The combination of a bacterium and other microbe into the first eukaryote was a big advance in evolutionary history; it made possible the huge variety of different body shapes and sizes we see today. This is thanks to the bacterial endosymbiont, the mitochondrion, taking on specialized metabolic tasks for the cell.

 

We already knew about endosymbionts that help with oxygen respiration, with photosynthesis (chloroplasts), and with amino acid synthesis (certain endosymbionts in insects). But bacteria have other metabolic abilities that are very useful in certain conditions; do these bacteria ever team up with other organisms? The answer is yes! In this study, ciliates were discovered at the bottom of a lake in oxygen-free waters. These protists have an bacterial endosymbiont that helps them respire, not oxygen, but nitrate instead, generating more energy than most anaerobic ciliates.