Ocean-dwelling chemoheterotrophic bacteria that generate energy via aerobic respiration are dependent on the presence of photoautotrophs like cyanobacteria because:

Because, photoautotrophs serves as their carbon sources.
Chemoheterotrophic bacteria are those bacteria that are incapable of producing their own food, they depend on photoautotrophs, which are capable of making their own food by trapping energy from the sun. Thus, photoautotrophs serves as source of food for the chemoheterotrophic bacteria.