Diatom sexual reproduction. Light micrograph of two diatoms undergoing conjugation, a type of sexual reproduction. This is scalariform conjugation, where two cells align laterally, instead of end-to-end. During conjugation a tube forms between the cells and gametes (sex cells) move into the tube and combine, forming offspring. Diatoms are a group of photosynthetic single- celled algae. They form an important part of the plankton at the base of the marine food chain.