U.S. Environmental Protection Agency
EPA Science Advisory Board
Background Information

Document NameGrowth and grazing rates of bacteria sub-populations with different apparent DNA content in the Gulf of Mexico
Document AuthorJochem, F. J.
Lavrentyev, P.J.
First, M. R.
Short DescriptionMar Biol 145:1213-122
CategorySubgroup 1: Characterization of the Cause(s) of Hypoxia
Publication Year2004

Abstract: 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.