AME 40:259-267 (2005)  -  doi:10.3354/ame040259

ABSTRACT: This study introduces isothermal microcalorimetry as a method to describe microbial degradation of copepod fecal pellets through measurements of the metabolic heat-flow produced by the degrading microbial community. We monitored the heat-flow for 20 to 24 h from samples of 300 to 1500 fecal pellets from a laboratory-cultivated calanoid copepod, Acartia tonsa Dana, fed on the chryptophyte Rhodomonas salina in excess. The microbes grew at 0.095 to 0.24 h^–1. Specific degradation rates were calculated using 3 different literature values of the carbon:volume ratio of fecal pellets. The mean ± SD specific degradation rates, R_d, were estimated at 0.023 ± 0.015 h^–1 (for carbon:volume ratio of 0.39 pgC µm^–3) and at 0.047 ± 0.038 h^-1 (for carbon:volume ratio of 0.25 pgC µm^–3), defined as the proportion of fecal pellet carbon completely oxidised to CO₂ h^–1. The lowest carbon:volume ratio (0.085 pgC µm^–3) resulted in >100% fecal pellet carbon degradation within 20 h. These degradation rates are very high, presumably reflecting a fully active bacterial population. The limiting factor for microbial growth was not inorganic nutrients (N and P), but labile dissolved organic carbon (DOC). We compared heat-flow in situations with and without oxygen, and found it to be 1 order of magnitude lower under anoxic conditions. This indicates the presence of anaerobic or facultative bacteria associated with the fecal pellets. Possible applications of the methodology are discussed.

KEY WORDS: Copepod fecal pellets · Degradation · Microbial community · Isothermal microcalorimetry