We assess progress toward the protection of 50% of the terrestrial biosphere to address the species-extinction crisis and conserve a global ecological heritage for future generations. Using a map of Earth's 846 terrestrial ecoregions, we show that 98 ecoregions (12%) exceed Half Protected; 313 ecoregions (37%) fall short of Half Protected but have sufficient unaltered habitat remaining to reach the target; and 207 ecoregions (24%) are in peril, where an average of only 4% of natural habitat remains. We propose a Global Deal for Nature—a companion to the Paris Climate Deal—to promote increased habitat protection and restoration, national- and ecoregion-scale conservation strategies, and the empowerment of indigenous peoples to protect their sovereign lands. The goal of such an accord would be to protect half the terrestrial realm by 2050 to halt the extinction crisis while sustaining human livelihoods.

The possibility of HC03-transport in the blue-green alga (cyanobac- It is generally accepted that various algal species are capable of transporting the bicarbonate ion across the cell membrane for use in photosynthesis (25), but substantive evidence is lacking in most cases. Raven (25) suggested that if the rate of photosynthesis by an alga is markedly higher at an alkaline pH, for a given CO2 concentration, than at a more acid pH it can be concluded that the bicarbonate ion crosses the cell membrane and contributes C for photosynthesis. By this criterion, Raven (24) case does not rest solely upon a comparison of photosynthetic rates at acid and alkaline pH values. Lucas (16) has shown that C. corallina at pH 9.0 has a rate of photosynthesis higher than can be supported solely by the fixation of CO2 derived from the spontaneous dehydration of HCO3 in the external medium. It is probable that these cells take up HC03-in exchange for OH-(generated by photosynthetic fixation of the transported HCO3) on distinct ion carriers in the cell membrane (17). Tailing (30) has shown that the algae Microcystis aeruginosa and Ceratium hirudinella both have photosynthetic rates at pH values of 10 or greater that are in excess of the rates that could be supported by the spontaneous dehydration of HC03-in the medium. Transport of HCO3-across cell membranes is thus indicated.In an interesting series of experiments Badger et al. (2) have shown that both Chlamydomonas reinhardtii and Anabaena variabilis, when grown at low ambient CO2 concentrations, can substantially accumulate inorganic C. This accumulation is not the result of a more alkaline intracellular pH relative to the external medium (2) and a "C02-concentrating mechanism" is indicated, presumably across the cell membrane.We have been studying the photosynthetic carbon metabolism of the blue-green alga Coccochloris peniocystis (7,14). This alga has been placed in the genus Synechococcus by Stanier et al. (29), along with other blue-green algae with cylindrical cells that undergo repeated binary fission in a single plane, which frequently results (as with C. peniocystis) in the formation of short chains of cells. Recently, Birmingham and Colman (4) have shown that C. peniocystis, at pH 7.9 and a low inorganic C concentration, has a rate of photosynthesis 5-fold that is supportable by CO2 production from the spontaneous dehydration of HCO3 in the medium. In this paper we report further investigations of HCO3-transport in C. peniocystis. MATERIALS AND METHODSOrganism and Growth Conditions. C. peniocystis Kutz (1548) was obtained as an axenic culture from the algal collection at Indiana University, Bloomington, and was cultured as previously described (21). The cell density at harvest was such that the Chl content was 6-1 1 ,ug/ml. Cells were harvested by centrifugation at 6,0)0g at 25 C and were then washed with the solution to be used in the subsequent experiment (usually 20 mm K2HPO4 phosphate adjusted to pH 8.0 with 1 N NaOH).

