Why was an expansion in plant life observed under warm temperature?

Read the following passage carefully and choose the most appropriate answer to the question out of the four alternatives.   Global temperature rises could cause the marine food web to collapse, devastating the livelihoods of tens of millions of people who rely on fisheries for food and income, scientists have warned. Warming oceans restrict vital energy flows between different species in the marine ecosystem, reducing the amount of food available for bigger animals - mostly fish - at the top of the marine food web, according to a study in the journal PLOS Biology. This could have “serious implications” for fish stocks, said Ivan Nagelkerken, a professor at University of Adelaide. The Adelaide scientists set up 12 large tanks, each holding 1,800 litres of water, in a temperature-controlled room to replicate complex marine food webs, and tested the effects of ocean acidification and warming over six months. Plant productivity increased under warmer temperatures but this was mainly due to an expansion of bacteria which fish do not eat, Prof. Nagelkerken said.