Final answer:
Prochlorococcus, a cyanobacterium, is mostly responsible for photosynthesis in aquatic habitats, contributing to about half of the world's oxygen. Light availability greatly influences the distribution and growth of these microscopic photoautotrophs in the water.
Step-by-step explanation:
The organism responsible for most of the photosynthesis in aquatic habitats is Prochlorococcus, a type of cyanobacteria. These microscopic photoautotrophs are believed to be the most abundant photosynthetic organism on Earth and are critically important in generating approximately half the world's oxygen.
In aquatic environments, the distribution of photosynthetic organisms such as Prochlorococcus is largely influenced by light availability, which is dependent on factors like water depth and transparency. Phytoplankton, another significant group of aquatic photosynthetic organisms, also play a crucial role in the food webs and oxygen production in the oceans, with their growth being influenced by the availability of nutrients like nitrogen and iron.