BOLIVINA SUBADVENA: ANOTHER PROKARYOTIC SYMBIONT-BEARING FORAMINIFER FROM A DYSOXIC SETTING

An extraordinarily high abundance of both modern and fossil specimens of the foraminiferal genus Bolivina often indicates organic enrichment and depleted oxygen conditions. However, Bolivina is typically not considered to be an “extremophile” genus. Results from ultrastructural analysis of live Bolivina subadvena, which were collected from the oxygen deficient Santa Barbara Basin, indicate that rod-shaped prokaryotic ectobionts are common and apparently associated with pore plugs. Transmission electron microscopy indicates firm attachment of the putative symbionts to the pore plug, often in a sheath of organic material and/or with fine fibrils. In addition, the foraminiferal host appears to have specialized intracellular structures that either facilitate metabolic exchange or promote bacterial adhesion. Plus, mitochondria appear to be concentrated near the pore plugs (e.g., Leutenegger & Hansen, 1979 Marine Biology, 54:11). It is, however, unclear if the mitochondria are aggregated due to the ectobionts or the pore plug. The firm attachment and abundance of prokaryotes (up to four per pore plug) suggested that they act as a screen between the environment and foraminiferal cytoplasm. Additional experimental studies are required to establish the physiological significance of this relationship.

While Bolivina is a particularly diverse genus, it is unclear if congeners of B. subadvena also have ectobionts. The presence of ectobionts may have a measurable affect on the stable isotopic signature of Bolivinid tests. For example, Stott et al. (2002 Geochem. Geophys. Geosys. 10.1029/2001GC000196) report a delta 13C of ~-1.5 per mil for B. subadvena in the Santa Monica Basin and a maximum specimen occurrence at ~1 mm depth, which has a pore-water delta 13C of ~-0.5 to -1.0 per mil. Thus, the ectobionts of B. subadvena may account for the offset between the pore-water and foraminiferal test (shell) delta 13C. Results suggest that caution should be used when using symbiont-bearing Bolivina species as paleoceanographic proxy recorders.

This project was supported by the W. Storrs Cole Memorial Research Award (Geological Society of America, to JMB), NASA Exobiology NRA-01-01-EXB-057 (to JMB), and NSF DEB0445181 (to SSB).