Structure, Bioadhesive Distribution and Elastic Properties of the Agglutinated Test of Astrammina rara (Protozoa: Foraminiferida)

ABSTRACT. The fine structure, elastic properties, and distribution of the fibrous, meshlike cement (bioadhesive) were studied for the test of the antarctic agglutinated foraminiferan Astrammina rara. Grain-size analysis of particles incorporated into the test compared with adjacent sediment indicates that A. rara is grain-size selective. Fractured tests curl inward, suggesting that the test is under tension—an impression substantiated by micromanipulation observations. Changes in test appearance were examined by scanning electron microscopy after sequential chemical treatments combined with ultrasonication. Organic fibrils securing fine-grained particulates on the test exterior were removed during initial sonication. A veil of fibrous organic material lining the test interior (i.e. inner organic lining) was removed by treatment with a nonionic detergent, revealing ligamentous cables of bioadhesive securely joining large grains. These cables are partially disrupted by treatment with sodium dodecyl sulfate, and further disrupted by disulfide reducing agents, suggesting that protein is an integral adhesive component. The large detrital grains incorporated into the test are arranged in an interlocked, optimally packed fashion. Together, these observations indicate that the seemingly simple spherical architecture of A. rara's test is in fact quite complex, consisting of large grains compressed by tensile cables of a proteinaceous bioadhesive, with additional rigidity supplied by fine particulate "mortar" deposited externally.