New Paper: Forams are what they eat.
Assisted by AI
Being heterotrophic like us, foraminifera consume, digest, and grow using nutrients derived from the breakdown of their food. In this paper, we track how nitrogen (N) from food is incorporated into living foraminifera. By feeding them copepods (tiny crustaceans) with specific N isotopic compositions, we were able to investigate the pathways through which food-derived N is translocated into foraminiferal biomass and, ultimately, into the organic layers embedded within their calcite shells.
We found that the N isotopic signals of both foraminiferal biomass and shell evolve towards those of their food sources, indicating the incorporation of food-derived N into foraminifera. For the biomass, its N isotopic signals are well modelled as a mixture between the original biomass and newly metabolised food intake, until they reach that of the food source but never exceed it. In addition, the increase in N content is comparable to the feeding rate. Together, these observations suggest a closed N system in foraminifera with minimal N leakage to the environment.
Similarly, the N isotopic signals in shells quickly approach those of the food sources. However, shell signals deviate from biomass signals and are constantly closer to those of the food. We attribute this discrepancy between biomass and shell N signals to their different incorporation pathways. Specifically, newly metabolised food-derived N is directly incorporated into the shell without first mixing with the foraminiferal bulk biomass.
Our findings validate the correlation between the N isotopic signals of foraminifera and their food sources, supporting the use of their shells as proxies to reconstruct past changes in nutrient sources and surface ocean nutrient cycling.