Effects of host cell extracts on cultures of endosymbiotic diatoms from larger foraminifera

Log phase endosymbiotic diatom species, Nitzschia valdestriata, N. laevis, N. panduriformis var. continua, N. frustulum var. subsalina and var. symbiotica, Amphora tenerima, Fragilaria shiloi and Navicula hanseniana, were aseptically exposed to an homogenate of freshly crushed foraminiferal host species (Amphistegina). To various degrees, depending upon endosymbiont species, the host homogenate affected the formation of new frustules of growing and dividing cells. F. shiloi was the most affected species. New cells were spheroids and had only vestiges of frustules or none at all. Fewer numbers of the other species tested (40 to 60%) were abnormal. The raphe and keel of affected Nitzschia spp. were usually curved and incomplete. Many cells were spherical. We infer that host substances are probably responsible for the maintenance of the frustule-less state in vivo and that if ingested potential endosymbionts escape digestion, they could become frustule-less after growth and cell division. Host homogenate also increased the levels of photosynthetate released by diatoms to their medium. N. valdestriata was stimulated to release 76% of its photosynthetate while other species tested were stimulated to release between one quarter to one half of their photosynthetate. Host homogenate was also effective in causing the release of 43% of the photosynthetate of a free-living diatom, Amphora sp., which was also tested in the experiment. The concentration of the metabolites released in the experiment was too low for us to identify by TLC.