Abstract
Ocean deoxygenation driven by global warming and eutrophication is a primary concern for marine life. Resistant animals may be present in dead zone sediments, however there is lack of information on their diversity and metabolism. Here we combined geochemistry, microscopy, and RNA-seq for estimating taxonomy and functionality of micrometazoans along an oxygen gradient in the largest dead zone in the world. Nematodes are metabolically active at oxygen concentrations below 1.8 µmol L−1, and their diversity and community structure are different between low oxygen areas. This is likely due to toxic hydrogen sulfide and its potential to be oxidized by oxygen or nitrate. Zooplankton resting stages dominate the metazoan community, and these populations possibly use cytochrome c oxidase as an oxygen sensor to exit dormancy. Our study sheds light on mechanisms of animal adaptation to extreme environments. These biological resources can be essential for recolonization of dead zones when oxygen conditions improve.
Highlights
Ocean deoxygenation driven by global warming and eutrophication is a primary concern for marine life
Scarce water circulation and high rates of degradation can eventually lead to water column hypoxia (≤63 μmol O2 L−1 or ≤2 mg O2 L−1) and anoxia[3]
Oceanic models anticipate a global decrease in the total oxygen inventory of up to 7% by 2100, with a number of oxygen minimum zones (OMZs) losing more than 4% oxygen per decade[5]
Summary
Ocean deoxygenation driven by global warming and eutrophication is a primary concern for marine life. Can persist for hundreds to thousands of years as in the case of certain stagnant bottom water of enclosed seas such as the Baltic and Black Seas[3,7] In these systems, bottom water close to the seafloors is regularly characterized by very low oxygen (≤22 μmol O2 L−1), which precludes life to most animals[7]. Bottom water close to the seafloors is regularly characterized by very low oxygen (≤22 μmol O2 L−1), which precludes life to most animals[7] These marine systems characterized by severe hypoxia or anoxia are often referred as dead zones[7]. Adaptation and community responses of benthic organisms to oxygen starvation have only recently started to be investigated[6,17], and the mechanism through which they survive long-term anoxia is one of the most intriguing questions in marine ecology
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