Study of the fate of bacterial production in mudflat : importance  and role within the benthic food web

Pierre-Yves PASCAL

University of La Rochelle
Avenue Michel Crépeau
17042 La Rochelle
France

PhD 2004-2007

Research advisor:

Nathalie NIQUIL: Centre de Recherche sur les Ecosystèmes Littoraux Anthropisés – UMR6217 - Unité Mixte de recherche CNRS, IFREMER, Université de La Rochelle ; Avenue Michel Crépeau 17042 La Rochelle France

Scientific advisor:

Chistine DUPUY: Centre de Recherche sur les Ecosystèmes Littoraux Anthropisés – UMR6217 - Unité Mixte de recherche CNRS, IFREMER, Université de La Rochelle ; Avenue Michel Crépeau 17042 La Rochelle France

Pierre RICHARD: Centre de Recherche sur les Ecosystèmes Littoraux Anthropisés – UMR6217 - Unité Mixte de recherche CNRS, IFREMER, Université de La Rochelle ; Place du Séminaire B.P. 5 17137 L'Houmeau France

This work is funded by Conseil Général de Charente-Maritime (17), the programme ECCO, the programme PNEC – Littoral Atlantique and the programme ANR – VASIREMI

Benthic bacteria - mudflat - tracer – food web - stable isotope - grazing – foraminifera – nematode – Hydrobia ulvae – inverse modelling – Marennes-Oléron

         The development and application of improved methods for measuring bacterial abundance and production have radically changed the perception of bacteria compartment in pelagic marine systems. Bacteria are known to play a role in the degradation of organic matter and the regeneration of nutrients. But it’s now admitted that an important part of primary production is consumed by higher trophic levels through the bacteria. The “microbial loop” model (e.g. Azam et al. 1983) consider bacteria as a “link” more than a “sink”, increasing the rate of primary production available for higher trophic levels. Therefore, in pelagic foodweb models developed, bacteria play frequently a major role (e.g. Vézina & Savenkoff 1999) .

            In marine soft sediments, bacterial abundance are relatively constant at around 10⁹ bacteria.ml^-1 porewater (Schmidt et al. 1998) , being thousand time more abundant than in pelagic systems. Moreover, high rates of production (Alkemade et al. 1992) have been measured in aquatic sediments (e.g. van Duyl & Kop 1990) . Those points have driven to a debate: is benthic bacterial production a carbon sink or link in benthic food webs? Due to technical limitations, studies dealing with benthic bacterivory are not developed as pelagic ones. Furthermore, results of this studies varies greatly (Kemp 1990) so it is hard to draw general conclusions about the importance of grazing as a fate for benthic bacterial production.

 - Elaborate a new method to quantify ingestion of bacteria by meiofauna (foramifera and nematodes) and macrofauna (Hydrobia ulvae). This technique is based on the pre-enrichment with stable isotope of benthic bacteria in culture medium. Those bacteria are then put in contact with their potential consumers. Evolution of gazers isotopic composition is linked with an ingestion of enriched bacteria and gives access to a bacterial ingestion rate.

- Determine bacterial ingestion rate of meiofauna and macrofauna in experimental conditions according to various biotic and abiotic factors.

- Determine bacterial ingestion rate of meiofauna and macrofaune in situ in the Brouage mudflat during the year 2006.

- Integrate data concerning the benthic bacterial compartment in an inverse model of the benthic food web in the Brouage mudflat (Leguerrier et al. 2003, Degré et al. 2006)

1 ) Sampling in Brouage muflat  ; 2) Ammonia tepida the most common species of foraminifera

Rossignol L, Dupuy C, Pascal PY, Debenay J-P (In press) Hydrobia ulvae: a deposit feeder for cleaning living hard-shelled foraminifera. Journal of Foraminiferal Research

Pascal PY, Dupuy C, Mallet C, Richard P, Niquil N. (In prep). ¹⁵N enriched bacteria method to quantify the bacterivory of benthic organism in sediment. Environmental microbiology.

Pascal PY, Niquil N, Dupuy C, Richard P,  Mallet C, Debenay J P Bacterivory of foraminifera (Ammonia Tepida) in intertidal mudflat according to various biotic and abiotic factors. ASLO summer meeting, 4-9 June 2006, Victoria (British Columbia, Canada)

Pascal P Y, The bacterial film, Festival of the very short film of scientific popularization, 2005, La Rochelle. Jury award, public award and high school award.

Alkemade R, Wielemaker A, de Jong SA, Sandee AJJ (1992) Experimental evidence for the role of bioturbation by the marine nematode Diplolaimella bruciei in stimulating the mineralization of Spartina anglica detritus. Marine Ecology Progress Series 90:149-155

Azam F, Fenchel T, Field JG, Gray J, Meyer-Reil LA, Thingstad F (1983) The ecological role of bacteria in the sea. Marine Ecology Progress Series 10:257-263

Degré D, Leguerrier D, Armynot du Chatelet E, Rzeznik-Orignac J, Auget J-C, Dupuy C, Marquis E, Fichet D, Struski C, Joyeux E, Sauriau P-G, Niquil N (2006) Comparative analysis of the food webs of two intertidal mudflats during two seasons using inverse modelling : Aiguillon Cove and Brouage Mudflat, France. Estuarine and Coastal Shelf Science 69:107-124

Kemp PF (1990) The fate of benthic bacterial production. Review Aquatic Science 2:109-124

Leguerrier D, Niquil N, Boileau N, Rzeznik J, Sauriau PG, Le Moine O, Bacher C (2003) Numerical analysis of the food web of an intertidal mudflat ecosystem on the Atlantic coast of France. Marine Ecology Progress Series 246:17-37

Schmidt JL, Deming JW, Jumars PA, Keil RG (1998) Constancy of bacterial abundance in surficial marine sediments. Limnology and oceanography 43:976-982

van Duyl FC, Kop AJ (1990) Seasonal patterns of bacterial production and biomass in intertidal sediments of the western Dutch Wadden Sea. Marine Ecology Progress Series 59:249-261

Vézina AF, Savenkoff C (1999) Inverse modeling of carbon and nitrogen flows in the pelagic food web of the  northest subartic Pacific. Deep-sea research II 46:2909-2939