Ultrastructure and distribution of kleptoplasts in benthic foraminifera from shallow-water (photic) habitats

Assimilation, sequestration and maintenance of foreign chloroplasts inside an organism is termed “chloroplast sequestration” or “kleptoplasty”. This phenomenon is known in certain benthic foraminifera, in which such kleptoplasts can be found both intact and functional, but with different retention times depending on foraminiferal species. In the present study, seven species of benthic foraminifera (Haynesina germanica, Elphidium williamsoni, E. selseyense, E. oceanense, E. aff. E. crispum, Planoglabratella opercularis and Ammonia sp.) were collected from shallow-water benthic habitats and examined with the transmission electron microscope (TEM) for cellular ultrastructure to ascertain attributes of kleptoplasts. Results indicate that all these foraminiferal taxa actively obtain kleptoplasts but organized them differently within their endoplasm. In some species, the kleptoplasts were evenly distributed throughout the endoplasm (e.g., H. germanica, E. oceanense, Ammonia sp.), whereas other species consistently had plastids distributed close to the external cell membrane (e.g., Elphidium williamsoni, E. selseyense, P. opercularis). Chloroplast degradation also seemed to differ between species, as many degraded plastids were found in Ammonia sp. and E. oceanense compared to other investigated species. Digestion ability, along with different feeding and sequestration strategies may explain the differences in retention time between taxa. Additionally, the organization of the sequestered plastids within the endoplasm may also suggest behavioral strategies to expose and/or protect the sequestered plastids to/from light and/or to favor gas and/or nutrient exchange with their surrounding habitats.