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 epifluorescence 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, bacterial 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 indicated by the variability in size and shape of cells within each group. Flow cytometry revealed 4 bacterial 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 significance of bacterial DNA types as revealed by flow cytometry is discussed in the context of published results.

Distribution of marine viruses in the Central and South Adriatic Sea

“…To standardise fluorescence intensity of cells, 1μm yellow-green beads were added (Level-III Epics Division of Coulter Corporation Hialeah, Florida). Two groups of bacteria were distinguished according to their relative green fluorescence as a proxy for nucleic acid content (Jochem, 2001), referred to as the high nucleic acid (HNA) and the low nucleic acid bacteria (LNA), and light scattering. Bacterial cell production was determined using the 3 H-Thymidine incorporation techniques (Fuhrman & Azam, 1980).…”

Morphology and LPS content for the estimation of marine bacterioplankton biomass in the Ionian Sea

“…The simultaneous occurrence of different bacterial morphotypes in marine environments has been well documented (Jochem, 2001;Acinas et al, 1999;Robarts et al, 1996;Viles and Sieraki, 1992). In the Ionian Sea, the bacterial community was mainly composed of cocci representing 54% of total bacteria.…”

“…Their contribution together with coccobacilli (71%) was comparable to those reported by Robarts et al (1996), Wiebe and Pomeroy (1972), Ferguson and Rublee (1976) in studies carried out in pelagic and coastal ecosystems. The predominance of cocci, including cyanobacteria, in oligotrophic and/or deep environments might be explained by the adaptation to a low organic substrate concentration due to their better surface-to-volume ratio than rodshaped and curved bacteria (Jochem, 2001). In contrast, the proportion of rods and curved bacteria (vibrios and spirillae) in the Ionian Sea was higher than the 13% reported by Robarts et al (1996) in the southeastern Mediterranean and the 10% detected by Jiao and Ni (1997) in the western and central Pacific.…”

“…A great number of methods have been proposed for biomass estimation, such as quantification of macromolecular components (i.e. muramic acid, ATP, specific lipids, DNA or proteins) (Jochem, 2001;Bratbak, 1993;Simon and Azam, 1989).…”

Spatial variability in Florida Bay particulate organic matter composition: combining flow cytometry with stable isotope analyses

“…SYBR DNA fluorescence was recorded in both the green FL1 (535±15 nm) and red FL3 (>650 nm) fluorescence channels, DNA clusters of bacteria were counted by logical gating from both FL1 vs FL3 and SSC vs FL1 dot plots, and histograms with gate borders set to minima in population fre- quency distribution. Measured sample volumes for estimates of bacterial abundance (cells ml )1 ) were calculated from measurement times (30 s) based on weight calibration of flow rates (0.2 ll s )1 ; Jochem, 2001). For bulk phytoplankton samples, 1 ml of each replicate sample was analyzed without staining, as phytoplankton pigments exhibit detectable autofluorescence following laser excitation (Hofstraat et al, 1994).…”