FATE OF PARTICLE-BOUND BACTERIA INGESTED BY CALANUS-PACIFICUS

Bacteria bound to particles can be ingested by marine calanoid copepod s, thus potentially providing a direct trophic link between bacteria a nd metazoa provided the microbes are digested by the animals. If some bacteria survive gut passage, they might play a role in fecal pellet s olubilization and fragmentation (via ectoenzymes) and decomposition. W e examined whether Calanus pacificus could assimilate bacterial biomas s and whether a significant fraction of the ingested bacteria passed i nto the fecal pellets alive. We fed adult female copepods on freeze-th awed diatoms (Cylindrotheca fusiformis and Chaetoceros sp.) which had been colonized by natural populations of seawater bacteria labeled wit h H-3-leucine. The copepods retained 26 to 31 % of the label 43 h afte r ingestion and subsequent gut clearance, indicating that a substantia l fraction of bacterial biomass bound to particulates of appropriate s ize can be directly available to this copepod as a source of material and energy. Bacteria were present at very high concentrations (10(9) t o 10(10) ml-1) in the fecal pellets of copepod fed on bacterized food, but were absent in the fecal pellets of copepods fed axenic food. Whe n fecal pellets freshly ejected by copepods fed bacterized food were i ncubated in seawater, bacteria abundances in them increased, yielding minimum specific growth rates of 0.08 h-1 (generation time g = 8.7 h) at 16-degrees-C and 0.19 and 0.20 h-1 (g = 3.6 and 3.5 h) at 22-degree s-C. Since bacteria grew within the fecal pellets, at least some of th e bacteria we found in the freshly egested fecal pellets must have sur vived gut passage. In order to determine whether bacteria imparted ami nopeptidase activity to copepod fecal pellets, the enzyme activity of fecal pellets produced on uncolonized and colonized freeze-thawed diat oms was compared. This comparison showed that bacteria in the fecal pe llets account for 68 to 84 % of the aminopeptidase activity of fecal p ellets, indicating a potential role for the interior bacteria in fecal pellet solubilization and fragmentation. We conclude that feeding of these copepods on attached bacteria not only serves to link the microb ial loop with the grazing food chain but, because of the passage of vi able bacteria into the fecal pellet, may also influence fecal pellet d egradation.