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Nitrogen-fixation by cyanobacterial symbionts in the oceanic diatoms Hemiaulus mernbranaceus, H. hauckii, and H. sinensis was documented in the Southwestern Atlantic Ocean. All Herniaulus cells with diatom chlorophyll autofluorescence contained fluorescent symbionts undetectable by standard light microscopy. Average cell-specific ethylene reduction rates (2.3 to 5.3 X 10-l3 M ethylene cell h-') were 2.2 to 5.2 tlmes lower than calculated rates from RhizosoleniaRichelia blooms in the central Pacific gyre and 26 times lower than results from fichelid-containing Rhizosolenia cultures. Calculations suggest that the N2 fixation associated with Herniaulus is 21 to 4 5 times greater than that from the Rhizosolenia-Richelia association in the southwestern Atlantic Ocean. Hemiaulus is a common and ubiquitous diatom genus typically dominating or CO-dominating the diatom community after seasonal stratification in warm oligotrophic seas (Guillard & Kilham 1977). A puzzling aspect of its ecology is its persistence over large areas dominated by nanoand pico-plankton despite low ambient nutrients. Periodic blooms of this genus occur in the central Pacific gyre and have been associated with the nitrogen-fixing Rhizosolenia-Richelia symbiosis (Venrick 1974). Cyanobacterial symbionts usually identified as Richelia intracellularis Schmidt are also known in Hemiaulus; however, the identity of the symbiont is uncertain (Sundstrom 1984). The presence of a heterocyst suggests N2-fixation is likely in this symbiosis (Kimor et al. 1978, Heinbokel 1986), but NZ-fixation by Hemiaulus symbionts has never been examined. Current address: Environmental Sciences Program, University of Massachusetts, Boston Harbor Campus, Boston, Massachusetts 02125-3393, USA Although the host is frequently seen, the symbiont is rarely reported. Kimor et al. (1978) noted that up to 62 % of the Hemiaulus membranaceus Cleve collected off southern California contained symbionts, although more extensive collections offshore contained only 16.1 to 18.6 % symbiotic Hemiaulus. Heinbokel (1986) noted that the Hemiaulus symbionts were frequently visible only under epifluorescent illumination, and that ca 80 % (n = 668) of the Hemiaulus hauckii and H. m e m branaceus in samples north of Hawaii contained symbionts. He suggested that symbiont abundance, and their role in oceanic nitrogen fixation, may have been underestimated due to the need to use epifluorescence to reliably identify the symbiont. In this report, I document NZ-fixation by the Hemiaulus symbioses, and present some additional observations on the cryptic nature of the symbiont. Methods and materials. Hen~iaulus cells were isolated by micropipet from surface tows (30 cm net, 20 pm mesh) north of St. Johns, U.S. Virgin Islands (13 Feb 1991; 18O36.43' N, 65O27.49' W), and north of Haiti (15 Feb 1991; 2Oo40.15'N, 72"54.43'W). Hemiaulus cells (130 to 200 each) were placed into 2 m1 screwcap vials equipped with a Teflon lined septum and containing 1 m1 of 0.45 pm filtered seawater. Care was taken to exclude trichomes of Trichodesmium and cells of Richelia-containing Rhizosolenia from the vials. Curved chains of H. hauckii were present, but were not examined for acetylene reduction due to the difficulty in picking the chains with the micropipet. A blank of filtered seawater was run concurrently. Vials were incubated at 26°C and 306 pE m-' S-'. Two experiments were run with each net tow collection O Inter-Research/Printed in Germany 0171-8630/91/0076/0201/$ 03.00 202 Mar. Ecol. Prog. Ser. 76: 201-204, 1991 and consisted of one vial of H. membranaceus and one vial of combined H. hauckii and H. sinensis. Acetylene was injected (0.2 ml) into the headspace and subsequent ethylene generation was measured on a Shimadzu Mini 2 gas chromatograph. Ethylene concentrations were corrected for liquid-phase solubility, and successive time series points were corrected for the sample volume withdrawn in the previous assays. The small headspace volume (1 ml) precluded replication of time series samples. Rates of ethylene evolution were determined from the slope of the time-series data. Symbiont enumeration was performed on a Zeiss Axiomat microscope equipped with epifluorescence capabilities using freshly collected samples. Approximately 10 to 100 cells of each species (depending on its abundance) were examined from the same net tow as the acetylene reduction experiments. Results and conclusions. Ethylene evolution occurred in all experimental vials containing Hemiaulus. Rates of ethylene evolution were linear for a t least 4 h (Fig. l ) . H. membranaceus-containing vials evolved ethylene at rates of 2.3 and 3.6 x 10-l3 m01 ethylene cell-' h-', and H. hauckii-H. sinensjs evolved ethylene at 3.0 and 5.3 x 10-l3 m01 ethylene cell-' h-'. No symbionts were visible under brightfield or Nomarski illumination; however, under epifluorescence, all Hemiaulus spp. except the curved morph of H. hauckii contained 1 or 2 symbionts (Fig. 2). The curved H. hauckii (n = 30) showed no host chlorophyll autofluorescence, contained no symbionts, and appeared dead. These observations are similar to Heinbokel's (1986) observations of ca 8 0 % symbiont-containing Hemiaulus cells north of Hawaii. Although H. hauckii and H. sinensis were not examined individually for acetylene reduction, it is reasonable to assume the symbionts of both species were fixing nitrogen. Average host cell-specific rates of ethylene evolution for the

Nitrogen-fixation by the cyanobacterial symbiont of the diatom genus Hemiaulus