Parmecium bursaria (Bob Blaylock, en.wikipedia.org).

     Findings recently published in the journal Current Biology suggest that many symbiotic relationships are more exploitative than mutually beneficial. Biologists from the universities of Exeter and York generated the findings after looking at the relationship between protozoa (Parmecium bursaria) and algae, where the protozoa hosts chlorophyll-containing algae and receives sugar and oxygen from it due to photosynthesis.

     "We found that for the host the benefits in symbiosis increased with light. Although symbiosis is very costly in the dark for the hosts, because the algae are useless, when you increase the light intensity then it becomes very beneficial to have algae because they give you lots of sugar," said Mike Brockhurst, professor of biology at the University of York. "Across all of the environments that we tested we never found any conditions where both species benefitted. For the algae it is always costly to be in symbiosis."

     "This new research has turned the assumption that symbiosis is mutually beneficial on its head," said Brockhurst. 

     The full article from futurity.org is available here, or here from phys.org.