New research suggests that a tiny phytoplankton that is an essential part of the marine food web may decline sharply as oceans warm. Prochlorococcus is the smallest and most abundant phytoplankton. A study published Monday in Nature Microbiology found that it could shrink by half in tropical oceans over the next 75 years if waters exceed 82 degrees Fahrenheit. Besides its crucial role in the marine food web, Prochlorococcus produces about one-fifth of the planet’s oxygen. The findings challenge previous assumptions that this phytoplankton would thrive in warmer waters. Researchers warn that the decline could have significant consequences for marine biodiversity and the global climate.