Accelerated biogenic silica dissolution by marine invertebrate digestion: in comparison with phosphorus and iron

Herbivore digestion in aquatic ecosystems is usually considered a method of nutrient repackaging rather than recycling, as recalcitrant and low-level nutrients are presumed for their egesta. We hypothesize that this opinion holds only for nutrients recycled by excretion and egestion, not for those elements recycled overwhelmingly by fecal decomposition. In this study, we compared the dissolution of biogenic silica (BSi), phosphorus (P) and iron (Fe) between two food items and fecal pellets of two marine invertebrates fed on artificial seawaters free of bacteria. Relative to raw food materials, the mass proportion in fecal pellets of BSi increased, while that of P and Fe decreased. During the 21 days of incubation, the total dissolution rate of BSi was 13.9-36.0 times higher in fecal pellets than food items, followed by P (1.5-4.2 times) and Fe (1.1-2.4 times). While the dissolution of BSi and Fe occurred mostly in the first few days, P was mostly released in the last ten days. Regarding BSi dissolution, a higher rate was observed in oyster Crassostrea gigas than the Echiuran Urechis unicinctus, but no significant difference was found between fecal pellets in either species under naturally available diatom food (Phaeodactylum tricornutum) and introduced terrestrial food (rice husk powder), respectively. Our results show direct evidence of digestion-associated nutrients mobilization. BSi dissolution after animal digestion may be similarly efficient to that caused by bacteria colonization in natural seawater.

Automated summary

Herbivore digestion in aquatic ecosystems plays an important role in nutrient cycling. This study provides evidence that the dissolution of biogenic silica, phosphorus, and iron in fecal pellets is higher than in raw food materials, indicating the efficient mobilization of these nutrients through digestion.