AME 38:31-40 (2005)  -  doi:10.3354/ame038031

ABSTRACT: Through ingestion a copepod introduces rich organic substrates into its guts and fecal pellets, where dense bacteria may exploit them and show fast growth. Thus, a copepod and its fecal pellets may be regarded as microbial hotspots in the ocean. This study investigated the effects of copepods’ feeding activities on the associated bacteria, using the Most Probable Number (MPN) method. Starved Acartia tonsa (calanoid copepod) carried a background bacteria population of 10³ to 10⁴ copepod^-1. When fed axenic cultures of algae Rhodomonas salina or Dunaliella tertiolecta, the bacterial abundance increased curvilinearly with the copepods’ ingestion rates. When fed axenic cultures of diatom Thalassiosira weissflogii, the bacterial abundance showed a hump-shaped response to the ingestion rate. These observations support a conceptual model of a balance between bacteria growth stimulated by the host’s feeding and bacteria loss through the host’s defecation. The equivalent bacterial population density associated with the copepods was orders of magnitude higher than typical population density of marine free-living bacteria. In a time series experiment, bacteria inside the copepods’ bodies increased significantly after 3 d of feeding, and the estimated bacterial growth rate was 0.89 d^-1, higher than the average growth rate for free-living pelagic bacteria. The copepods released a significant amount of bacteria via defecation when food was present, and the equivalent bacterial abundance associated with fecal pellets was 6.5 × 10⁸ bacteria ml^-1. Bacteria recovered from A. tonsa feeding on different diets showed different growth kinetics in enrichment cultures, suggesting that diet types may be a selective force for different bacterial communities inside the host’s body.

KEY WORDS: Acartia tonsa · Microbial hotspot · Bacteria · Colonization · Most Probable Number · Fecal pellets