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Influence of dinoflagellate diurnal vertical migrations on dimethylsulfoniopropionate and dimethylsulfide distribution and dynamics (St. Lawrence Estuary, Canada)1 Anissa Merzouk, Maurice Levasseur, Michael Scarratt, Sonia Michaud, and Michel Gosselin
Abstract: The influence of the diurnal vertical migration of the dinoflagellates Alexandrium tamarense and Scrippsiella trochoidea on dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) dynamics was studied during a 34-h Lagrangian experiment in the St. Lawrence Estuary in July 2000. Particulate DMSP (DMSPp), dissolved DMSP (DMSPd), and DMS exhibited diel patterns with minimum concentrations during the night and maximum concentrations around noon. DMSPp concentrations were correlated with the abundance of the two vertically migrating DMSP-rich dinoflagellates. The DMSPp:Chl a ratio exhibited similar diel variations, suggesting a light-induced de novo DMSP synthesis during the day. Diel variations of the DMS:Chl a ratio suggest that the accumulation of DMS around noon resulted from physiological responses of the algae and (or) bacteria to light. Biological gross DMS production and bacterial DMS consumption were decoupled, leading to rapid fluctuations in DMS. These results show that in systems dominated by DMSP-rich dinoflagellates containing DMSP lyases, DMS concentrations may vary by as much as a factor of 10 over a 24-h period. Such diel variations must be considered when estimating the contribution of such systems to the DMS sea to air flux. Résumé : Nous avons étudié l’influence de la migration verticale journalière des dinoflagellés Alexandrium tomarense et Scrippsiella trochoidea sur la dynamique du déméthylsulfoniopropionate (DMSP) et du sulfure de diméthyle (DMS) dans une expérience de type lagrangien de 34 h dans l’estuaire du Saint-Laurent en juillet 2000. Le DMSP particulaire (DMSPp), le DMSP dissous (DMSPd) et le DMS suivent tous des patterns journaliers avec des concentrations minimales durant la nuit et des concentrations maximales vers midi. Les concentrations de DMSPp sont en corrélation avec l’abondance des deux dinoflagellés à migration verticale qui sont riches en DMSP. Le rapport DMSPp:Chl a suit des variations journalières semblables, ce qui indique une néosynthèse de DMSP induite par la lumière durant le jour. Les variations journalières du rapport DMS:Chl a indiquent que l’accumulation de DMS vers midi résulte de réactions physiologiques des algues et (ou) des bactéries à la lumière. La production biologique brute de DMS et la consommation bactérienne de DMS sont déphasées, ce qui entraîne des fluctuations rapides de DMS. Ces résultats démontrent que dans les systèmes dominés par des dinoflagellés riches en DMSP et contenant des DMSP lyases, les concentrations de DMS peuvent varier par un facteur allant jusqu’à 10 au cours d’une période 24 h. On doit tenir compte de ces variations journalières lorsqu’on estime la contribution de tels systèmes au flux de DMS de la mer à l’atmosphère. [Traduit par la Rédaction]
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Introduction The production of the climatically active gas dimethylsulfide (DMS) in the ocean is controlled by several biological processes acting on different time scales. At mid- and
high latitudes, maximum DMS concentrations are generally measured during the summer when the plankton community is dominated by strong producers of dimethylsulfoniopropionate (DMSP) (the algal precursor of DMS) and by a rapid turnover rate of the algal biomass (Kwint and Kramer 1996;
Received 14 March 2003. Accepted 13 March 2004. Published on the NRC Research Press Web site at http://cjfas.nrc.ca on 22 June 2004. J17404 A. Merzouk2 and M. Levasseur. Université Laval, Département de biologie (Québec-Océan), Pavillon Alexandre-Vachon, Québec, QC G1K 7P4, Canada. M. Scarratt and S. Michaud. Fisheries and Oceans Canada, Maurice Lamontagne Institute, 850 Route de la mer, Mont-Joli, QC G5H 3Z4, Canada. M. Gosselin. Institut des sciences de la mer à Rimouski (ISMER), Université du Québec à Rimouski, 310 Allée des Ursulines, Rimouski, QC G5L 3A1, Canada. 1
This paper is part of the proceedings of the Third International Symposium on Biological and Environmental Chemistry of DMS(P) and Related Compounds, held in Rimouski (Québec), 26–28 September 2002. 2 Corresponding author (e-mail:
[email protected]). Can. J. Fish. Aquat. Sci. 61: 712–720 (2004)
doi: 10.1139/F04-066
© 2004 NRC Canada
