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Microbial growth rates and grazing losses and the role of bacteria with high and low apparent DNA content
Photo- and heterotrophic pico- and nanoplankton in the Mississippi River plume:
distribution and grazing activity The abundance of pico- and nanophytoplankton, bacteria and heterotrophic
nanoflagellates, and grazing rates on phototrophic pico- and nanoplankton and bacterioplankton were
assessed along a salinity gradient (0.2–34.4) in the Mississippi River plume in May 2000. Grazing rates
were established by serial dilution experiments, and analysis by flow cytometry allowed differentiation
of grazing rates for different phytoplankton subpopulations (eukaryotes, Synechococcus spp.,
Prochlorococcus spp.). Grazing rates on phytoplankton tended to increase along the salinity
gradient and often approached or exceeded 1 day–1. Phytoplankton net growth rates (growth – grazing)
were mostly negative, except for positive values for eukaryotic nanoplankton in the low-salinity,
high chlorophyll region. Grazing pressure on bacteria was moderate (~0.5 day–1) and bacteria gained
positive net growth rates of ~0.3 day–1. Eukaryotic nanophytoplankton were the major phototrophic
biomass and protozoan food source, contributing 30–80% of the total consumed carbon. Bacteria
were the second most important food source at 9–48% of the total consumed carbon. Synechococcus
spp. and Prochlorococcus spp. remained an insignificant portion of protozoan carbon consumption,
probably due to their low contribution to the total pico- and nanoplankton biomass. Group-specific
grazing losses relative to standing stocks suggest protozoan prey preference for eukaryotes over
bacteria. Protozoan grazers exerted a major grazing pressure on pico- and nanophytoplankton, but
less so on bacteria.
Published in: Journal of Plankton Research 25: 1201-1214; 2003
Growth and grazing rates of bacteria groups with different apparent DNA content in the
Gulf of Mexico Growth rates and grazing losses of bacterioplankton were assessed by serial dilution
experiments in surface waters in the Mississippi River plume, the northern Gulf of Mexico, a Texas
coastal lagoon (Laguna Madre), southeast Gulf of Mexico surface water, and the chlorophyll subsurface
maximum layer in the southeast Gulf of Mexico. Bacteria were quantified by flow cytometry after DNA
staining with SYBR Green, which allowed for discrimination of growth and grazing rates of four bacteria
subpopulations distinguished by their apparent DNA content and cell size (light scatter signal). Total
bacteria growth rates (0.2–0.9 day-1) were mostly balanced by grazing losses, resulting in net growth
rates of -0.18 to 0.45 day-1. Growth rates of DNA subpopulations varied within experiments, sometimes
substantially. In most, but not all, experiments, the largest bacteria with highest DNA content
exhibited the highest growth rates, but a relationship between DNA content and growth rates or grazing
losses was absent. Small bacteria with the lowest DNA content showed positive growth rates in most
experiments, sometimes higher than growth rates of bacteria containing more DNA, and were grazed upon
actively. Low-DNA bacteria were not inactive and were an integral part of the microbial food web.
Published in: Marine Biology 145: 1213-1225; 2004
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