Factors limiting heterotrophic bacterial production in the southern Pacific Ocean

F Van Wambeke,G Alarcón,P Raimbault,C Guieu,T Moutin,S Bonnet

doi:10.5194/bg-5-833-2008

Abstract

Abstract. The role of potential factors limiting bacterial growth was investigated along vertical and longitudinal gradients across the South Eastern Pacific Gyre. The effects of glucose, nitrate, ammonium and phosphate additions on heterotrophic bacterial production (using leucine technique) were studied in parallel in unfiltered seawater samples incubated under natural daily irradiance. The enrichments realized on the subsurface showed three types of responses. From 141° W (Marquesas plateau) to approx 125° W, bacteria were not bottom-up controlled, as confirmed by the huge potential of growth in non-enriched seawater (median of enhancement factor×39 in 24 h). Within the Gyre (125° W–95° W), nitrogen alone stimulated leucine incorporation rates (median×4.2), but rapidly labile carbon (glucose) became a second limiting factor (median×37) when the two elements were added. Finally from the border of the gyre to the Chilean upwelling (95° W–73° W), labile carbon was the only factor stimulating heterotrophic bacterial production. Interaction between phytoplankton and heterotrophic bacterial communities and the direct versus indirect effect of iron and macronutrients on bacterial production were also investigated in four selected sites: two sites on the vicinity of the Marquesas plateau, the centre of the gyre and the Eastern border of the gyre. Both phytoplankton and heterotrophic bacteria were limited by availability of nitrogen within the gyre, but not by iron. Iron limited phytoplankton at Marquesas plateau and at the eastern border of the gyre. However 48 h enrichment experiments were not sufficient to show any clear limitation of heterotrophic bacteria within Marquesas plateau and showed a limitation of these organisms by labile carbon in the eastern border of the Gyre.

Highlights

Heterotrophic bacteria generally meet their energy and elemental needs from utilisation of organic matter, which includes essential elements like C, N, P and Fe
In oligotrophic environments, organic material devoted to bacterial growth is provided continuously through regenerating processes by higher size-class organisms, so that bottom up control is influenced by preying organisms
4.1 Abundance or production to track limitation?. Because both abundance and leucine incorporation rates have been followed, one question is arising: What is the best indicator for tracking factors limiting heterotrophic bacterial growth? The increase in leucine incorporation rates, when present, is either due to stimulation of a greater percentage of active population, or the stimulation of the specific growth rate of individual cells, or some combination of these two processes, whereas bacterial abundance is regulated by grazing when intact sea water is manipulated, and should increase more rapidly in pre-filtered samples

Summary

Introduction

Heterotrophic bacteria generally meet their energy and elemental needs from utilisation of organic matter, which includes essential elements like C, N, P and Fe. in oligotrophic environments, elemental needs are sometimes not satisfied only by utilization of organic matter and heterotrophic bacteria can compete with phytoplankton for mineral nutrients like N, P or Fe (Kirchman, 1994; Tortell et al, 1999; Thingstad, 2000). To examine factors limiting heterotrophic bacterial growth, seawater samples are generally amended with various components (organic molecules, macro nutrients, iron), alone or in combination. After 24– 48 h, some bacterial parameters are examined, the main one primarily being bacterial production (either with thymidine or leucine technique). Different elements have been shown to stimulate bacterial production : phosphorus in the Atlantic Ocean (Sargasso Sea: Cotner et al, 1997, Gulf of Mexico: Pomeroy et al, 1995) and in the Mediterranean Sea (Eastern: Zohary and Robarts, 1998; Thingstad et al, 2005; Western: Van Wambeke et al, 2002), nitrogen in the South West Pacific Ocean (French Polynesia: Torreton et al, 2000), labile organic carbon in the Equatorial and Subarctic Pacific (Kirchman, 1990; Kirchman and Rich, 1997), iron in the Southern ocean (Pakulski et al, 1996, Tortell et al, 1996).

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