Biogeochemical tracers of the marine cyanobacterium Trichodesmium

Abstract

We examined the utility of several biogeochemical tracers for following the fate of the planktonic diazotrophic cyanobacterium Trichodesmium in the sea. The presence of a (CIO) fatty acid previously reported was observed in a culture of Trichodesmium but was not found in natural samples. This cyanobacterium had high concentrations of C14 and C16 acids, with lesser amounts of several saturated and unsaturated C18 fatty acids. This composition was similar to that of other marine cyanobacteria. The major hydrocarbon identified was the C17n-alkane, which was present in all samples from the five stations examined. Sterols common to algae and copepods were observed in many samples along with hopanoids representative of bacteria, suggesting a varied community structure in colonies collected from different stations. We found no unique taxonomic marker of Trichodesmium among the sterols. Measurements of the σ15N and σ13C in Trichodesmium samples from the SW Sargasso and NW Caribbean Seas averaged −0.4960 (range from −0.7 to −0.25960) and −12.9%0 (range from −15.2 to −11.9960), respectively, thus confirming previous observations that this cyanobacterial diazotroph has both the lowest σ15N and highest σ13C of any marine phytoplankter observed to date. A culture of Trichodesmium grown under diazotrophic conditions had a σ15N between −1.3 and −3.6960. Our results support the supposition that the relatively low σ15N and high σ13C values observed in suspended and sediment-trapped material from some tropical and subtropical seas result from substantial input of C and N by Trichodesmium.