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Dacey et al. 1998; Kettle et al. 1999). The summer period is also marked by important variations in DMS concentrations on time scales of several days, with DMS peaks coinciding with the collapse of algal blooms and the release of DMSP in the environment (Matrai and Keller 1993; Dacey et al. 1998; Van Duyl et al. 1998). Recent studies have also reported large fluctuations in DMSP and DMS concentrations on hourly time scales. Causes for these rapid variations are not well understood but there are indications that they can result from vertical migration of dinoflagellates, physiological responses to the light regime, and impact of light on bacterial utilization of DMSP and DMS. The diurnal vertical migration of dinoflagellates may generate short-term (hourly) variations in particulate DMSP (DMSPp) concentrations. Some phytoplankton species, and dinoflagellates in particular, can perform diurnal vertical migrations, allowing them to efficiently use nutrients at depth during the night and solar radiation close to the surface during the day (Margalef 1978; Cullen and McIntyre 1998). Dinoflagellates are known for their high DMSP content (Keller et al. 1989), and short-term variations in the concentrations of DMSPp in surface waters of the Mediterranean Sea have been previously associated with vertical migration of dinoflagellates (Belviso et al. 2000). Several dinoflagellate species also possess DMSP lyase, the enzyme responsible for the cleavage of DMSP into DMS and acrylate. Consequently, dinoflagellates are often associated with high DMSP lyase activity (Steinke et al. 2002) and high DMS concentrations (Scarratt et al. 2002) in the ocean. Diel variations in the location of the dinoflagellates in the water column may thus result in important changes in both DMSP and DMS pool size. DMSP and DMS variations on hourly time scales may also reflect physiological responses of the algae to the environment. During a 24-h on-deck microcosm experiment, Simó et al. (2002) showed that DMSP synthesis by phytoplankton varied in parallel with primary production and reached a maximum at noon, suggesting that DMSP biosynthesis is a diurnal process coupled to photosynthesis. A close coupling between DMSP production and primary production has also been observed in the field (Archer et al. 2001; Simó et al. 2002). This coupling may result from the physiological properties of DMSP. It has been proposed that DMSP synthesis could act as an overflow mechanism allowing the dissipation of energy and reduced sulfur in high light but nutrient-limiting conditions (Stefels 2000). More recently, the increase in DMSP generally measured in microalgae submitted to harmful levels of UV radiation or nutrient limitation was attributed to the potential antioxidant properties of DMSP (Sunda et al. 2002). Since DMS shares the same antioxidant property as DMSP, DMSP lyase containing algae may produce more DMS under stressing conditions (Sunda et al. 2002). Diel variations in the intensity of solar radiation (and particularly UV) may thus have indirect effects on seawater DMSP and DMS concentrations by controlling algal DMSP and DMS synthesis. Short-term variations in DMS concentrations may also result from variations in bacterial DMS production and consumption. Bacteria are thought to be responsible for most of the DMSP cleavage into DMS in the oceans. Bacteria use DMSP as a carbon or a sulfur source and their uptake of
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DMSP may vary rapidly depending on substrate availability and their physiological state (Kiene et al. 2000). Because some bacteria also consume DMS (Kiene 1992; Kiene and Bates 1990), the net bacterial production of DMS reflects the balance between the two processes, which are generally positively correlated (Simó et al. 1995; Wolfe et al. 1999). Simó and Pedros-Alió (1999) demonstrated for the first time that bacterial DMSP and DMS consumption may vary during the day. During a 24-h on-deck microcosm experiment using water from the North Atlantic, they measured decreases in bacterial DMSP consumption at dawn and DMS consumption in the afternoon and at night. Causes for these variations were not identified and the demonstration of their occurrence in situ is still lacking. Each year, blooms of the dinoflagellates Alexandrium tamarense and Scrippsiella trochoidea co-occur in the St. Lawrence Estuary during the summer months. Previous studies have shown elevated DMSP quotas (Keller et al. 1989; Levasseur et al. 1995) and presence of DMSP lyases (Niki et al. 2000; Wolfe et al. 2002) in strains of these two species. In July 2000, a cruise was conducted in the lower St. Lawrence Estuary with the main objective of determining the capacity of A. tamarense to perform vertical migration (J. Fauchot, Institut des Sciences de la Mer de Rimouski, Université du Québec à Rimouski, 310 Allée des Ursulines, Rimouski, QC G5L 3A1, Canada, unpublished data). We took this opportunity to acquire information on the influence of the diurnal migration of these DMSP-rich dinoflagellates on the distribution and cycling of DMSP and DMS.
