* Corresponding author. Present address. Freshwater Biological Laboratory, University of Copenhagen, 51 Helsing+rsgade, DK-3400 Hiller+d, Denmark. Fax: #45-48-24-14-76. E-mail address: bkb9457@vip.cybercity.dk (L. Riemann)  Present Address: Monterey Bay Aquarium Research Institute, P.O. Box 628, Moss Landing, CA 95039-0628, USA. Deep-Sea Research II 46 (1999) 1791}1811 Bacterial community composition during two consecutive NE Monsoon periods in the Arabian Sea studied by denaturing gradient gel electrophoresis (DGGE) of rRNA genes Lasse Riemann*, Grieg F. Steward, Laura B. Fandino, Lisa Campbell, Michael R. Landry, Farooq Azam Marine Biology Research Division, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0202, USA Department of Oceanography, Texas A&M University, College Station, TX 77843-3146, USA Department of Oceanography, University of Hawaii at Manoa, Honululu, HI 96822, USA Received 9 September 1998; received in revised form 18 December 1998; accepted 8 January 1999 Abstract Horizontal and vertical variations in bacterial community composition were examined in samples collected during two Joint Global Ocean Flux Study (JGOFS) Arabian Sea cruises in 1995. The cruises, 11 months apart, took place during two consecutive NE Monsoon periods (January and December). Bacteria were harvested by "ltration from samples collected in the mixed layer, mid-water, and deep sea at stations across the study area. Total bacterial community genomic DNA was analyzed by PCR ampli"cation of 16S rRNA gene fragments, followed by denaturing gradient gel electrophoresis (DGGE). In total, 20 DGGE bands re#ecting unique or varying phylotypes were excised, cloned and sequenced. Amplicons were dominated by bacterial groups commonly found in oceanic waters (e.g., the SAR11 cluster of -Proteobacteria and cyanobacteria), but surprisingly none of the sequenced amplicons were related to -Proteobacteria or to members of the Cytophaga-Flavobacter-Bacteroides phylum. Amplicons related to magnetotactic bacteria were found for the "rst time in pelagic oceanic waters. The DGGE banding patterns revealed a dominance of +15 distinguishable amplicons 0967-0645/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 9 9 ) 0 0 0 4 4 - 2