Tracing carbon and nitrogen incorporation and pathways in the microbial community of a photic subtidal sand

V Evrard,B Veuger,M Huettel,Plm Cook,Jj Middelburg

doi:10.3354/ame01248

Abstract

The pathways and fate of C and N in the microbial compartment of a coastal permeable sandy sediment in the photic zone were studied in a pulse-chase experiment. 13 C-bicarbonate and 15 N-nitrate were added to the water column on top of 5 sediment cores incubated in 4 transparent and 1 opaque laboratory chambers. After 9 h of labelling in the light and dark, stable isotope incorpora- tion by microphytobenthos (MPB) and bacteria was quantified over a period of 4 d through the analy- sis of phospholipid-derived fatty acids (PLFA) and hydrolysable amino acids (HAA). 13 C was fixed by MPB, and more than 50% was directed to the production of extracellular polymeric substance (EPS). MPB 15 N incorporation was similar in the dark and in the light. Bacterial activity appeared to have 2 effects that depended on sediment depth: at the sediment surface, there was a steady increase of label in the bacterial PLFA, suggesting metabolism of label-containing EPS; in the subsurface layers, uniform enrichment started immediately after the labelling procedure, indicating continuous incor- poration of inorganic C by chemoautotrophic bacteria (Cbac). This experiment demonstrates the efficient transfer of inorganic C and N to the benthic community through the activities of photo- and chemoautotrophic microorganisms, and the role of EPS as a carrier of energy to the benthic microbial food web.

Highlights

Coastal sediments have traditionally been considered donor-controlled ecosystems in which supplied detritus of variable quality is mineralised
D + L-alanine (L-Ala is a stable constituent of the hydrolysable amino acids (HAA) pool of all organisms), as the 13C or 15N D/L-Ala incorporation ratio (13C or 15N incorporation in DAla/13C or 15N incorporation in D + L-Ala) to obtain an MPB composition, biomass and production
13C and 15N incorporation into total hydrolysable amino acids (THAA) was assessed for the 0–1 cm layer (Fig. 5a). 13C and 15N incorporation into THAA followed similar dynamics, but was much lower than incorporation into the bulk sediment (Fig. 4a,b)

Summary

Introduction

Coastal sediments have traditionally been considered donor-controlled ecosystems in which supplied detritus of variable quality is mineralised. During the last 2 decades, there has been a growing awareness that primary production by microphytobenthos (MPB) can contribute significantly to organic C production (MacIntyre et al 1996) and moderate sediment C flows in the coastal zone (Middelburg et al 2000). This key role of MPB in coastal primary production and benthic C flows has been documented in intertidal flats that receive abundant light during exposure (MacIntyre et al 1996, Underwood & Kromkamp 1999). Gattuso et al (2006) suggested that net benthic primary production may take place on more than 33% of the global shelf area This implies that the contribution of MPB to the global C cycle may have been underestimated until now. D/L-Ala incorporation ratios for 13C and 15N were well above the racemisation background value of ~0.017 (representing D-Ala produced by racemisation during sample hydrolysis), implying a high contribution of bacteria to label incorporation (Fig. 5b)

Methods

Results

Conclusion