C 5 glycolipids of heterocystous cyanobacteria track symbiont abundance in the diatom Hemiaulus hauckii across the tropical North Atlantic

Diatom–diazotroph associations (DDAs) include marine heterocystous cyanobacteria found as exosymbionts and endosymbionts in multiple diatom species. Heterocysts are the site of N₂ fixation and have thickened cell walls containing unique heterocyst glycolipids which maintain a low oxygen environment within the heterocyst. The endosymbiotic cyanobacterium <i>Richelia intracellularis</i> found in species of the diatom genus <i>Hemiaulus</i> and <i>Rhizosolenia</i> makes heterocyst glycolipids (HGs) which are composed of C₃₀ and C₃₂ diols and triols with pentose (C₅) moieties that are distinct from limnetic cyanobacterial HGs with predominantly hexose (C₆) moieties. Here we applied a method for analysis of intact polar lipids to the study of HGs in suspended particulate matter (SPM) and surface sediment from across the tropical North Atlantic. The study focused on the Amazon plume region, where DDAs are documented to form extensive surface blooms, in order to examine the utility of C₅ HGs as markers for DDAs as well as their transportation to underlying sediments. C₃₀ and C₃₂ triols with C₅ pentose moieties were detected in both marine SPM and surface sediments. We found a significant correlation between the water column concentration of these long-chain C₅ HGs and DDA symbiont counts. In particular, the concentrations of both the C₅ HGs (1-(O-ribose)-3,27,29-triacontanetriol (C₅ HG₃₀ triol) and 1-(O-ribose)-3,29,31-dotriacontanetriol (C₅ HG₃₂ triol)) in SPM exhibited a significant correlation with the number of <i>Hemiaulus hauckii</i> symbionts. This result strengthens the idea that long-chain C₅ HGs can be applied as biomarkers for marine endosymbiotic heterocystous cyanobacteria. The presence of the same C₅ HGs in surface sediment provides evidence that they are effectively transported to the sediment and hence have potential as biomarkers for studies of the contribution of DDAs to the paleo-marine N cycle.

Abstract licence: CC BY

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