Algal symbiosis in larger foraminifera

Abstract

Foraminifera with endosymbiotic algae abound in shallow tropical and semitropical seas. Diverse groups of contemporary foraminifera are the hosts for a wide variety of endosymbiotic algae (diatoms, dinoflagellates, unicellular chlorophytes, unicellular rhodophytes and/or cyanobacteria) or their plastids suggesting that foraminifera are particularly good partners for the establishment of symbioses. The fossil record supports this idea. Since the Pennsylvanian there have been evolutionary bursts of symbiont bearing lineages of foraminifera in shallow, well-illuminated tropical and semi-tropical seas. Two factors predispose symbiosis in the group: 1) their general cameral subdivision (this compartmentalizes and separates different cellular activities: e.g. digestion is spatially separated from symbionts) and 2) asexual reproduction insures vertical transmission of symbionts. Host-symbiont specificity in diatom-bearing foraminifera is not finical; the same host species can harbor any one of several dozen diverse species of pennate diatoms. Nitzschia frustulum symbiotica is the most common of the diatom symbionts, being found in ∼30% of the associations. Nanofrustulum shiloi, Nitzschia laevis, Nitzschia panduriformis and Amphora spp. are also more common than the other symbiont species. Often a second species of diatom can be isolated from the same host. Experiments demonstrate that some endosymbiotic diatom species can replace others. Red cyanobacteria have been found in dinoflagellate-bearing soritines. Specimens of Marginopora vertebralis from Lizard Island also host small numbers of prymnesiids. Many questions about host-symbiont relationships remain to be explored. Calcification of symbiont-bearing species is enhanced in the light. Foraminifera seem selective in the species of algae they assimilate. A number of species (Archais angulatis, Sorites marginalis, Amphisorus hemprichii, and Amphistegina spp) cannot grow if they are starved, even when incubated in the light, suggesting that algal photosynthesis alone does not satisfy their needs. Starved Heterostigina depressa, in contrast, grew in the light in the absence of obvious feeding on algae, but feeding on bacteria was not ruled out. Each host species grows within a range of light intensity. Symbiont-bearing foraminifera migrate toward or away from light sources if conditions permit them to do so. Both field observations and laboratory experiments suggest that larger foraminifera, as a group, grow best in oligotrophic conditions. Growth of hosts with their symbionts in the laboratory is balanced in illuminated chemostats that continuously supply low concentrations of nutrients. ©2006 Balaban.