Yet how is a lichen 'more than the sum of fungus and alga'?

I don't know anything about lichen, but the below is what I assume "more than the sum of" in this context means:

"The symbiosis between the mycobiont and the photobiont creates an organism that is more than the sum of its parts, in other words, a lichen is an emergent property. Lets take a step back to examine this statement. On the one hand, neither the photobiont nor the mycobiont can withstand intense UV radiation, dessication, or extreme temperatures. But on the other hand, when the photobiont and mycobiont work together within the context of the lichen symbiosis, they create an organism that can withstand living in outer space – thats more extreme temperature and radiation (not to mention vacuum exposure) than is experienced on Earth! Lichen can even grow within rocks (endolithic lichen)! These are conditions that would kill a fungus or algae."