Symbiosis, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 26 (1999) 299-312 Balaban, Philadelphia/Rehovot 299 zyxwvutsrqpon r zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIH B,,)h ' Bacterial Endosymbio nts i n the Agglut i na ti ng F or aminiferan zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Spiculidendron corallicolum R iitzl e r and Richa r ds on, 1996 SUSAN L. RICHARDSONl,2* and KLAUS RUTZLER3 IDepartment of Ecology and Evolutionary Biology, Yale University, New Haven, CT 06540; 2Present address: Department of Geology and Geophysics, MS-08, The Woods Hole Oceanographic Institution, Woods Hole, MA 02543, Tel. +508-289-4910, Fax. +508-457-2183; and 3Department of Invertebrate Zoology, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, USA Received October 10, 1998; Accepted February 26, 1999 Abstr a ct The cytoplasm of the Caribbean agglutinating foraminiferan Spiculidendron corallicolum Rutzler and Richardson, 1996, consistently contains gram-negative, non-photosynthetic bacteria that show ultrastructural similarities to marine nitrifying bacteria. In some micrographs, bacteria occur in densities as high as 21 cells per 60 μm3 of cytoplasm, indicating that an individual foraminiferan may contain as many as 2.2 x lQ9 bacterial endosymbionts. The bacteria are free-living in the cytoplasm, spherical to ovoidal in shape (-0.6 x 0.7 μm), and devoid of extracellular structures. Ultrastructurally, the bacteria are characterized by a discrete fibrillar nucleoid, a carboxysome-like electron dense inclusion (100 nm), and intracytoplasmic lamellae (100 nm x length of the cell). The presence of dividing individuals within the cytoplasm and the absence of this group within the foraminiferans' digestive vacuoles, indicate that the bacteria are endosymbionts and not ingested food organisms. The discovery of prokaryotic endosymbionts in what has been considered to be a basal foraminiferal clade, indicates that endosymbiosis may have played a more significant role in the evolution and diversification of foraminiferans than has previously been suspected. Keywords: Astrorhizidae', bacterial endosymbionts, Belize, endosymbiosis, foraminiferans *The author to whom correspondence should be sent. 0334-5114 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA I 99 I $05 .50 ©1999 Balaban