Burkill¹, Peter H., and Phillip Williamson², on behalf of AMBITION participants

¹Plymouth Marine Laboratory, Plymouth PL1 3DH, UK (now at Southampton Oceanography Centre, Southampton SO14 3ZH, UK), Tel: +44 1603 593111, Fax: +44 1603 507714, E-mail address: p.williamson@uea.ac.uk, and ²NERC/School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK

Analysing the microbial biodiversity of the Indian Ocean (AMBITION): cruise overview

Biogeochemical fluxes in the Arabian Sea are strongly influenced by autotrophic and heterotrophic microorganisms. The AMBITION research cruise was directed at elucidating the main players and processes in the surface mixed layer, the sub-surface chlorophyll maximum, and deeper oxygen-depleted waters. Sampling and experiments were carried out along a 5500 km transect, mostly along 67°E, from Victoria, Seychelles to Muscat, Oman during inter-monsoon conditions (30 Aug - 29 Sept 2001). As expected, there were strong south-to-north gradients of upper ocean nutrients (nitrate from <1 nmol litre^-1 to ~3 µmol litre^-1) and chlorophyll a (from 19 to 180 ng litre^-1), accompanied by major changes in primary production and microbial community structure. Characterisation techniques included microscopic sorting; HPLC pigment analyses; analytical flow cytometry; isotopic uptake (using ¹³C, ¹⁴C, ¹⁵N and ³⁵S); and analyses of genetic diversity and expression using 16S rDNA and rRNA probes, to identify major picoeukaryote taxa and specific metabolic capabilities (nitrogen fixation and methyl bromide utilisation). Microzooplankton biomass exceeded microphytoplankton biomass (>15μm) at most stations, whilst a high diversity of prokaryotic nitrogen-fixers were found throughout the transect. In addition to cyanobacteria closely related to Trichodesmium, such diazotrophs included Katagnymene spiralis; two groups of Synechococcus spp; phylotypes similar to uncultured γ-proteobacteria (previously described from the tropical Atlantic and Pacific); and others related to Vibrio/Klebsiella, Marichromatium pupuratum, Chlorobium, Desulfovibrio, and gram-positive anaerobes. AMBITION involved researchers from five institutions, supported by the NERC Marine & Freshwater Microbial Biodiversity thematic programme.