Monounsaturated fatty acids and their oxidation products were quantified in surficial sediments (0–1 cm) dominated by sympagic (ice-associated) material released at the end of the ice melt collected ...in summer in central and eastern Baffin Bay during the 2016 GreenEdge campaign. Sympagic algae preservation towards bacterial mineralization was monitored based on intact and oxidized C16:1ω7 (palmitoleic) acid, and oxidation products of C18:1ω7 (vaccenic) and C16:1ω5 acids provided insights on the photooxidative and autoxidative alterations of bacteria present in these sediment samples. Preservation of sympagic algal material appeared to be highest at the stations that were relatively unaffected by copepod grazing and that contained strongly autoxidized (and thus inactive) bacteria. Analysis of sinking particles collected with a drifting trap showed an intense flux of highly photooxidized ice algae in early July that was dominated by Navicula spp. and associated with bacteria that had also been strongly altered by photooxidative processes. It is proposed that subsequent homolytic decomposition of the hydroperoxides resulting from this intense photooxidation may have driven the strong autoxidation of sympagic algae and bacteria observed in the sediments. The lack of colonization of sympagic material by active benthic bacteria observed at some of the stations investigated was attributed to its high content in deleterious autoxidative hydroperoxides and free fatty acids (reaching for example 107% and 22% of residual palmitoleic acid, respectively, at station 605).
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•Sediments and sinking particles from central and eastern Baffin Bay were analyzed.•Ice algae released at the end of the ice melt strongly dominate these sediments.•Highly photooxidized ice algae and bacteria sink in the water column in early July.•Preservation of ice algal material is well correlated with bacteria oxidation state.•Hydroperoxides and FFAs prevent ice algae colonization by benthic bacteria.
•Strong biotic and abiotic degradation of TOM on the Beaufort Shelf observed.•Autoxidation state of TOM appeared to correlate with salinity.•Contact of TOM with seawater induces ...autoxidation.•Photooxidation of TOM on land favors its autoxidation in seawater.•Summary of POM degradation in the Beaufort Sea proposed.
Sterols and their biotic and abiotic degradation products were quantified in suspended particulate matter (SPM) from surface waters in the Mackenzie River mouth to the Beaufort Sea shelf (Canadian Arctic). 24-Ethylcholesterol (sitosterol) and 24-methylcholesterol (campesterol) appeared to be extensively degraded by bacterial and especially autoxidative degradation in the samples. Degradation was most extensive in some samples from the outer boundaries of the plume, which exhibited much higher sitosterol/campesterol ratio values than previously observed in studies of the Beaufort Sea. The lack of reactivity of specific planktonic sterols such as cholesterol, 24-methylcholesta-5,22E-dien-3β-ol (epi-brassicasterol) and 24-methylenecholesterol and the good correlation between the abundances of sitosterol, campesterol and dehydroabietic acid (DHAA, a biomarker of Pinaceae resin) oxidation products allowed us to attribute the main origin of these two sterols to terrigenous vascular plants. A good correlation was observed between the extent of autoxidation and salinity, suggesting that the free radical oxidation is enhanced via contact with seawater. Laboratory incubation of Mackenzie River SPM in Milli-Q water and seawater confirmed this proposal. To explain the specific induction of autoxidation on vascular plant-derived material, a mechanism involving homolytic cleavage of photochemically produced hydroperoxides resulting from the senescence of higher plants on land is proposed. Cleavage could be catalyzed by redox-active metal ions released from SPM in the mixing zone of riverine water and marine water. The greatest extent of degradation observed at outer boundaries of the plume is attributed to preferential settling of lithic material relative to less dense higher plant debris increasing the proportion of highly degraded vascular plant material in the SPM. The results are important for this ecologically vulnerable region, where destabilization of permafrost by global warming is expected to increase the input of terrigenous C to coastal seas. Autoxidation, which until now has received little attention, plays a key role in the degradation of vascular plant-derived lipids in surface waters and should be taken into consideration during future studies of terrigenous organic matter degradation.
