B. Moriceau 1, i, 1, O. Ragueneau 2, M. Garvey , U. Passow 1 1
1 C-gruppe, Alfred Wegener Institut, Bremerhaven, Germany
2,i UMR 6539, IUEM, Technopôle Brest-Iroise, 29280 Plouzané, France
i
Actual address
Résumé : A la fin d’un bloom, de nombreuses diatomées sédimentent sous la forme d’agrégats. L’agrégation des diatomées influence le recyclage de la silice biogénique (BSiO2) dans les eaux océaniques de surface, la sédimentation et la préservation de la BSiO2 sur le plancher océanique, car les diatomées agrégées sédimentent rapidement le long de la colonne d’eau, ce qui laisse peu de temps à la dissolution. De plus, l’agrégation a un impact direct sur la vitesse de dissolution de la BSiO2. Les expériences en laboratoire présentées dans cette étude, ont exploré l’influence de l’agrégation sur la vitesse de dissolution de la BSiO2. Des agrégats monospécifiques ont été formés à partir de trois espèces différentes de diatomées, des Chaetoceros decipiens, des Skeletonema costatum, et des Thalassiosira weissflogii. Les vitesses de dissolution de la BSiO2 des cellules de diatomées de la même culture ont été mesurées pour des cellules agrégées et libres, puis comparées. Des paramètres généralement associés à la dissolution de la BSiO2 ont été mesurés en parallèle: les concentrations en silice dissoute (DSi) au voisinage des cellules, la viabilité (rapport entre le nombre de cellules vivantes et de cellules mortes), la quantité de particules d’exopolymère transparentes (Tolosa et al.) et les concentrations bactériennes. Les vitesses de dissolution initiales des frustules de diatomées étaient significativement plus faibles pour les cellules agrégées (4.6 an-1) que pour les cellules libres (14 an-1). Les vitesses de dissolution plus lentes de la BSiO2 des agrégats ont été attribuées (1) aux concentrations élevées en DSi dans les agrégats (9 and 230 µM)
comparativement au milieu environnant les cellules libres, (2) à une plus forte viabilité des cellules agrégées et (3) à un nombre de bactéries par diatomées plus faible dans les agrégats. Les variations des vitesses de dissolution entre les différents agrégats sont expliquées par les concentrations en TEP variables en fonction des agrégats.
Evidence for reduced biogenic silica dissolution rates in
diatom aggregates
B. Moriceau 1, i, 2, O. Ragueneau 2, M. Garvey , U. Passow 1 1
1 C-gruppe, Alfred Wegener Institut, Bremerhaven, Germany
2,i UMR 6539, IUEM, Technopôle Brest-Iroise, 29280 Plouzané, France
i
Actual address
Abstract- At the end of a bloom, diatoms settle as large aggregates. Aggregation of diatoms influences the recycling of silica in the upper ocean, and the sedimentation and preservation of biogenic silica (BSi) at the seafloor, because aggregated diatoms sink rapidly through the water column, leaving little time for dissolution. Additionally, aggregation may directly impact dissolution rates of opal. Data of laboratory studies investigating the influence of aggregation on BSi dissolution rates are represented in this study. Monospecific aggregates of three different species of diatoms, Chaetoceros decipiens, Skeletonema costatum, and Thalassiosira weissflogii .were produced in the laboratory. Biogenic silica (BSi) dissolution rates of aggregates and freely suspended diatoms from the same batch culture were measured and compared. Variables associated with dissolution were measured in parallel: dissolved silicon (DSi) concentration in the vicinity of cells, viability (ratio of living cells to dead cells), the amount of transparent exopolymer particles (TEP) and bacterial concentration. Initial dissolution rates of diatom frustules were significantly lower for aggregated diatoms (4.6 yr-1) than for freely suspended diatoms (14 yr-1). Lower dissolution rates in aggregates were attributed (1) to elevated DSi concentrations (9 and 230 µM), (2) to a higher viability of cells and (3) to a lower number of bacteria per diatom. Variations in dissolution rates among different aggregates were accredited to varying amounts of TEP per aggregate.
