Biliproteins and phycobilisomes from cyanobacteria and red algae at the extremes of habitat.

This review considers the properties of biliproteins from cyanobacteria and red algae that grow in extreme habitats. Three situations are presented: cyanobacteria that grow at high temperatures; a red alga that grows in acidic conditions at high temperature; and an Antarctic red alga that grows in the cold in dim light conditions. In particular, the properties of their biliproteins are compared to those from organisms from more usual environments. C-phycocyanins from two cyanobacteria able to grow at high temperatures are found to differ in their stabilities when compared to C-phycocyanin from mesophilic algae. They differ in opposite ways, however. One is more stable to dissociation than the mesophilic protein, and the other is more easily dissociated at low temperatures. The thermophilic proteins resist thermal denaturation much better than the mesophilic proteins. The most thermophilic cyanobacterium has a C-phycocyanin with a unique blue-shifted absorption maximum which does not appear to be part of the adaptation of the cyanobacterium to high temperature. The C-phycocyanin from the high-temperature red alga is able to resist dissociation better than mesophilic C-phycocyanins. Electron micrographs show the phycobilisomes of these algae. The Antarctic alga grows under ice at some distance down the water column. Its R-phycoerythrin has a novel absorption spectrum that gives the alga an improved ability to harvest blue light. This may enhance its survival in its light-deprived habitat.