Assaying picoplankton distribution by flow cytometry of underway samples collected along a meridional transect across the Atlantic Ocean

Abstract

The composition of picoplankton in near-surface samples collected underway at least every 6 h from a ship sailing from the Falkland Islands to the British Isles was determined by flow cytometry, using both autofluorescence markers and staining with the DNA dye SYBR Green I. Prochlorococcus (0.1 to 0.3 × 106 cells ml-1) dominated the phytoplankton in the oligotrophic northern (24° to 38°N) and southern (8° to 34°S) gyre regions. In the equatorial region (8°S to 24°N) the more productive waters supported 10 to 100 times more picoeukaryote algae and Synechococcus than in the gyres. The collection of samples underway proved to be a very economical method of obtaining a good indication of the concentration of heterotrophic bacteria and of these 3 classes of picophytoplankton in the surface mixed layer (coefficient of correlation, r2 = 0.88 to 0.98) throughout a very long transect, but correlated less well with picoplankton standing stocks in the top 200 m of the water column (r2 = 0.39 to 0.88). Daily variations in the flow cytometry profiles of stained Prochlorococcus observed over a distance of some 8000 km of the tropical and subtropical Atlantic were interpreted as an indication of their synchronised cell division in surface waters. Their estimated average minimum growth rate (0.15 d-1) was similar in both southern and northern oligotrophic gyres and in equatorial waters.