Seasonal monitoring of lipid degradation processes in the western English Channel links bacterial 10S-DOX enzyme activity to free fatty acid production by phytoplankton

Abstract

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