Material and methods The study was conducted aboard the CCGS Martha L. Black in the lower St. Lawrence Estuary (Fig. 1) on 8 and 9 July 2000. A water mass with high concentrations of A. tamarense and S. trochoidea was localized through a series of short rosette casts. The water mass was then marked with a drifting buoy and followed for 34 h during calm conditions (see wind speeds, Fig. 2d). Samples were collected every 2 h near the drifting buoy from an inflatable boat (Zodiac) launched from the main ship (stations T0–T16, Fig. 1). The inflatable boat was used to minimize mixing of the water mass during sampling. Vertical profiles of temperature, salinity, and in vivo Chl a fluorescence were obtained for all stations with a conductivity–temperature–depth (CTD) meter and fluorometer. Water was collected with a 5-L Niskin bottle at the first subsurface in vivo fluorescence maximum (between 2 and 6 m). This sampling depth corresponded to the maximum abundance of dinoflagellates in the mixed layer. The water collected for DMS(P) concentrations and rates determinations was taken directly from the Niskin bottle and prescreened on 202-µm Nitex mesh to remove large grazers. For DMS(P) concentration determination, water samples were gently transferred into 66-mL brown polyethylene bottles, leaving no headspace. For the incubations, larger volumes (10 L) of water were collected every 4 h and transferred gently into 10-L insulated containers, leaving no headspace. Water samples were also collected for nutrients analyses and phytoplankton species identification. © 2004 NRC Canada
714 Fig. 1. Map of the lower St. Lawrence Estuary with the trajectory of the drifting buoy and the locations of the sampling stations.
Can. J. Fish. Aquat. Sci. Vol. 61, 2004
and Taylor 1993). Two unamended incubation bottles were immediately used for time zero values for both the control and the MBE treatment. The remaining bottles (four unamended controls and four containing MBE) were incubated in the dark in a thermostatic bath within 2.4 °C of in situ temperature. After 2 and 4 hours, duplicate bottles of both control and MBE treatments were gently mixed, gravity filtered, and analysed for DMSPp, DMSPd, and DMS concentrations, as described above. DMS production rates were determined from linear regressions of DMS concentrations versus time. The rates of bacterial DMS consumption were obtained from the difference in DMS production rates in the bottles with inhibitor (gross DMS production) and without inhibitor (net DMS production).
Results Phytoplankton abundances Phytoplankton identification and enumeration were performed on samples preserved with acid Lugol using a settling column and inverted microscope. Total phytoplankton species were identified and enumerated for the water samples used for rate determinations, i.e., every 4 h at the subsurface fluorescence maximum. DMS(P) concentrations DMSPp, dissolved DMSP (DMSPd), and DMS concentrations were determined for all stations by gravity filtering 66mL water samples through 47-mm Whatman GF/F filters (0.7 µm). For DMS samples, 24-mL serum bottles were filled with filtrate leaving no headspace, sealed with butyl septa, and crimped with aluminium seals. DMS samples were analysed within minutes of sampling. For DMSPd samples, 24-mL serum bottles were filled with 23 mL of filtrate and 1 mL of 10 mol·L–1 KOH solution, and sealed. For DMSPp samples, the filters were put in 24-mL serum bottles containing 23 mL of deionized water and 1 mL of 10 mol·L–1 KOH solution and sealed. All DMSP samples were stored at 4 °C in the dark and analysed within 4 weeks of collection. Sulfur gas analyses were performed using a purge and trap system coupled to a Varian 3400 gas chromatograph (see Scarratt et al. (2000) for details). DMSP samples were calibrated with a 5 ng DMSP·mL–1 standard prepared in 14-mL serum bottles containing 0.6 mL of 10 mol·L–1 KOH solution. DMS samples were calibrated against 46- to 912-µL injections of helium containing approximately 12 ng DMS·mL–1, prepared using a permeation