Abstract
Degradation of palmitoleic acid (C16:1ω7), the main fatty acid component of sea ice-associated (sympagic) diatoms, was monitored in Arctic sea ice at the beginning of ice melting and in the underlying sinking particles and superficial bottom sediments. In sea ice, degradation of sympagic algae involved biotic oxidation induced by 10S-DOX-like lipoxygenase of unknown salinity-stressed attached bacteria, while photo- and autoxidation were limited. In the water column, strong hydratase and Z/E isomerase activity were observed. Hydration of unsaturated fatty acids seems to be a detoxification strategy, which is essential for bacterial survival when associated with free fatty acid-rich environments such as ice algae. In contrast, Z/E isomerisation of palmitoleic acid was attributed to the release of Fe2+ ions during radical-induced damage of the active site of the bacterial 10S-DOX-like lipoxygenase and Z/E isomerases. Due to the poor physiological state of their attached bacteria resulting from salinity stress in brine channels or toxicity of free ice algae fatty acids, sympagic algae appeared to be only very weakly biotically degraded within the water column. In bottom sediments, free radicals resulting from 10S-DOX-like lipoxygenase activity induced a strong autoxidation of the ice algal material. The presence in bottom sediments of a significant proportion of oxidation products resulting from 10S-DOX-like lipoxygenase activity attested to the strong contribution of sea ice-derived OM released during the early stages of ice melt prior to deposition in the sediments. However, on the basis of the highest fatty acid photooxidation state observed in these sediments, an additional contribution of highly photooxidized material (ice algal material released at the end of ice melting or open water phytoplankton) seems likely. The degradation of hydroperoxides, resulting from biotic and abiotic degradation of palmitoleic acid, appeared to involve: (i) homolytic cleavage of the peroxyl group affording the corresponding hydroxy- and oxoacids, (ii) reduction to the corresponding hydroxyacids by peroxygenases, (iii) heterolytic proton-catalysed cleavage and (iv) conversion to allylic 1,4-diols by diol synthases and hydroperoxide isomerases.
Highlights
The thinning and retreat of Arctic sea ice, which is one of the most striking consequences of recent climate change, will likely have a significant impact on Arctic ecosystem functioning in the future (Wassmann et al, 2011)
Similar profiles of palmitoleic acid oxidation products were observed previously in other sea ice samples (Amiraux et al, submitted for publication) and in sinking particles underlying melting sea ice (Amiraux et al, 2017), in the Canadian Arctic, and attributed to the involvement of a specific bacterial enzymatic process under the hypersaline conditions found in brine channels
We detected an intense 10S-DOX-like lipoxygenase activity in sea ice, attributed to the involvement of a specific bacterial enzymatic process under the hypersaline conditions found in brine channels during the first stages of ice melting
Summary
The thinning and retreat of Arctic sea ice, which is one of the most striking consequences of recent climate change, will likely have a significant impact on Arctic ecosystem functioning in the future (Wassmann et al, 2011). A climate-change-mediated shift in primary producers would impact the structure and function of the sea floor community, which is strongly dependent upon the deposition of organic material from the overlying water column for its energy requirements (McMahon et al, 2006). Under the effect of global warming the carbon sink potential of ice algae (resulting from their strong aggregation and the stress state of their associated bacteria, should be gradually replaced by the carbon source potential of open water phytoplankton (weakly aggregated and mineralized before the bottom) (Amiraux et al, 2017). Determining the fate of sympagic (ice-associated) derived particulate organic matter (POM) following its release from sea ice during spring melting is an important research objective (Tedesco and Fettweis, 2012), but partitioning processes such as growth of the water column phytoplankton community, grazing, remineralization and export is complicated and currently not well constrained. The material that is not grazed or re-mineralized during its descent through the water column, can feed the benthos (Boetius et al, 2013) or be stored in sediments
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