Microbial food webs and metabolic state across oligotrophic waters of the Mediterranean Sea during summer

Biogeosciences, 8, 1839–1852, 2011 www.biogeosciences.net/8/1839/2011/ doi:10.5194/bg-8-1839-2011 © Author(s) 2011. CC Attribution 3.0 License. Biogeosciences Microbial food webs and metabolic state across oligotrophic waters of the Mediterranean Sea during summer U. Christaki1 , F. Van Wambeke2 , D. Lefevre2 , A. Lagaria1,3 , L. Prieur4,8 , M. Pujo-Pay5,6 , J.-D. Grattepanche1 , J. Colombet7 , S. Psarra3 , J. R. Dolan4,8 , T. Sime-Ngando7 , P. Conan5,6 , M. G. Weinbauer4,8 , and T. Moutin9 1 INSU-CNRS, UMR8187 LOG, Laboratoire d’Océanologie et des Géosciences, Université Lille Nord de France, ULCO, 32 avenue Foch, 62930 Wimereux, France 2 INSU-CNRS, UMR6117, LMGEM, Laboratoire de Microbiologie, Géochimie et Ecologie Marines, Université de la Méditerranée, Centre d’Océanologie de Marseille, Campus de Luminy Case 901, 13 288 Marseille cedex 9, France 3 Hellenic Centre for Marine Research, Institute of Oceanography, 71003 Heraklion, Crete, Greece 4 INSU-CNRS, UMR7093 LOV, Laboratoire d’Océanographie de Villefranche, Observatoire Océanologique de Villefranche-sur-Mer, 06 238 Villefranche-sur-Mer, France 5 INSU-CNRS, UMR7621, Lab. d’Océanographie Microbienne, Observatoire Océanologique, 66651 Banyuls/mer, France 6 UPMC Univ Paris 06, UMR7621, Laboratoire d’Océanographie Microbienne, Observatoire Océanologique, 66651 Banyuls/mer, France 7 INSU-CNRS, UMR6023, LMGE, Laboratoire Microorganismes: Génome et Environnement – 63177 Aubière cedex, France 8 UPMC Univ Paris 06, UMR7093, Laboratoire d’Océanographie de Villefranche, Observatoire Océanologique de Villefranche-sur-Mer, 06230 Villefranche-sur-Mer, France 9 INSU-CNRS, UMR6535, LOPB, Laboratoire d’Océanographie physique et biogéochimique Université de la Méditerranée, Centre d’Océanologie de Marseille, Campus de Luminy Case 901, 13 288 Marseille cedex 9, France Received: 6 December 2010 – Published in Biogeosciences Discuss.: 10 January 2011 Revised: 8 June 2011 – Accepted: 10 June 2011 – Published: 12 July 2011 Abstract. The abundance and activity of the major members of the heterotrophic microbial community – from viruses to ciliates – were studied along a longitudinal transect across the Mediterranean Sea in the summer of 2008. The Mediterranean Sea is characterized by a west to east gradient of deepening of DCM (deep chlorophyll maximum) and increasing oligotrophy reflected in gradients of biomass and production. However, within this well documented longitudinal trend, hydrological mesoscale features exist and likely influence microbial dynamics. Here we present data from a W-E transect of 17 stations during the period of summer stratification. Along the transect the production and fate of organic matter was investigated at three selected sites each one located in the centre of an anticyclonic eddy: in the Algero-Provencal Basin (St. A), the Ionian Basin (St. B), and the Levantine Basin (St. C). The 3 geographically distant eddies showed low values of the different heterotrophic compartments of the microbial food web, and except for viruses in site C, all integrated (0–150 m) stocks were higher in reference stations loCorrespondence to: U. Christaki () cated in the same basin outside the eddies. During our study the 3 eddies showed equilibrium between GPP (Gross Primary Production) and DCR (Dark Community Respiration). Integrated PPp (Particulate Primary Production) values at A, B and C varied from ∼140 to ∼190 mg C m−2 . 1 Introduction The Mediterranean Sea is one of the most oligotrophic marine systems in the world. The basin-wide cyclonic circulation of nutrient-depleted water (Dugdale and Wilkerson, 1988), hot, dry climate and low land run-off contribute to the low productivity of the sea. The Mediterranean also exhibits a marked west to east gradient of oligotrophy seen in an increasing nutrient depletion from west to east (Krom et al., 1991), declines in chlorophyll concentrations (Ignatiades et al., 2009) and rates of primary production (Moutin and Raimbault, 2002; Turley et al., 2000). The hypothesis of phosphorus limitation of primary production in the Mediterranean has inspired numerous studies dealing with microbial processes in its open waters, and resulted in the Published by Copernicus Publications on behalf of the European Geosciences Union. 1840 U. Christaki et al.: Microbial food webs and metabolic state across oligotrophic waters establishment of large-scale patterns of abundance and activity for different planktonic food web components (reviewed in Siokou et al., 2010). The Mediterranean is generally considered to be an oligotrophic ecosystem characterized by a microbe-dominated food web. The dominance of small heterotrophs and small phototrophs in this region is consistent with a scenario of little energy transfer to high trophic levels (cf. review by Siokou et al., 2010). Thus, microbial heterotrophic activity is an important energy pathway in the planktonic food web and in particular in the eastern Mediterranean, where most of the organic carbon produced is consumed and respired (Regaudie-de-Gioux et al., 2009). Up to 90–95 % of primary production is sustained by internal recycling of organic matter during the stratified period (Moutin and Raimbault, 2002). Turley et al., (2000) hypothesised that a large portion of primary production is directly channeled to heterotrophic prokaryotes through exudation and/orFig.1 lysis of nutrient-stressed phytoplankton. Probably the most convincing evidence of P limitation of heterotrophic prokaryotes resulted from the CYCLOPS in-situ P-fertilization experiment conducted in May 2002 in the Cyprus-Eddy. In this experiment, prokaryotic heterotrophic production increased in response to P addition whereas phytoplankton biomass diminished (Thingstad et al., 2005). Nonetheless, the Mediterranean Sea while often attributed the label “oligotrophic”, shows considerable variability over a wide range of temporal and spatial scales. This variability is reflected in the microbial components of the planktonic food web. For example, in the west the Almeria-Oran front is an area of high primary production (Videau et al., 1994; Van Wambeke et al., 2004) compared to surrounding waters, while the Cyprus-Eddy in the east, is a zone of low phytoplankton production (Psarra et al., 2005). Our study was conducted within the framework of the BOUM cruise (Biogeochemistry from Oligotrophic to the Ultra-oligotrophic Mediterranean) in the summer of 2008. The first objective was a complete description of the microbial food web, and in particular the heterotrophic components, along a W-E transect of 17 stations of the Mediterranean Sea during the period of summer water-column stratification. Our second objective was to estimate rates of the production and fate of organic matter, in 3 geographically distant oligotrophic environments located at the centre of anticyclonic eddies. The cores of persistent eddies are relatively isolated from surrounding waters, th (...truncated)