end of the DUNE-2-R1 experiment, suggesting a switch from an initial N or NP co-limitation towards a severe P limitation. We also show that the contribution of new production to PP strongly increased after wet dust deposition events, from initially 15% to 60-70% 24 h after seeding, indicating a switch from a regenerated-production based system to a new-production based system. DUNE experiments show that wet and dry dust deposition events induce contrasting responses of the phytoplanktonic community due to differences in the atmospheric supply of bioavailable new nutrients. Our results from original mesocosm experiments demonstrate that atmospheric dust wet deposition greatly influences primary productivity and algal biomass in LNLC environments through changes in the nutrient stocks, and alters the NO3-/DIP ratio, leading to a switch in the nutrient limitation of the phytoplanktonic activity

Contrasted Saharan dust events in LNLC environments: impact on nutrient dynamics and primary production

International audienceThe response of the phytoplanktonic community (primary production and algal biomass) to contrasted Sa-haran dust events (wet and dry deposition) was studied in the framework of the DUNE ("a DUst experiment in a low-Nutrient, low-chlorophyll Ecosystem") project. We simu-lated realistic dust deposition events (10 g m −2) into large mesocosms (52 m 3). Three distinct dust addition experiments were conducted in June 2008 (DUNE-1-P: simulation of a wet deposition; DUNE-1-Q: simulation of a dry deposition) and 2010 (DUNE-2-R1 and DUNE-2-R2: simulation of two successive wet depositions) in the northwestern oligotrophic Mediterranean Sea. No changes in primary production (PP) and chlorophyll a concentrations (Chl a) were observed after a dry deposition event, while a wet deposition event resulted in a rapid (24 h after dust addition), strong (up to 2.4-fold) and long (at least a week in duration) increase in PP and Chl a. We show that, in addition to being a source of dis-solved inorganic phosphorus (DIP), simulated wet deposition events were also a significant source of nitrate (NO − 3) (net in-creases up to +9.8 µM NO − 3 at 0.1 m in depth) to the nutrient-depleted surface waters, due to cloud processes and mixing with anthropogenic species such as HNO 3 . The dry deposi-tion event was shown to be a negligible source of NO − 3 . By transiently increasing DIP and NO − 3 concentrations in N–P starved surface waters, wet deposition of Saharan dust was able to relieve the potential N or NP co-limitation of the phy-toplanktonic activity. Due to the higher input of NO − 3 relative to DIP, and taking into account the stimulation of the bio-logical activity, a wet deposition event resulted in a strong increase in the NO − 3 /DIP ratio, from initially less than 6, to over 150 at the end of the DUNE-2-R1 experiment, suggest-ing a switch from an initial N or NP co-limitation towards a severe P limitation. We also show that the contribution of new production to PP strongly increased after wet dust de-position events, from initially 15 % to 60–70 % 24 h after seeding, indicating a switch from a regenerated-production based system to a new-production based system. DUNE ex-periments show that wet and dry dust deposition events in-duce contrasting responses of the phytoplanktonic commu-nity due to differences in the atmospheric supply of bioavail-able new nutrients. Our results from original mesocosm ex-periments demonstrate that atmospheric dust wet deposition Published by Copernicus Publications on behalf of the European Geosciences Union. 4784 C. Ridame et al.: Phytoplanktonic response to Saharan dust events greatly influences primary productivity and algal biomass in LNLC environments through changes in the nutrient stocks, and alters the NO − 3 /DIP ratio, leading to a switch in the nu-trient limitation of the phytoplanktonic activity