Mass spectrometry has been used to confirm the presence of an active transport system for CO2 in Synechococcus UTEX 625. Cells were incubated at pH 8.0 in 100 micromolar KHCO3 in the absence of Na+ (to prevent . Upon illumination the ceUls rapidly removed almost all the free CO2 from the medium. Addition of carbonic anhydrase revealed that the CO2 depletion resulted from a selective uptake of CO2. rather than a total uptake of all inorganic carbon species. CO2 transport stopped rapidly (<3 seconds) when the light was turned off. lodoacetamide (3.3 millimolar) completely inhibited CO2 fixation but had little effect on CO2 transport. In iodoacetamide poisoned cells, transport of CO2 occurred against a concentration gradient of about 18,000 to 1. Transport of CO2 was completely inhibited by 10 micromolar diethylstilbestrol, a membrane-bound ATPase inhibitor. Studies with DCMU and PSI light indicated that CO2 transport was driven by ATP produced by cyclic or pseudocyclic photophosphorylation. Low concentrations of Na+ (<100 microequivalents per liter), but not of K+, stimulated CO2 transport as much as 2.4-fold. Unlike Na+-dependent HC03-transport, the transport of CO2 was not inhibited by high concentrations (30 milliequivalents per liter) of Li'. During illumination, the CO2 concentration in the medium remained far below its equilibrium value for periods up to 15 minutes. This could only happen if CO2 transport was continuously occurring at a rapid rate, since the continuing dehydration of HC03-to CO2 would rapidly raise the CO2 concentration to its equilibrium value if transport ceased. Measurement of the rate of dissolved inorganic carbon accumulation under these conditions indicated that at least part of the continuing CO2 transport was balanced by HCO3-efflux.Photosynthesis by cyanobacteria can occur when the CO2 concentration in the extracellular medium is so low that CO2 fixation via Rubisco2 could not occur were it not for the presence of 'CO2-concentrating' mechanisms (1,2,9,13,16,19,21,25,28 (1,2,9,18,29). For a given DIC concentration, the rate of DIC accumulation was faster under the nonequilibrium conditions (high CO,/HCO3-) than under equilibrium conditions (high HCO3-/C0.), thus indicating a lower Kmn for CO2 transport than for HCO3 transport (1. 2, 9, 29).Miller and Canvin (17) provided further evidence for a CO,-transport capacity, distinct from the HCO3-transport capacity, when they made use of the observation that HCO-transport in rapidly growing cells of Synechococcus UTEX 625 is inhibited by the absence of Na+ in the extracellular medium (8,17,22). Cells that were incubated in the absence of Na + were stimulated to accumulate normal levels of intracellular DIC by the addition of CA (17). It was postulated that, in the absence of the CA, the rate of supply of CO2 to the CO,-transport system was limited by the rate of HCO3-dehydration to CO2 in the extracellular medium. The DIC transport occurring in the presence of CA was not inhibited by the addition of Li+, whereas the Na+-dependent...

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Mass spectrometric measurements of dissolved free 13CO2 were used to monitor CO2 uptake by air grown (low CO2) cells and protoplasts from the green alga Chlamydomonas reinhardtli. In the presence of 50 micromolar dissolved inorganic carbon and light, protoplasts which had been washed free of extemal carbonic anhydrase reduced the 13CO2 concentration in the medium to close to zero. Similar resuits were obtained with low CO2 cells treated with 50 micromolar acetazolamide. Addition of carbonic anhydrase to protoplasts after the period of rapid CO2 uptake revealed that the removal of CO2 from the medium in the light was due to selective and active CO2 transport rather than uptake of total dissolved inorganic carbon. demonstrated for these organisms (2, 4, 9, 15). In the case of cyanobacteria, both HCO3-and CO2 are substrates for active transport (2,3,6,7,14,15) with CO2 being selectively and preferentially used by the cells (2,6,16). In Chlamydomonas, HC03-is actively transported (4, 25, 29), but CO2 uptake has been considered to be passive (18,20). Carbon dioxide, however, is taken up from the medium faster than HCO3-by Chlamydomonas (13,28,29) and several authors (13,29) have considered the possibility of active CO2 transport.Studies on the DIC transport mechanism of green algae are complicated by their cellular compartmentation. Recently, it was shown that isolated chloroplasts of low C02 Chlamydomonas reinhardtii were able to accumulate DIC (19) and a model was presented where the only active DIC transport mechanism was located on the chloroplast envelope (18,19). In that model the plasma membrane was suggested to be only a diffusion barrier for CO2 generated by external carbonic anhydrase. In contrast, by comparison of the apparent affinities for DIC of whole cells and purified chloroplasts, Suiltemeyer et al. (26) came to the conclusion that active transport by the chloroplast alone may not be responsible for the photosynthetic characteristics of whole cells.Another difficulty in examining the DIC species taken up by whole cells is the presence of an external carbonic anhydrase (10) which catalyzes the rapid equilibrium between CO2 and HCO3-, thus making a direct discrimination between CO2 and HCO3-uptake impossible (7, 13). However, using inhibitors for external carbonic anhydrase or the cell-wall less mutant CW-15, some authors came to the conclusion that CO2 and not HCO3-(13) or that both C02 and HCO3- (29) were actively transported.Confusion about which DIC species is actively taken up from the medium may also be caused by methods which only measure total rates of transport rather than transport of CO2 or HC03-individually. Using MS, which measures free dissolved gases in liquid, several authors presented direct eviGreen algae and cyanobacteria possess a high apparent affinity for DIC3 when grown at low DIC concentrations (low CO2 cells: 2,5,9,17), and DIC accumulation has been '