Buffer sensitivity of photosynthetic carbon utilisation in eight tropical seagrasses

Some of the mechanisms involved in inorganic carbon (Ci) acquisition by tropical seagrasses from the western Indian Ocean were described by Bjork et al. (Mar Biol 129:363-366, 1997). However, since then, it has been found that an additional, buffer-sensitive, system of Ci Litilisation may operate in some temperate seagrasses (Hellblom et al. in Aquat Bot 69:55-62, 2001, Hellblom and Axelsson in Photos Res 77:173-191, 2003); this buffer sensitivity indicates a mechanism in which electrogenic H+ extrusion may form acidic diffusion boundary layers, in which either HCO3--H+ is co-transported into the cells, or where HCO3 is converted to CO2 (as catalysed by carbonic anhydrase) prior to uptake of the latter Ci form. Because a buffer was used in the 1997 study, we found it important to reinvestigate those same eight species, taking into account the direct effect of buffers on this potential mode of Ci acquisition in these plants. In doing so, it was found that all seagrass species investigated except Cymodocea serrulata were sensitive to 50 mM TRIS buffer of the same pH as the natural seawater in which they grew (pH 8.0). Especially sensitive were Halophila ovalis, Halodule wrightii and Cymodocea rotundata, which grow high up in the intertidal zone (only ca. 50-65% of the net photosynthetic activity remained after the buffer additions), followed by the submerged Enhalus acoroides and Syringodium isoetifolium (ca. 75% activity remaining), while Thalassia hemprichii and Thalassodendron ciliatum, which grow in-between the two zones, were less sensitive to buffer additions (ca. 80-85% activity remaining). In addition to buffer sensitivity, all species were also sensitive to acetazolamide (AZ, an inhibitor of extracellular carbonic anhydrase activity) such that ca. 45-80% (but 90% for H. ovalis) of the net photosynthetic activity remained after adding this inhibitor. Raising the pH to 8.8 (in the presence of AZ) drastically reduced net photosynthetic rates (0-14% remaining in all species); it is assumed that this reduction in rates was due to the decreased CO2 concentration at the higher pH. These results indicate that part of the 1997 results for the same species were due to a buffer effect on net photosynthesis. Based on the present results, it is concluded that (1) photosynthetic Ci acquisition in six of the eight investigated species is based on carbonic anhydrase catalysed HCO3- to CO2 conversions within an acidified diffusion boundary layer, (2) C. serrulata appears to support its photosynthesis by extracellular carbonic anhydrase catalysed CO2 formation from HCO3- without the need for acidic zones, (3) H. ovalis features a system in which H+ extrusion may be followed by HCO3--H+ co-transport into the cells, and (4) direct, non-H+ -mediated, uptake of HCO3- is improbable for any of the species.