N2 fixation in eddies of the eastern tropical South Pacific Ocean

Mesoscale eddies play a major role in controlling ocean biogeochemistry. By impacting nutrient availability and water column ventilation, they are of critical importance for oceanic primary production. In the eastern tropical South Pacific Ocean off Peru, where a large and persistent oxygen deficient zone is present, mesoscale processes have been reported to occur frequently. However, investigations on their biological activity are mostly based on model simulations, and direct measurements of carbon and dinitrogen (N2) fixation are scarce. We examined an open ocean cyclonic eddy and two anticyclonic mode water eddies: a coastal one and an open ocean one in the waters off Peru along a section at 16° S in austral summer 2012. Molecular data and bioassay incubations point towards a difference between the active diazotrophic communities present in the cyclonic eddy and the anticyclonic mode water eddies. In the cyclonic eddy, highest rates of N2 fixation were measured in surface waters but no N2 fixation signal was detected at intermediate water depths. In contrast, both anticyclonic mode water eddies showed pronounced maxima in N2 fixation below the euphotic zone as evidenced by rate measurements and geochemical data. N2 fixation and carbon (C) fixation were higher in the young coastal mode water eddy compared to the older offshore mode water eddy. A co-occurrence between N2 fixation and biogenic N2, an indicator for N loss, indicated a link between N loss and N2 fixation in the mode water eddies, which was not observed for the cyclonic eddy. The comparison of two consecutive surveys of the coastal mode water eddy in November and December 2012 revealed also a reduction of N2 and C fixation at intermediate depths along with a reduction in chlorophyll by half, mirroring an aging effect in this eddy. Our data indicate an important role for anticyclonic mode water eddies in stimulating N2 fixation and thus supplying N offshore

Two subpopulations of Crocosphaera watsonii have distinct distributions in the North and South Pacific

Crocosphaera watsonii is a unicellular nitrogen (N2)-fixing cyanobacterium with ecological importance in oligotrophic oceans. In cultivated strains there are two phenotypes of C. watsonii (large and small cells) with differences that could differentially impact biogeochemical processes. Recent work has shown the phenotypes diverged through loss or addition of type-specific genes in a fraction of their genomes, whereas the rest of the genomes were maintained at 99–100% DNA identity. Previous molecular assays for C. watsonii abundances targeted the conserved regions and therefore could not differentiate between phenotypes, so their relative distributions in natural communities were unknown. To determine phenotype distributions, this study developed and applied type-specific quantitative polymerase chain reaction assays to samples from the North and South Pacific. Abundances of both Crocosphaera types declined sharply with depth between 45 and 75 m in both sites. In surface water small cells were 10–100 times more abundant than large cells in the N. Pacific, whereas in the S. Pacific the two phenotypes were nearly equal. Evidence for large cell aggregation was only found in N. Pacific samples. The differences in C. watsonii sub-populations in the North and South Pacific have direct implications for biogeochemistry and carbon export in oligotrophic gyres

Effects of nutrient enrichment on surface microbial community gene expression in the oligotrophic North Pacific Subtropical Gyre

Marine microbial communities are critical for biogeochemical cycles and the productivity of ocean ecosystems. Primary productivity in the surface ocean is constrained by nutrients which in part are supplied by mixing with deeper water. Little is known about the time scales, frequency, or impact of mixing on microbial communities. We combined in situ sampling using the Environmental Sample Processor and a small-scale mixing experiment with lower euphotic zone water to determine how individual populations respond to mixing. Transcriptional responses were measured using the MicroTOOLs (Microbiological Targets for Ocean Observing Laboratories) microarray, which targets all three domains of life and viruses. The experiment showed that mixing substantially affects photosynthetic taxa as expected, but surprisingly also showed that populations respond differently to unfiltered deep water which contains particles (organisms and detritus) compared to filtered deep water that only contains nutrients and viruses, pointing to the impact of biological interactions associated with these events. Comparison between experimental and in situ population transcription patterns indicated that manipulated populations can serve as analogs for natural populations, and that natural populations may be frequently or continuously responding to nutrients from deeper waters. Finally, this study also shows that the microarray approach, which is complementary to metatranscriptomic sequencing, is useful for determining the physiological status of in situ microbial communities

Study of dinitrogen fixation in N deficient environments : Contribution of diazotrophic unicellular and control by nutrient availability