In a few recent studies, the action of a bacterial dioxygenase (10S-DOX) on palmitoleic acid was observed within some polar and estuarine settings. To add further mechanistic information regarding ...the action of this enzyme in marine settings, we measured a range of lipids (sterols, fatty acids and the chlorophyll phytyl side chain) and their biotic and abiotic degradation products in water samples collected in 2018 from two depths (5 m and 25 m) at the temperate oceanographic time series site L4, located in the western English Channel. Lipid distributions indicated a dominance of diatoms and copepods during the spring bloom, while a peak in dinoflagellate activity was evident in samples collected from late summer/autumn, both outcomes being consistent with taxonomic data reported previously for the same sampling site and interval. Monitoring of lipid oxidation products characteristic of different degradation pathways showed a relatively weak effect of photo- and autoxidation processes, with these acting mainly on the more reactive lipids (i.e. chlorophyll and polyunsaturated fatty acids). In contrast, monitoring of biotic degradation processes revealed significant quantities of 10S-hydroxyhexadec-8(E)-enoic acid in samples collected at the end of April (reaching 40% of the residual parent palmitoleic acid), attributed to the involvement of bacterial 10-dioxygenase (10S-DOX) activity during the spring bloom. We propose that this enzyme could be utilised by bacteria to detoxify free fatty acids released by wounded diatoms in the presence of copepods
•Lipid and their oxidation products were quantified in spm collected at the L4 station.•Abiotic degradation processes appeared to act mainly on chlorophyll and PUFAs.•High amounts of 10S-hydroxyhexadec-8(E)-enoic acid were observed in April samples.•A bacterial 10-dioxygenase (10S-DOX) activity seems to act during the spring bloom.•10S-DOX could be utilised by bacteria to detoxify FFAs released by wounded diatoms.
Rationale
Ferulic and p‐coumaric acids are important biological and structural components of plant cell walls and possess antioxidant and antimicrobial properties. These phenolic acids are widespread ...in environmental samples. However, when they are present at very low concentrations or in very complex lipid extracts, their identification and quantification can be challenging.
Methods
The electron ionization mass spectrometry (EI‐MS) fragmentation pathways of ferulic and p‐coumaric acid trimethylsilyl (TMS) derivatives were investigated. These pathways were deduced by (i) low‐energy collision‐induced dissociation (CID) gas chromatography (GC)/EI‐MS/MS, (ii) accurate mass measurement, and (iii) 13C labelling. These compounds were then characterized and quantified in multiple reaction monitoring (MRM) mode in total lipid extracts of deposited atmospheric particles using highly specific transitions based on the main fragmentation pathways elucidated.
Results
Low‐energy CID‐MS/MS analyses, accurate mass measurement and 13C labelling enabled us to elucidate EI‐MS fragmentations of ferulic and p‐coumaric acid TMS derivatives. Some specific fragmentations proved useful for subsequent characterization and quantification of these compounds. As an application of some of the described fragmentations, trace amounts of these phenolic acids were characterized and quantified in MRM mode in wet‐ and dry‐deposited atmospheric particles containing low proportions of organic matter.
Conclusions
EI‐MS fragmentations of ferulic and p‐coumaric acid TMS derivatives exhibit specific fragment ions that can be very useful for the quantification of trace amounts of both phenolic acids in environmental samples.
•Increasing salinity favored lipoxygenase induction in terrestrial higher plant debris.•Lipoxygenases initiated autoxidative degradation of terrestrial POM in estuaries.•Autoxidative degradation of ...terrestrial POM enhanced at low and high latitude.•In temperate zones autoxidative damage of terrestrial POM more limited.
There exists a substantial amount of research on abiotic (e.g. photochemical) degradation pertaining to organic matter (OM) in the marine realm. While recent research has shown its importance in the degradation of terrestrial particulate OM (TPOM), the mechanisms involved in the induction of autoxidation in estuaries remain unclear. In this study, we propose for the first time the involvement of lipoxygenase (LOX) activity in the induction of autoxidation in mixed waters. The observation of unusual profiles of palmitoleic acid oxidation products and the presence of jasmonic acid in suspended particulate matter (SPM) collected close to the Rhône River, as well as in samples from the Mackenzie and Amazon rivers, is attributed to strong LOX activity. We show the role played by salinity in the induction of this LOX activity and provide an explanation for the differences in estuarine autoxidation level. At high latitude, lower temperatures and irradiance favor photooxidative damage to higher plant debris and, consequently, hydroperoxide production. High hydroperoxide content strongly contributes to LOX activation in mixed waters. The high resulting LOX activity enhances alkoxyl radical production and thus autoxidation. On the contrary, at low latitude, photooxidative effects are limited, and riverine autoxidation is favored. The higher hydroperoxide content of TPOM may, as a consequence, thereby also contribute to a high level of LOX activity and autoxidation in estuaries. In temperate zones, land and riverine photooxidative and autoxidative damage is limited, unlike estuaries where we observed significant LOX-induced and autoxidative damage.
