Discrepancies between feeding ecology and trophic position based on nitrogen isotopic composition in endosymbiont-bearing benthic foraminifers

Abstract Symbiotic relationships of microorganisms including eukaryotic algae and prokaryotes affect holobiont nitrogen metabolism and provide survival advantages in extreme environments; however, the influences of different symbionts on nitrogen metabolism in host organisms remain unclear. By tracing the nitrogen isotopic composition (δ15N) of amino acids (AAs), it is possible to constrain the biosynthetic sources and trophic interactions. We targeted reef-dwelling large benthic foraminifers hosting species-specific symbiotic algae to identify the trophic positions of multiple species with distinct feeding strategies. We measured δ15N in AAs of bulk organic matter and of foraminiferal shells and compared the feeding strategy of each species with its estimated trophic position (TP). Estimated TPs based on δ15N in AAs were inconsistent with respect to species-specific feeding strategies. In particular, Amphisorus kudakajimensis, which depends on heterotrophic feeding, showed light δ15NGlu and δ15NPhe and TP of ~ 1. A shared nutrient source for both host and symbionts and an interactive supply of trophic compounds may explain the low trophic position. The mixotrophic or heterotrophic TPs of Calcarinidae hosting endosymbiotic diatom with limited feeding (TP = 1.5–2.3) may be affected by bacterial heterotrophic processes. Differences in symbiotic algae (zooxanthellae and diatoms) may have influenced host’s nutritional strategies through functional variations in nitrogen metabolism. Large variations in δ15NGlu and δ15NPhe within single species suggested that large benthic foraminifers exploit multiple sources of nitrogen. Additional culture studies using different nitrogen sources could provide insights into detailed nitrogen metabolism of organisms with endosymbionts and role of associated organisms.