The induction of pyrenoid synthesis by hyperoxia and its implications for the natural diversity of photosynthetic responses in Chlamydomonas

In algae, it is well established that the pyrenoid, a component of the carbon-concentrating mechanism (CCM), is essential for efficient photosynthesis at low CO(2). However, the signal that triggers the formation of the pyrenoid has remained elusive. Here, we show that, in Chlamydomonas reinhardtii, the pyrenoid is strongly induced by hyperoxia, even at high CO(2) or bicarbonate levels. These results suggest that the pyrenoid can be induced by a common product of photosynthesis specific to low CO(2) or hyperoxia. Consistent with this view, the photorespiratory by-product, H(2)O(2), induced the pyrenoid, suggesting that it acts as a signal. Finally, we show evidence for linkages between genetic variations in hyperoxia tolerance, H(2)O(2) signaling, and pyrenoid morphologies.