We determined, and compared, the abiotic stress state of algae and their attached bacterial communities following their release into the water column during the Arctic sea ice melt season using ...specific lipid markers that are characteristic of type II photo-oxidation processes, together with those associated with cis-trans isomerase and 10S-DOX-like lipoxygenase activity, which are indicative of salinity stress. More specifically, parent lipids and some of their oxidation products were quantified in sinking particles (from sediment traps) collected from the Beaufort Sea, Resolute Passage and the Davis Strait (Canadian Arctic) in 2009, 2012 and 2015, respectively. Our data show that salinity- and light-induced bacterial stress processes are temporally decoupled, with the former occurring at the beginning of ice melting and the latter observed during the final stages of ice melt and in subsequent open waters. The salinity-induced bacterial stress seen during the early stages of ice melting contrasts the healthy state of sea ice algae under the same hypersaline conditions. As sea ice melt progresses, brine channels become hyposaline, thus reducing the bacterial salinity stress, while ice algae become susceptible to photodegradation by singlet oxygen (1O2), despite relatively low temperatures and irradiance. However, the presence of high amounts of extracellular polymeric substances (EPS) encapsulating the algae likely prevents the diffusion of 1O2 to the attached bacteria. Photo-oxidative stress is enhanced further in open waters due to the low amounts of EPS. The lower amounts of active bacteria in the Arctic compared to temperate or tropical regions is thus attributed to the combined action of efficient salinity stress in spring and photo-oxidative stress in summer. Outcomes from this study also imply that surface sediments from the Canadian Arctic contain relatively high quantities of sea ice-derived organic matter that has undergone a strong degree of salinity-induced stress.
•Salinity and light bacterial stresses are temporally decoupled during ice melting.•10S-DOX and CTI activity, two relevant biomarkers of bacterial salinity stress•Seafloor sediment organic matter seems almost exclusively composed of sea ice biota.
Comparison of Type II photosensitized oxidation of lipids (the photodynamic effect) and photodegradation of chlorophyll (sensitizer photobleaching) in samples of particulate matter collected ...previously from locations representing a diverse range of latitudes reveals an enhancement of the photooxidation of lipids at the expense of chlorophyll photodegradation in the polar regions. The efficiency of the photodynamic effect appears to be particularly high in sinking particles collected under sea ice and is attributed to the rapid settling of highly aggregated sympagic algae to depths of low light transmission favouring the photodynamic effect at the expense of photobleaching of the sensitizer. Paradoxically, the low efficiency of Type II photosensitized oxidation of lipids observed in temperate and equatorial regions is associated with high solar irradiances in these regions. Type II photosensitized oxidation of lipids in senescent phytoplankton seems thus to be strongly dependent of the intensity of solar irradiance.
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•Photodynamic effect efficiency in POM is compared at different latitudes.•Efficiency of lipid photooxidation is low in temperate and equatorial regions.•Type II photosensitized oxidation of lipids is highest in polar regions.•Photodynamic effect is particularly efficient in particles sinking under sea ice.•Photooxidation of lipids in senescent phytoplankton is favoured at low irradiance.
