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However , across large areas of the oceans such as the Southern Ocean , phytoplankton are limited by the lack of the micronutrient iron. This has led to some scientists advocating iron fertilization as a means to counteract the accumulation of human-produced carbon dioxide ( CO ) in the atmosphere .
 Southern Ocean, micronutrient
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
Answer:
 Southern Ocean, micronutrient