Phosphorous is not particularly abundant on Earth. Yet Terran life needs the element. A similar element that could replace phosphorous is arsenic. Of course, arsenic is poisonous to us. This is because it is so similar to phosphorous. It may be possible that life could have evolved on other worlds to use arsenic instead of phosphorous. This is because arsenic can do everything that phosphorous does (in the way of structural bonding and energy storage). It could also be used to drive metabolism. On such a world, phosphorous would be the poison because it would interfere with those functions, much as arsenic interferes with the functions of phosphorous in life on Earth.
Is it possible for lifeforms to be poisonous to each other because of their basic chemistry? Would it be dangerous for arsenic based lifeforms and phosphorous-based lifeforms to simply touch each other or even to life in the same space?
References: “Are Aliens Among Us?” by Paul Davies, Scientific American December 2007