CHARACTERIZATION OF NITROGEN AND CARBON STABLE ISOTOPES IN EPIPHYTIC FORAMINIFERAL MORPHOTYPES

Epiphytic foraminifera are important components of the seagrass-meadow biota. These foraminifera previously 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 foraminifera. We propose further modifications to increase the ecological resolution. Thus, the A(star) morphotype splits into leaf-encrusting forms (AF(star)) and rhizome encrusting taxa (AR(star)). 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-star 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 (Sa Foradada and Sant Elm) and one from Madagascar were analysed. The most abundant morphotype reported in Mallorca localities was B (38% +/- 4.3 in Sa Foradada and 45% +/- 4.2 in Sant Elm), followed by AF(star) (34% +/- 4.6 in Sa Foradada and 41% +/- 1.0 in Sant Elm). In Madagascar, the most abundant morphotype is D-star (45% +/- 10), and symbiont-bearing morphotypes (LM and LR) were considerably more abundant than at the Mediterranean locations. Among all samples, the delta N-15 values ranged between 0.5 and 3 parts per thousand; snc values varied between -18 and -0.9 parts per thousand. An MDS statistical analysis showed that variability in the delta N-15 and delta C-13 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(star) and AR(star)) 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 delta N-15 and delta C-13 values of the motile B and D-star morphotypes are highly variable, indicative of diverse food sources, including cyanobacteria, fungi, microalgae and particulate organic matter (phytodetritus). The delta N-15 in the C morphotype are more enriched and delta C-13 more depleted (3 parts per thousand and -10 parts per thousand, respectively) than in the sessile morphotypes. Consistent with observations of other epiphytic, sessile organisms, cyanobacteria seem to be a very important food source.