Final answer:
The development of two linked photosystems in chloroplasts allows the oxidation of water (H₂O), overcoming the limitations of anoxygenic photosynthesis by generating oxygen as a byproduct, which is essential for aerobic life.
Step-by-step explanation:
The evolution of photosynthesis from anoxygenic to oxygenic has allowed for the oxidation of water (H₂O) to replace the reaction center electron in the photosystem, thereby generating oxygen as a byproduct. The two linked photosystems, photosystem I (PSI) and photosystem II (PSII), work together to achieve this. PSII splits water to supply the needed electron and generate oxygen, while PSI uses light energy to transfer this electron eventually to NADP+ to form NADPH. The non-cyclic photophosphorylation involving both photosystems overcomes the limitations of anoxygenic photosynthesis, which does not produce oxygen and relies on a single photosystem.
In anoxygenic photosynthesis, other reduced compounds like hydrogen sulfide (H₂S) or thiosulfate serve as electron donors, so oxygen is not produced. Conversely, in oxygenic photosynthesis, the splitting of water provides not only electrons to continue the cycle but also protons for ATP synthesis and oxygen as a crucial byproduct for aerobic respiration in the ecosystem.