Ce travail de thèse a pour but d'étudier la fixation d'azote marine dans les environnements riches mais déficitaires en azote (N), comparé au phosphore (P) dans un rapport N:P<16, grâce à l'utilisation d'approches complémentaire en culture in vitro et sur le terrain in situ. La première partie de ce travail a consisté à évaluer la réponse de la fixation d'azote de la cyanobactérie unicellulaire diazotrophe Crocosphaera watsonii face à des concentrations micromolaires en azote inorganique dissous (DIN) supposées inhiber l'activité de fixation d'azote : - suite à un apport sporadique, ou, - après une longue période d'acclimatation. Les résultats de ces études n'ont pas permis d'observer une inhibition des activités de fixation d'azote de cet organisme, laissant supposer que ce processus pourrait être actif dans une zone de l'Océan ayant ces même caractéristiques biogéochimiques : le Sud Est Tropical de l'Océan Pacifique (ETSP). En effet, cette zone est une des trois plus grandes zones de minimum d'oxygène (OMZ) de l'Océan et d'intenses processus de pertes de N (dénitrification et anammox) y ont lieu, résultant en un déficit de N par rapport au P. Des études présumaient que des processus inverses, de gains de N par la fixation d'azote, pourraient y être actifs mais aucune mesure à l'échelle du bassin n'y avait été faite car la fixation d'azote n'était supposée se produire que dans les environnements oligotrophes, comme les gyres subtropicaux. Dans le cadre d'un projet international, des missions océanographiques ont pu avoir lieu dans cette zone en Février 2010 pendant un évènement El Niño et en Mars-Avril 2011 pendant un évènement La Niña.The objectif of these thesis was to study dinitrogen fixation in marine environments rich but deficient of nitrogen (N) compared to phosphorus (P) in a ratio N:P<16, by using complementary approaches in culture in vitro and in the field in situ. The first part of this work was to evaluate the response of nitrogen-fixing unicellular Cyanobacteria Crocosphaera watsonii faced with micromolar concentrations of dissolved inorganic nitrogen (DIN) supposed to inhibit nitrogen fixation activity : - after sporadic input, or, - after a long period of acclimatization. The results of these studies have failed to observe the inhibition of nitrogen fixation activities of this organism, suggesting that this process could be active in an area of the Ocean with these same biogeochemical characteristics : the Eastern Tropical South Pacific (ETSP). Indeed, this area is one of the three largest oxygen minimum zones (OMZ) of the Ocean, where intense processes of N losses (denitrification and anammox) took place, resulting in a deficit of N compared to P. Studies assumed that the inverse process, gain of N by nitrogen fixation, could be active in the ETSP but no measurements across the basin have been performed because nitrogen fixation was assumed to occur only in oligotrophic environments, such as the subtropical gyres. In the framework of an international project, cruises took place in this area in February 2010 during a El Niño event and in March-April 2011 during a La Niña event. Results of these two cruises have confirmed that nitrogen fixation was unexpectedly active with an intensity comparable to those reported in oligotrophic areas

Phytoplanktonic response to contrasted Saharan dust deposition events during mesocosm experiments in LNLC environment

International audienceThe response of the phytoplanktonic community (primary production and algal biomass) to contrasted Saharan dust events (wet and dry deposition) was studied in the framework of the DUNE "a DUst experiment in a low-Nutrient, low-chlorophyll Ecosystem" project. We simulated realistic dust deposition events (10 g m-2) into large mesocosms (52 m3). Three distinct experimental dust additions were conducted in June 2008 (DUNE-1-P: simulation of a wet deposition, DUNE-1-Q: simulation of a dry deposition) and 2010 (DUNE-2-R1, -R2: simulation of 2 successive wet depositions) in the northwestern oligotrophic Mediterranean Sea. No changes in primary production (PP) and chlorophyll a concentration (Chl a) were observed after a dry deposition event while a wet deposition event resulted in a rapid (24 h after dust additions), strong (up 2.4 fold) and long (at least a week duration) increase in PP and Chl a. We show that in addition to being a source of dissolved inorganic phosphorus (DIP), simulated wet deposition events were also a significant source of NO3- (net increases up to +9.8 μM NO3- at 0.1 m depth) to the nutrient depleted surface waters due to cloud processes and mixing with anthropogenic species such as HNO3. The dry deposition event was shown to be a negligible source of NO3-. By transiently increasing DIP and NO3- concentrations in P-N starved surface waters, wet deposition of Saharan dust was able to relieve the potential N or NP co-limitation of the phytoplanktonic activity. Due to the higher input of NO3- relative to DIP, a wet deposition event resulted in a strong increase in the NO3-/DIP ratio from initially 3-/DIP ratio leading to a switch in the nutrient limitation of the phytoplanktonic activity

