CHARACTERIZATION OF NITROGEN AND CARBON STABLE ISOTOPES IN EPIPHYTIC FORAMINIFERAL MORPHOTYPES (Invited Presentation)

Epiphytic foraminifera were categorized, based upon their ecological and feeding strategies, into four morphotypes that were subsequently modified to include a new morphotype for the symbiont-bearing forms. We propose further modifications to increase the ecological resolution. Thus, the A* morphotype splits into leaf-encrusting forms (AF*) and rhizome encrusting taxa (AR*). Similarly, the symbiont-bearing morphotype has been separated into Large Miliolids (LM) that host a variety of algal symbionts, and Large Rotalids (LR) that exclusively host diatoms. B and C morphotypes remain as they were originally defined, whilst D* morphotype does not include symbiont-bearing taxa and represents opportunistic forms. To determine the trophic strategy of the epiphytic morphotypes, the cytoplasmic nitrogen and carbon stable-isotope signals from two localities of Mallorca (Spain) and one from Madagascar were analysed. The most abundant morphotype reported in Mallorca was B, followed by AF*. In Madagascar, the most abundant morphotype is D*, and symbiont-bearing morphotypes (LM and LR) were considerably more abundant than at the Mediterranean locations. Among all samples, the δ15N values ranged between 0.5 and 3‰; δ13C values varied between ‐18 and ‐0.9‰. An MDS statistical analysis showed that variability in the δ15N and δ13C isotopes is associated with differences among the morphotypes and likely reflects their feeding strategies. A SIMPER analysis of the isotopic composition revealed minimal differences within the sessile (AF* and AR*) and within the symbiont-bearing (LM and LR) morphotypes, indicating similar trophic strategies within each pair, largely based upon cyanobacteria as a food source. These foraminifera perform “farming” of (cyano)bacteria, fungi and diatoms, which constitute the essential components of their diet. The LM-LR morphotypes also receive organic carbon from their algal symbionts. The δ15N and δ13C values of the motile B and D* morphotypes are highly variable, indicative of diverse food sources, including cyanobacteria, fungi, microalgae and particulate organic matter (phytodetritus). The δ15N in the C morphotype are more enriched and δ13C more depleted (3‰ and ‐10‰, respectively) than in the sessile morphotypes.