Final answer:
Among the choices given, pteropods and hard corals are the marine organisms most directly affected by ocean acidification due to their reliance on calcium carbonate for their structural integrity.
Step-by-step explanation:
All the following are organisms likely to be directly affected by ocean acidification except for nonphotosynthetic organisms that do not rely heavily on calcareous structures. Pteropods and hard corals are among the marine organisms that are most vulnerable to ocean acidification. Both rely on calcium carbonate for their shells and skeletons. Pteropods, tiny sea snails, will start to see their shells dissolve as acidity increases, which can already be observed in regions like the North Atlantic and off Alaska. Hard corals' calcification processes are hindered by rising CO2 levels, which change the water's pH. As a result, corals are more susceptible to bleaching, and their ability to form calcium carbonate skeletons is reduced, which impacts not only the coral itself but also the myriad of species that depend on coral reefs for habitat and food.
Non-photosynthetic organisms are not directly mentioned in relation to acidification impact on their ability to grow, thus making the choice not as clear. However, the question specifies organisms that are 'mostly likely to be directly affected' by ocean acidification, which implies a direct physiological impact due to changes in calcification processes.