Sensitivity of Trichodesmium erythraeum and Crocosphaera watsonii abundance and N-2 fixation rates to varying NO3- and PO43- concentrations in batch cultures

Batch cultures of Trichodesmium erythraeum, strain IMS101, and Crocosphaera watsonii, strain WH8501, were grown under metal- and vitamin-replete conditions to evaluate differences in diazotroph abundance and N-2 fixation rates as well as biomass C:N:P ratios resulting from changes in the concentrations of nitrate (NO3-) and phosphate (PO43-) in culture media. Holding light levels and temperature constant, variations in culture NO3- and PO43- concentrations included (N:P ratios in mu M) 0:0.5, 5:1, 8:0.5, and 16:1. The abundance of both diazotrophs was greatest in the 16:1 and 5:1 N:P ratio treatments (i.e. those grown with 1 mu M PO43-) while the highest N-2 fixation rates for both diazotrophs were observed in the 0:0.5 treatment (i.e. those grown in NO3--free media). Measurable but reduced (similar to 25 to 50% of the rates in cultures grown with no NO3-) N-2 fixation rates were evident in both T. erythraeum and C. watsonii cultures grown with up to 16 mu M NO3-. These results indicate that while diazotrophs grown in the presence of NO3- have significantly lower N-2 fixation rates than those not chronically exposed to NO3-, these lower per cell N-2 fixation rates are compensated for by a greater abundance of diazotrophs in treatments with 1 mu M PO43- and result in comparable volume-integrated rates of N-2 fixation. Additionally, N-2 fixation rates for T. erythraeum and C. watsonii were comparable when normalized to carbon (biomass). Finally, the exponential-phase C:N:P biomass ratios of both diazotrophs were similar to each other as well as to previous studies and varied little among the treatments but increased, often significantly, between exponential and stationary growth phases

Simple approach for the preparation of 15− 15N2-enriched water for nitrogen fixation assessments: evaluation, application and recommendations

Recent findings revealed that the commonly used 15N2 tracer assay for the determination of dinitrogen (N2) fixation can underestimate the activity of aquatic N2-fixing organisms. Therefore, a modification to the method using pre-prepared 15−15N2-enriched water was proposed. Here, we present a rigorous assessment and outline a simple procedure for the preparation of 15−15N2-enriched water. We recommend to fill sterile-filtered water into serum bottles and to add 15−15N2 gas to the water in amounts exceeding the standard N2 solubility, followed by vigorous agitation (vortex mixing ≥ 5 min). Optionally, water can be degassed at low-pressure (≥950 mbar) for 10 min prior to the 15−15N2 gas addition to indirectly enhance the 15−15N2 concentration. This preparation of 15−15N2-enriched water can be done within 1 h using standard laboratory equipment. The final 15N-atom% excess was 5% after replacing 2–5% of the incubation volume with 15−15N2-enriched water. Notably, the addition of 15−15N2-enriched water can alter levels of trace elements in the incubation water due to the contact of 15−15N2-enriched water with glass, plastic and rubber ware. In our tests, levels of trace elements (Fe, P, Mn, Mo, Cu, Zn) increased by up to 0.1 nmol L−1 in the final incubation volume, which may bias rate measurements in regions where N2 fixation is limited by trace elements. For these regions, we tested an alternative way to enrich water with 15−15N2. The 15−15N2 was injected as a bubble directly to the incubation water, followed by gentle shaking. Immediately thereafter, the bubble was replaced with water to stop the 15−15N2 equilibration. This approach achieved a 15N-atom% excess of 6.6 ± 1.7% when adding 2 mL 15−15N2 per liter of incubation water. The herein presented methodological tests offer guidelines for the 15N2 tracer assay and thus, are crucial to circumvent methodological draw-backs for future N2 fixation assessments