experimental evidence for one harmful effect of too much CO2

experimental evidence showing that a species of algae important for locking carbon out of the biosphere will stop doing so if we let CO2 levels get so high as to cause too much acidification of the oceans;

http://phys.org/news/2016-07-wrought-acidic-oceans-marine-life.html

Of course, this is inadvertently also evidence, if any were needed, that too much CO2 can interfere with marine life.