Abstract Fecal pellets produced by marine zooplankton contribute substantially to transporting biologically fixed carbon from the sunlit sea surface into deeper water layers. Their occurrence and composition are often heterogeneous, and pellets may undergo various changes while sinking, making it difficult to quantitatively assess their mechanisms of production and transformation or determine their contribution to carbon flux. To address this knowledge gap, we characterized freshly produced fecal pellets from taxonomically identified organisms in terms of morphology, carbon content, sinking velocity, taxonomic composition, and rates of aerobic respiration and denitrification. The pellet's heterogeneous eukaryotic compositions suggested non‐selective feeding. Microbial communities were a composite of organisms, many of which could be linked to eukaryotic source materials. Small pellets showed disproportionally high sinking velocities and carbon contents, which could be attributed to the pellets' porosity increasing with size, likely due to less compact packaging. Low carbon remineralization rates of ~ 2% d −1 showed stronger links with surface‐area‐related parameters than with pellet composition, suggesting that respiration was limited by surface accessibility or diffusion. However, we did not detect denitrification, and a simple model confirmed the likely absence of anoxic micro‐niches. These results underline the important contribution of fecal pellets to carbon transport and the critical role of their structural integrity and membrane casings in protecting biological material from remineralization.
Karthäuser et al. (Sun,) studied this question.