MEPS 325:15-27 (2006)  -  doi:10.3354/meps325015

Mesocosm study of particle dynamics and control of particle-associated bacteria by flagellate grazing

Kam W. Tang1,*, Hans-Peter Grossart2, Emily M. Yam1, George A. Jackson3, Hugh W. Ducklow1, Thomas Kiørboe4

1Virginia Institute of Marine Science, 1208 Greate Road, Gloucester Point, Virginia 23062, USA
2IGB-Neuglobsow, Alte Fischerhuette 2, Stechlin, 16775, Germany
3Department of Oceanography, Texas A&M University, College Station, Texas 77843, USA
4Danish Institute for Fisheries Research, Kavalergården 6, Charlottenlund, 2920, Denmark

ABSTRACT: The goal of this study was to attain an integrated understanding of the dynamics of attached microbial communities in nature as governed by the component processes of particle formation, colonization, detachment, growth and interspecific interactions among the microbes. The study was conducted in mesocosms in a Danish fjord over a 2 wk period in April to May 2005. Despite nutrient additions, chlorophyll and particle concentrations were low and no distinct phytoplankton blooms were observed. Particle volume concentration was dominated by particles >100 µm. The abundance of attached bacteria was weakly but positively correlated with total particle volume. Phylogenetic composition of attached bacteria became increasingly different from that of free bacteria through time. Growth rates of free and attached bacteria were 0.31 and 0.023 d–1, respectively. Colonization experiments with model agar aggregates showed that the bacterial community had limited apparent diffusivity but strong chemotaxis, which resulted in an up to 8-fold increase in bacterial colonization rates with organic-enriched aggregates. Estimated residence time of attached bacteria on aggregates was 21 min or less. Grazing mortality derived from temporal changes in attached bacterial abundance in the presence or absence of grazing activities ranged from 0 to 9.8 d–1. In comparison, grazing mortality of free bacteria was 0.06 to 0.91 d–1, suggesting that the bacterivorous flagellates were more adapted to grazing on surfaces than in free suspension. Very few attached flagellates were observed in the experiments, and microscopic observations revealed that many flagellates were attached only temporarily to surfaces. Our study showed that grazing mortality is a critical but often underappreciated factor in regulating marine snow bacterial population abundance. The loose association of bacterivorous flagellates with aggregates requires that their role be evaluated by direct grazing measurements instead of simple quantification of attached flagellate abundance.

KEY WORDS: Marine snow · Aggregation · Colonization · Grazing mortality · Bacteria · Flagellates · Chemotaxis

Full text in pdf format