Biogeochemical tracers of the marine cyanobacterium Trichodesmium

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 (C-10) fatty acid previously reported was observed in a culture of Trichodesmium but was not found in natural samples. This cyanobacterium had high concentrations of C-14 and C-16 acids, with lesser amounts of several saturated and unsaturated C-18 fatty acids. This composition was similar to that of other marine cyanobacteria. The major hydrocarbon identified was the C-17 n-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 delta(15)N and delta(13)C in Trichodesmium samples from the SW Sargasso and NW Caribbean Seas averaged -0.4 parts per thousand (range from -0.7 to -0.25 parts per thousand) and -12.9 parts per thousand (range from -15.2 to -11.9 parts per thousand), respectively, thus confirming previous observations that this cyanobacterial diazotroph has both the lowest delta(15)N and highest delta(13)C of any marine phytoplankter observed to date. A culture of Trichodesmium grown under diazotrophic conditions had a delta(15)N between -1.3 and -3.6 parts per thousand. Our results support the supposition that the relatively low delta(15)N and high delta(13)C values observed in suspended and sediment-trapped material from some tropical and subtropical seas result from substantial input of C and N by Trichodesmium. (C) 1997 Elsevier Science Ltd.