Morphology and DNA content of bacterioplankton in the northern Gulf of Mexico: analysis by epifluorescence microscopy and flow cytometry

Abstract

The distribution of pelagic bacteria was assessed along 2 offshore - onshore transects in the northwestern Gulf of Mexico in July and October 1999 and along a salinity gradient (0.2 to 34.4 ‰) in the Mississippi River plume in May 2000. Cell abundance was estimated by epifluores- cence microscopy after DAPI staining and by flow cytometry after DNA staining with SYBR Green I. Total bacterial counts by both techniques corresponded well. Bacterial abundance ranged from 0.9 × 10 6 to 1.35 × 10 6 cells ml -1 in the upper 200 m of the water column in the northwestern Gulf and from 0.1 × 10 6 to 2.05 × 10 6 cells ml -1 in the Mississippi River plume. Bacteria exhibited surface maxima in July 1999 but subsurface maxima in the upper half of the chlorophyll maximum in October 1999 and off the Louisiana shelf break in May 2000. Stations with a thin layer of low-salinity plume water exhibited an additional bacterial maximum at the surface. Within the Mississippi River plume, bacte- rial abundance decreased with increasing salinity, and their maximum abundance preceded the chlorophyll maximum along the salinity gradient. Three morphotypes of bacteria were distinguished by epifluorescence microscopy: cocci, rod-shaped bacteria, and curved bacteria. Cocci (40 to 60% of total bacteria; counts corrected for Prochlorococcus spp.) were the most common morphotype. Rods and curved bacteria had similar shares (18 to 25%) and presented multi-species consortia as indi- cated by the variability in size and shape of cells within each group. Flow cytometry revealed 4 bac- terial subpopulations distinguished by their DNA content, none of which seem to reflect a specific morphotype. Whereas regional differences in the contribution of the distinguished DNA types to total bacterial abundance were low in the open Gulf, a switch in predominance from low-DNA to high- DNA cells below the subsurface chlorophyll maximum was obvious in all profiles. The ecological sig- nificance of bacterial DNA types as revealed by flow cytometry is discussed in the context of pub- lished results.