Abstract
Abstract. 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 Richelia intracellularis found in species of the diatom genus Hemiaulus and Rhizosolenia 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 Hemiaulus hauckii 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.
Highlights
Cyanobacteria are cosmopolitan oxygenic photoautotrophs that play an important role in the global carbon and nitrogen cycles
Temperature was uniformly high across the cruise track (> 27 ◦C in the euphotic zone) with the 25 ◦C isotherm deepening along the cruise track (Fig. 3a)
This was the only species for which there was a correlation with C5 HG32 triol but not C5 HG30 triol. This diatom–diazotroph associations (DDA) has not been cultured and nothing is known about the heterocyst lipid composition of this species. These results suggest C5 HG30 triol may be synthesized by this species
Summary
Cyanobacteria are cosmopolitan oxygenic photoautotrophs that play an important role in the global carbon and nitrogen cycles. Free-living heterocystous cyanobacteria are rare in the open ocean (Staal et al, 2003); heterocystous taxa are abundant as both exosymbionts and endosymbionts in diatoms (Foster et al, 2011; Gómez et al, 2005; Luo et al, 2012; Villareal, 1991; Villareal et al, 2011, 2012).
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