Pyrenoids are specialized structures found within the chloroplasts of certain algae and some plants, playing a crucial role in carbon fixation during photosynthesis. They are typically composed of protein and are associated with the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). Pyrenoids help enhance the efficiency of carbon capture in aquatic environments, especially in species that thrive in low CO2 conditions. Their presence is significant in the study of algal physiology and carbon cycling in ecosystems.