tube (KinTek). The quantification limits of the method were 0.04 nmol·L–1 for DMS and DMSPd and 0.008 nmol·L–1 for DMSPp. DMS production and consumption rates DMS production and bacterial DMS consumption rates were determined every 4 h during on-deck dark incubations using water from the subsurface fluorescence maximum. Ten incubation bottles (66-mL brown polyethylene bottles with screwcaps), prerinsed with HCl, were filled with seawater, leaving no headspace. Four bottles were amended with methyl butyl ether (MBE) (30 µmol·L–1 final concentration), a specific inhibitor of bacterial DMS consumption (Visscher
Oceanographic setting The Lagrangian experiment was characterized by sunny weather (Fig. 2a) and very low wind speeds on 8 July (< 2 m·s–1) with a gradual increase on 9 July (Fig. 2b). The physical and biochemical characteristics of the water column during the study were typical of summer conditions for the St. Lawrence Estuary (Levasseur et al. 1984; Therriault and Levasseur 1985). The surface mixed layer, best delimited by the depth of the thermocline (Fig. 2d), varied between 10 and 15 m and was characterized by less saline (27–29 psu) and warmer (10–12 °C) water (Figs. 2c and 2d). In the upper 15 m of the water column, macronutrients concentrations were low (NO3 < 0.3 µmol·L–1, PO4 < 0.3 µmol·L–1, and SiO4 < 1.5 µmol·L–1; data not shown) and Chl a concentrations calculated from fluorescence were lower than 15 mg·m–3 (Fig. 2e). A sharp, deep chlorophyll maximum was observed at the thermocline (Fig. 2e). Dinoflagellates, DMS(P) concentrations, and DMS(P):Chl a ratios Throughout the time series, the phytoplankton community was dominated by diatoms (36–64% of total phytoplankton cell abundances), with flagellate and dinoflagellate contributions ranging from 24% to 40% and from 4% to 12% of the total cell abundances, respectively (data not shown). Most of the diatoms were concentrated in the deep fluorescence maximum located at the thermocline at approximately 10–15 m. The dinoflagellate assemblage was dominated by the species A. tamarense, S. trochoidea, and Heterocapsa triquetra, which were found mostly in the mixed layer where they formed a distinct subsurface fluorescence peak during the day, always shallower than the deep fluorescence maximum. The three dinoflagellate species exhibited diurnal vertical migrations during the sampling period (J. Fauchot, Institut des Sciences de la Mer à Rimouski, Université du Québec à Rimouski, 310 Allée des Ursulines, Rimouski, QC G5L 3A1, Canada, unpublished data). Their concentrations were low during the night in the upper mixed layer, increased during the morning to reach 60 000 cells·L–1 at 1000, and decreased more or less gradually during the afternoon to reach very low concentrations again during the night (Fig. 3a). DMSPp concentrations varied between 4 and 122 nmol·L–1 and exhibited a diel pattern with maximum values measured © 2004 NRC Canada
Merzouk et al. Fig. 2. Solar irradiance, wind speed, and structure of the water column during the Lagrangian experiment: (a) solar irradiance; (b) wind speed; (c) salinity (psu); (d) temperature (°C); (e) Chl a concentration (mg·m–3) and sampling depths (circles). Grey areas represent the night.
during the day (Fig. 3b). DMSPd concentrations varied between 2 and 11 nmol·L–1 and exhibited diel variations on 8 July with the highest concentrations during the day (Fig. 3c). DMS concentrations ranged from 0.6 to 6.2 nmol·L–1 and exhibited important short-term variations (Fig. 3d). DMS concentrations were below 2 nmol·L–1 during the night, increased during the morning and reached 4.5 nmol·L–1 around noon, decreased during the afternoon to reach 0.5 nmol·L–1 at 1800, and increased again to reach 4 nmol·L–1 during the evening. The evening peak in DMS concentrations was followed by a rapid decrease during the night. Among all phytoplankton species present, DMSPp concentrations exhibited significant correlations only with the
715 Fig. 3. Temporal variations of dinoflagellates abundance and DMS(P) pools at the surface (
