Plastid terminal oxidase acts as an alternative electron sink in a marine cyanobacterium Arthrospira sp.

In the marine cyanobacterium Arthrospira sp. under high light, the electron transport activity of photosystem (PS) II was much higher than the activities of PSI and the whole chain, indicating the existence of an alternative electron sink in PSII. Under high light, the addition of n-propyl gallate (PG), an inhibitor of plastid terminal oxidase (PTOX), decreased photosynthetic electron transport significantly as compared with that under low light. A significant residual level of photosynthetic electron transport remained in the presence of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) under high light. The extent of DBMIB insensitive electron transport was close to that of PG sensitive electron transport, suggesting that the PTOX acted as an alternative electron sink, accounting for 27% of total PSII electron transport in Arthrospira sp. cells under high light.