Institut Méditerranéen d’Océanologie
Accueil du site > Nos résultats > Paru dans les médias > Publication MIO : Cryptic oxygen cycling in anoxic marine zones dans (...)

Publication MIO : Cryptic oxygen cycling in anoxic marine zones dans PNAS

Version imprimable de cet article

Auteurs : Emilio Garcia-Robledo, Cory C. Padilla, Montserrat Aldunate,, Frank J. Stewart, Osvaldo Ulloa, Aurélien Paulmier, Gerald Gregori (MIO) and Niels Peter Revsbech

Edited by David M. Karl, University of Hawaii, Honolulu, HI, and approved June 21, 2017 (received for review December 2, 2016

Significance

Anoxic marine zones (AMZs) create expansive habitats for microbes whose anaerobic metabolisms help drive global nutrient cycles, for example, by removing nitrogen from the oceans by producing N2 gas. AMZ cycles may also be shaped by oxygen intrusion from outside the AMZ, creating opportunities for aerobic microbial metabolisms. Here we show that aerobic processes in AMZs are linked to oxygen production within the anoxic zone. Oxygen is produced during daytime in a layer of photosynthetic cyanobacteria near the top of the AMZ and then rapidly consumed by aerobic processes without accumulating. Oxygen turnover and carbon fixation rates are comparable to those of microbial N2 production, suggesting an important role for internal oxygen cycling in AMZ transformations of matter and energy.

Abstract

Oxygen availability drives changes in microbial diversity and biogeochemical cycling between the aerobic surface layer and the anaerobic core in nitrite-rich anoxic marine zones (AMZs), which constitute huge oxygen-depleted regions in the tropical oceans. The current paradigm is that primary production and nitrification within the oxic surface layer fuel anaerobic processes in the anoxic core of AMZs, where 30–50% of global marine nitrogen loss takes place. Here we demonstrate that oxygenic photosynthesis in the secondary chlorophyll maximum (SCM) releases significant amounts of O2 to the otherwise anoxic environment. The SCM, commonly found within AMZs, was dominated by the picocyanobacteria Prochlorococcus spp. Free O2 levels in this layer were, however, undetectable by conventional techniques, reflecting a tight coupling between O2 production and consumption by aerobic processes under apparent anoxic conditions. Transcriptomic analysis of the microbial community in the seemingly anoxic SCM revealed the enhanced expression of genes for aerobic processes, such as nitrite oxidation. The rates of gross O2 production and carbon fixation in the SCM were found to be similar to those reported for nitrite oxidation, as well as for anaerobic dissimilatory nitrate reduction and sulfate reduction, suggesting a significant effect of local oxygenic photosynthesis on Pacific AMZ biogeochemical cycling.

Voir en ligne : http://www.pnas.org/content/114/31/8319