Photorespiration, or C₂ photosynthesis, is generally considered a futile cycle that potentially decreases photosynthetic carbon fixation by more than 25%. Nonetheless, many essential processes, such as nitrogen assimilation, C₁ metabolism, and sulfur assimilation, depend on photorespiration. Most studies of photosynthetic and photorespiratory reactions are conducted with magnesium as the sole metal cofactor despite many of the enzymes involved in these reactions readily associating with manganese. Indeed, when manganese is present, the energy efficiency of these reactions may improve. This review summarizes some commonly used methods to quantify photorespiration, outlines the influence of metal cofactors on photorespiratory enzymes, and discusses why photorespiration may not be as wasteful as previously believed.

Rubisco, the most prevalent protein on the planet, initiates the conversion of CO ₂ into carbohydrates during photosynthesis. Responses of this process to atmospheric CO ₂ fluctuations daily, seasonally, and over millennia is still poorly understood. We have hypothesized that higher plants maintain carbon-to-nitrogen homeostasis by adjusting the balance of magnesium and manganese in chloroplasts to alter their relative carbon fixation and nitrogen assimilation rates. The following study examined the influence of magnesium and manganese on rubisco carboxylation and oxygenation in protein purified from two ecotypes of Plantago lanceolata : one adapted to the high atmospheric CO ₂ that occurs near a natural CO ₂ spring and the other adapted to more typical CO ₂ atmospheres that occur nearby. The plastid DNA coding for the large unit of rubisco were similar in both ecotypes. The kinetics of rubiscos from the two ecotypes differed more when they were associated with manganese than magnesium. Specificity for CO ₂ over O ₂ ( S _c/o ) for rubiscos from both ecotypes were higher when the enzymes were bound to magnesium than manganese. This disparity may account for the adaptation of this species to different CO ₂ environments.