Homotrema rubrum: Symbiosis Identified by Chemical and Isotopic Analyses

Abstract

The red coloration of the foraminiferan Homotrema rubrum (Lamarck) is due to the remnants ofphotosynthetic pigments. The presence of these pigments suggests a symbiotic relationship between Homotrema and some type of alga. This is most probably a green alga, based upon the chlorophyll content (chl a and b) found in the test. Symbiosis is also supported by examination of the carbon isotopic composition, which shows a decrease in the heavy carbon isotope in the foraminiferal carbonate compared to coexisting carbonates. This decrease is due to recycling of photosynthetically produced carbon dioxide from the algal symbiotes existing in Homotrema. These isotopic deviations should exist in the test or shell of all carbonate precipitating host organisms, and can be used to determine possible symbiotic relationships in other organisms, such as Paleozoic corals and fusulinids. The chamber addition in radial alignment and the incorporation in the wall of calcium carbonate inclusions in Homotrema rubrum represent new morphological observations. This test morphology indicates the possibility that symbiosis may also influence test morphology in Homotrema. Similarity of Homotrema to other radial-walled foraminiferans indicates that coevolution of foraminifers and algae resulted in a change to an endosymbiotic life style for the alga and a change in wall structure for the foraminiferan. Coevolution of foraminifers and alga is analogous to the relationship between hermatypic corals and their symbionts, both ecologically and morphologically, and may indicate similar conditions for development.