Endospores and Epulopiscium: Some Epulopiscium-like surgeonfish symbionts form mature endospores at night. These spores possess all of the characteristic protective layers seen in B. subtilis endospores and also contain large amounts of dipicolinic acid. These are the largest endospores described thus far, with the largest being over 4000 times larger than a Bacillus subtilis endospore.
The formation of endospores may help maintain the symbiotic association between these Epulopiscium-like symbionts and their surgeonfish hosts. Since endospore formation coincides with periods in which the host surgeonfish is not actively feeding, the cells do not need to compete for limited nutrients present in the gut at night. The protective properties of the endospores also allow them to survive passage to new surgeonfish hosts. The fish may also benefit from this relationship because it is able to maintain stable microbial populations that assist in digestion and may receive a nutritional gain from microbial products released during spore germination.
Daily life cycle of endospore-forming Epulopiscium-like symbionts
Endospore formation in some Epulopiscium-like symbionts follows a daily cycle:
A) Polar septa are formed at the poles of the cell.
B) Forespores become engulfed.
C) Forespores gradually increase in size within the mother cell through the day.
D) In late afternoon, final preparations for endospore dormancy.
E) Endospores mature and remain dormant throughout most of the night.
F) Just before sunrise, the endospores germinate and are released from mother cell to repeat the cycle.