Answer: The correct answer for the blank is- photosynthetic bacteria.
Photosynthetic bacteria ( also called cyanobacteria or blue green algae) are the microbes, which are primarily found in seawater. These were the first bacteria that created oxygen in the early atmosphere of the earth planet.
These bacteria contain chlorophyll pigment, which help in synthesizing food ( in the form of carbohydrate) using carbon dioxide and water in the presence of sunlight. Oxygen is released in surrounding in this process. This process is called photosynthesis.