Krill Excretion Boosts Microbial Activity in the Southern Ocean

Javier Arístegui,Juan L. Gómez-Pinchetti,Isabel Reche,Carlos M. Duarte,Kerrie Swadling

doi:10.1371/journal.pone.0089391

Abstract

Antarctic krill are known to release large amounts of inorganic and organic nutrients to the water column. Here we test the role of krill excretion of dissolved products in stimulating heterotrophic bacteria on the basis of three experiments where ammonium and organic excretory products released by krill were added to bacterial assemblages, free of grazers. Our results demonstrate that the addition of krill excretion products (but not of ammonium alone), at levels expected in krill swarms, greatly stimulates bacteria resulting in an order-of-magnitude increase in growth and production. Furthermore, they suggest that bacterial growth rate in the Southern Ocean is suppressed well below their potential by resource limitation. Enhanced bacterial activity in the presence of krill, which are major sources of DOC in the Southern Ocean, would further increase recycling processes associated with krill activity, resulting in highly efficient krill-bacterial recycling that should be conducive to stimulating periods of high primary productivity in the Southern Ocean.

Highlights

Antarctic krill (Euphasia superba), with a biomass estimated at 379 million tonnes [1], is one of the most abundant animals on Earth and the central node of the Antarctic food web, supporting the large biomass of megafauna characteristic of this ecosystem [2,3]
The results of our experiments demonstrated very large effects of krill excretion products on bacterial growth, production and respiration, in spite of the variability in the initial conditions of the experiments
Direct ammonium inputs produced moderate increases in bacterial production and respiration rates, which were non significant compared to controls (Tables 2–4)

Summary

Introduction

Antarctic krill (Euphasia superba), with a biomass estimated at 379 million tonnes [1], is one of the most abundant animals on Earth and the central node of the Antarctic food web, supporting the large biomass of megafauna characteristic of this ecosystem [2,3]. Smetacek [7] hypothesized that krill activity may stimulate phytoplankton growth, thereby conditioning the ecosystem to maintain high productivity. Tovar-Sanchez et al [8] provided additional evidence for this notion by showing that krill release large amounts of limiting elements, including Fe, P and N, in the Southern Ocean, thereby creating the conditions to support subsequent algal blooms. Recent evidence shows that in addition to releasing large amounts of inorganic nutrients [8], krill are an important source of organic materials to the water column, both particulate (faecal pellets), which may sink out of the mixed layer rapidly [13,14,15], and dissolved [16,17]. The release of large amounts of dissolved organic matter by krill activity may stimulate bacterial communities in the Southern Ocean, further contributing to the role of krill in accelerating recycling processes in the ecosystem

Methods

Results

Conclusion