KEY WORDS: Diatom aggregate – Biogenic silica dissolution – Silica recycling – Marine snow
INTRODUCTION
In many areas of the ocean, diatoms are the most important primary producers: Firstly they commonly form the basis of productive food webs (Silver et al. 1978, Cushing 1989, Thornton 2002) and secondly, diatoms are responsible for a large fraction of the vertical export of carbon (Goldmann 1993, Dugdale et al. 1995, Nelson et al. 1995, Ragueneau et al. 2000, Ragueneau et al. 2001, Armstrong et al. 2002, Thornton 2002, Sanchez-Vidal et al. in press).
One characteristic of diatoms is their requirement for dissolved silicon (DSi) to form their frustules. Recycling of biogenic silica within the mixed layer is the most important source of silicic acid in the open ocean (Dugdale et al. 1995, Nelson et al. 1995, Ragueneau et al. 2000). On average, 50% of the biogenic silica produced in the euphotic zone dissolves in the first 100 m of the water column (Nelson et al. 1995, Ragueneau et al. 2000, Ragueneau et al. 2002). BSi recycling depends on biological variables including bacterial number and activity (Kamatani 1982, Patrick et Holding 1985, Bidle et Azam 1999, 2001), on chemical variables like DSi concentrations (Van Cappellen et Qiu 1997a, 1997b) and on physical variables like temperature (Kamatani 1982, Bidle et al. 2002). These variables individually have a direct effect but can also interact to affect BSi dissolution. The recycling of the BSi will have different impacts depending on where it occurs. In the euphotic layer the DSi produced by BSi recycling is available for renewed growth, whereas recycling below the euphotic zone strengthens the silica pump and the DSi is lost for new production, except when transported back to the euphotic zone by physical processes.
A second characteristic of diatom cells is their excretion of dissolved polysaccharides that form transparent exopolymer particles (TEP). TEP increase the stickiness of cells which then aggregate (Alldredge et al. 1993, Passow et al. 1994, Passow et Alldredge 1995). Whereas most individual cells are recycled within the euphotic zone, two processes are known to rapidly export
diatom cells out of the upper layer: aggregation, which increases the sinking velocity of diatoms by two orders of magnitude (Alldredge et Gotschalk 1988, Alldredge et Silver 1988) and production of faecal pellets by zooplankton such as copepods. The sinking velocity of silica-rich faecal pellets is 30-fold faster than that of a single cell (12-77 m d-1 Wassmann et al. 1999, Turner 2002). Punctual observations of fresh organic matter and diatoms lying on the seafloor at a depth of more than 4000 m (Graf 1992, describe in Smith et al. 1996, review in Thornton 2002), underline the existence of a rapid vertical transport of diatom blooms. The rapid transport of diatoms to depths transfers the BSi recycling below the euphotic zone strengthening the silica pump described by Dugdale et al. (1995) and also allows fresh material reach the sediment and impact for the benthic biology and geochemical cycles (Ragueneau et al. 2002).
Within the European Program ORFOIS (ORigin and Fate of biogenic particle fluxes in the Ocean and their Interactions with the atmospheric CO2 concentration as well as the marine Sediment), our goal is to improve parameterization of particle fluxes through the water column, focusing on the Si component of these fluxes. Very little is known about the silica cycle below the euphotic zone as in situ observations are difficult in this zone. Determination of the dissolution rate below the euphotic zone needs to take into account that fast sinking diatom frustules at this depth are enclosed in aggregates or faecal pellets.
Schultes et al. (submitted) and Gallinari et al. (in prep) have measured the BSi dissolution of diatom frustules embedded in copepod faecal pellets. These experiments show that dissolution rate of diatom cells are reduced if cells are enclosed in intact faecal pellets. First measurements of BSi dissolution rates of aggregated versus non-aggregated diatoms indicated that aggregation also decreased BSi dissolution rates, but results were ambiguous, as continuous aggregation and fragmentation obscured results (Passow et al. 2003). Here we present unequivocal results of laboratory measurements of BSi dissolution of aggregated compared to non-aggregated cells under otherwise identical conditions. Furthermore, we address the role of TEP concentration,
bacterial abundance and DSi concentration in the vicinity of the diatoms as well as the viability of diatoms in BSi dissolution rates.