Hi all. I think we don't even have to make a cyanobacteria to do the job. Even E.coli can grow on minimal media with only mineral salts and glycerol, which is symmetric, as a single source of organic carbon. So does mirrored E.coli. And there is plenty of potential food for the Mirror Life in our environment. Although triglicerides (fats) are technically not symmetric (if all three radicals are different) their (+) and (-) forms may be digested by enzyme of any chirality, leading to glycerol(symmetric) and fat acids (also symmetric). Glycine, citric acid, acetic acid and many other molecules are all symmetric and can be the source of carbon. A potential pathogen doesn't have to be able to digest its host completely (and none of them actually do). From the point of view of a mirrored microbe we are Petri dishes, with a relatively inert medium soaked with tasty solution of low-molecular food. Natural nucleosides and nucleotides contain D-sugars, but nucleotide bases are symmetric and could be used as a building block for RNA and DNA of Mirror life.
Non-photosynthetic Mirror life can deplete oceans and atmosphere of CO2 as well. Their lipids can be digested, but D-proteins are much more prone to hydrolysis, and carbohydrates of the cell wall will be probably almost undigestible. Something like 100 kg/m^2 (quick calculation, could be wrong) of deposits on the seabed and it's over.