In some previous studies, the ratio between a di-unsaturated highly branched isoprenoid (HBI) lipid termed IPSO25 and a structurally related tri-unsaturated counterpart (HBI III) (viz. IPSO25/HBI ...III) has been used as a proxy measure of variable sea ice cover in the Antarctic owing to their production by certain sea ice algae and open water diatoms, respectively. To investigate this further, we quantified selected lipids and their photo- and autoxidation products in samples of suspended particulate matter (SPM) collected at different water depths in the polynya region west of the Dalton Iceberg Tongue (East Antarctica). The results obtained confirm the high efficiency of photo- and autoxidation processes in diatoms from the region. The systematic increase of the ratio IPSO25/HBI III with water depth in the current samples appeared to be dependent on the sampling site and was due to both (i) a relatively higher contribution of ice algae to the deeper samples resulting from their increased aggregation and therefore higher sinking rate, or (ii) a stronger abiotic degradation of HBI III during settling through the water column. Analyses of samples taken from the water-sediment interface and some underlying near-surface sediments revealed a further increase of the ratio IPSO25/HBI III, indicative of further differential oxidation of the more unsaturated HBI. Unfortunately, specific oxidation products of HBI III could not be detected in the strongly oxidized SPM and sediment samples, likely due to their lability towards further oxidation. In contrast, oxidation products of HBI III were detected in weakly oxidized samples of phytoplanktonic cells collected from Commonwealth Bay (also East Antarctica), thus providing more direct evidence for the involvement of photo- and/or autoxidation of HBI III in the region. This oxidative alteration of the ratio IPSO25/HBI III between their source and sedimentary environments might need to be considered more carefully when using this parameter for palaeo sea ice reconstruction purposes in the Antarctic.
•Evidence of abiotic degradation of various lipids in SPM samples from East Antarctica.•Photo- and autoxidative degradation processes acted more intensively in diatoms.•Oxidation products of HBI triene could be detected in weakly oxidized samples.•Increasing IPSO25/HBI triene ratio can result from preferential triene oxidation.
During sea-ice melt in the Arctic, primary production by sympagic (sea-ice) algae can be exported efficiently to the seabed if sinking rates are rapid and activities of associated heterotrophic ...bacteria are limited. Salinity stress due to melting ice has been suggested to account for such low bacterial activity. We further tested this hypothesis by analyzing samples of sea ice and sinking particles collected from May 18 to June 29, 2016, in western Baffin Bay as part of the Green Edge project. We applied a method not previously used in polar regions—quantitative PCR coupled to the propidium monoazide DNA-binding method—to evaluate the viability of bacteria associated with sympagic and sinking algae. We also measured cis-trans isomerase activity, known to indicate rapid bacterial response to salinity stress in culture studies, as well as free fatty acids known to be produced by algae as bactericidal compounds. The viability of sympagic-associated bacteria was strong in May (only approximately 10% mortality of total bacteria) and weaker in June (average mortality of 43%; maximum of 75%), with instances of elevated mortality in sinking particle samples across the time series (up to 72%). Short-term stress reflected by cis-trans isomerase activity was observed only in samples of sinking particles collected early in the time series. Following snow melt, however, and saturating levels of photosynthetically active radiation in June, we observed enhanced ice-algal production of bactericidal compounds (free palmitoleic acid; up to 4.8 mg L–1). We thus suggest that protection of sinking sympagic material from bacterial degradation early in a melt season results from low bacterial activity due to salinity stress, while later in the season, algal production of bactericidal compounds induces bacterial mortality. A succession of bacterial stressors during Arctic ice melt helps to explain the efficient export of sea-ice algal material to the seabed.
Photodegradation is a natural process that strongly affects the chromophoric fraction of dissolved organic matter (DOM), especially in surface water of the oceans. In the euphotic zone, the ...concentration and quality of DOM are mostly dependent on primary production by phytoplankton. The effect of photodegradation on algal DOM has not been investigated as much as on terrestrial DOM. In this study, we explored the effect of different spectral regions (i.e., full sun spectrum, visible light, 295–800 nm, 305–800 nm, and 320–800 nm) on algal exudates by Emiliania huxleyi, a ubiquitous coccolithophore. The optical properties (absorption and fluorescence) of algal DOM were investigated before and after irradiation with the different spectral regions. The absorption and fluorescence spectra were compared before and after irradiation. The results showed an increase in the effect of photobleaching with increasing irradiation energy for all of the absorbance indices. Similarly, the protein-like fluorescence decreased at increasing irradiation energy. The humic-like fluorescence, which was the most affected, did not show a linear trend between photobleaching and irradiation energy, which suggested that irradiation mainly determined a change in these molecules’ quantum yield.
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