Deep-&n Research I, Vol. 44, No. 1, pp. 27-38, 1997 0 1997 Ekvier Science Ltd Pergamon PII: s0967-0637(96)ooo91-x Printed in Grea~Britain. All rights reserved 0967-0637/97$17.00+0.00 Biogeochemical tracers of the marine cyanobacterium Trichodesmium EDWARD J. CARPENTER,* H. RODGER HARVEY,? BRIAN FRYI and DOUGLAS G. CAPONE? (Received 18 April 1995; in revisedform 6 February 1996; accepted 9 September 1996) 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 (C,,) fatty acid previously reported was observed in a culture of Trichodesmium but was not found in natural samples. This cyanobacterium had high concentrations of Cl4 and C16acids, with lesser amounts of several saturated and unsaturated C,s fatty acids. This composition was similar to that of other marine cyanobacteria. The major hydrocarbon identified was the Cl7 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 615N and 613C in Trichodesmium samples from the SW Sargasso and NW Caribbean Seas averaged -0.4% (range from -0.7 to -0.25%0) and - 12.9% (range from - 15.2 to - 11.9%), respectively, thus confirming previous observations that this cyanobacterial diazotroph has both the lowest 6”N and highest 613C of any marine phytoplankter observed to date. A culture of Trichodesmium grown under diazotrophic conditions had a 6”N between - 1.3 and - 3.6%. Our results support the supposition that the relatively low 615N and high 613C values observed in suspended and sediment-trapped material from some tropical and subtropical seas result from substantial input of C and N by Trichodesmium. 0 1997 Elsevier Science Ltd. All rights reserved INTRODUCTION The planktonic marine cyanobacterium Trichodesmium spp. is an important component of tropical and subtropical seas. It is abundant in the plankton and introduces new nitrogen to the euphotic zone via N2 fixation (Carpenter, 1983). In open waters of the tropical Atlantic Ocean it is a major source of new N, possibly exceeding that of N03-flux across the thermocline (Carpenter and Romans, 1991). In the Caribbean Sea it constitutes about 60% of the total phytoplankton standing stock in the water column (Carpenter and Price, 1977), and can be important in biogeochemical cycles in the Pacific (Karl et al., 1995) and Indian Oceans as well (Capone et al., submitted). Thus, while it appears to be a major component of * Marine Sciences Research Center, State University of New York, Stony Brook, New York, NY 11794-5000, U.S.A. t Chesapeake Biological Laboratory, University of Maryland System, Center for Environmental and Estuarine Studies, Solomons, MD 20688, U.S.A. $ Biology Department, Florida International University, University Park, Miami, FL 33199, U.S.A. 27