This investigation examines the occurrence of carbon concentrating mechanisms (CCMs) in eight species of the acellular green marine macroalgal genus Caulerpa. The measurements made were of the ␦ 13 C of organic matter, extracellular carbonic anhydrase activities, pH compensation values, and the inorganic C dependence of light-saturated photosynthesis rates. The data suggest that the pyrenoid-containing C. cactoides and C. geminata, and probably C. scalpelliformis (which lacks pyrenoids) have CCMs. Net diffusive influx of CO 2 fulfills the inorganic carbon requirements of the other species for which pH-drift data are available, i.e. C. flexilis, C. longifolia, C. obscura and C. brownii. No pH drift data are available for C. trifaria and no information is available as to whether it has pyrenoids, although ␦ 13 C data suggest the absence of a CCM in this species. The three species showing evidence of CCMs have the lowest affinities for inorganic C of the eight species tested. This apparently paradoxical finding has precedence for marine red-macroalgae, and requires that the selective significance of the CCMs in these organisms is not that of increased inorganic C affinity, but is perhaps associated with the ability to both suppress photoinhibition and to photosynthesize at higher seawater pH values.

The tissue previously described as paraveinal mesophyll in soybean leaves is shown to have the characters of bundle sheath rather than mesophyll cells and is renamed "extended bundle sheath" (EBS) tissue. Its presence was surveyed by leaf clearing in 66 species of legumes of all three subfamilies. A complete extended bundle sheath system similar to that previously described in soybean was identified in 21 of the species. This system is a paradermally oriented tissue, one cell deep, between the spongy and palisade mesophylls, consisting of extended bundle sheath cells, which join each other across the interveinal space either directly or via bridging cells of somewhat similar shape and size. A newly recognized, attenuated extended bundle sheath system, in which bundle sheath cells extend but do not form a continuum except in very narrow interveinal spaces, is described; it was found in 32 species. Extended bundle sheath tissue was absent from 13 of the species. The presence or form of extended bundle sheath tissue does not follow traditional taxonomic divisions. Extended bundle sheath systems were also found in 3 of 5 nonlegume species.

The lower limits of photosynthetically useable radiation at which growth and photosynthesis can occur establish the lower boundaries for the extent of photolithotrophy in the biosphere. Photolithotrophic growth denotes the capacity to grow with photons as the sole energy input. Slippage in terms of photosynthetic energy conversion implies a less than theoretical stoichiometry of energy-transduction process(es) such as the dissipation of intermediates of O(2) evolution and of ATP synthesis (H(+)/e(-) and H(+)/ATP ratios). Slippage is particularly important in limiting the growth of photolithotrophic organisms at very low photon fluence rates. We found that Dunaliella tertiolecta and Phaeodactylum tricornutum avoid such reductions in photon use efficiency by increasing the size and number of their photosynthetic units, respectively, and by altering Q(A) reduction kinetics on the reducing side of PS II. P. tricornutum is also less susceptible to slippage in terms of the breakdown of intermediates in its O(2) evolution pathway than D. tertiolecta. Minimizing H(+) leakage through the CF(0)-CF(1) ATP synthetase (and other H(+ )porters) is also discussed briefly. In combination, strategies employed by P. tricornutum effectively allow it to grow and photosynthesize at lower rates of energy input than D. tertiolecta, consistent with our observations. Differences in the responses of the photosynthetic apparatus of these two marine microalgae are mechanistic and probably representative of evolutionary divergences associated with strategies for dealing with environmental perturbations.

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