On-line mass spectrometry: membrane inlet sampling.

Significant insights into plant photosynthesis and respiration have been achieved using membrane inlet mass spectrometry (MIMS) for the analysis of stable isotope distribution of gases. The MIMS approach is based on using a gas permeable membrane to enable the entry of gas molecules into the mass spectrometer source. This is a simple yet durable approach for the analysis of volatile gases, particularly atmospheric gases. The MIMS technique strongly lends itself to the study of reaction flux where isotopic labeling is employed to differentiate two competing processes; i.e., O(2) evolution versus O(2) uptake reactions from PSII or terminal oxidase/rubisco reactions. Such investigations have been used for in vitro studies of whole leaves and isolated cells. The MIMS approach is also able to follow rates of isotopic exchange, which is useful for obtaining chemical exchange rates. These types of measurements have been employed for oxygen ligand exchange in PSII and to discern reaction rates of the carbonic anhydrase reactions. Recent developments have also engaged MIMS for online isotopic fractionation and for the study of reactions in inorganic systems that are capable of water splitting or H(2) generation. The simplicity of the sampling approach coupled to the high sensitivity of modern instrumentation is a reason for the growing applicability of this technique for a range of problems in plant photosynthesis and respiration. This review offers some insights into the sampling approaches and and the experiments that have been conducted with MIMS.

Multiple Rubisco forms in proteobacteria: their functional significance in relation to CO2 acquisition by the CBB cycle.

Rubisco is the predominant enzymatic mechanism in the biosphere by which autotrophic bacteria, algae, and terrestrial plants fix CO(2) into organic biomass via the Calvin-Benson-Basham reductive pentose phosphate pathway. Rubisco is not a perfect catalyst, suffering from low turnover rates, a low affinity for its CO(2) substrate, and a competitive inhibition by O(2) as an alternative substrate. As a consequence of changing environmental conditions over the past 3.5 billion years, with decreasing CO(2) and increasing O(2) in the atmosphere, Rubisco has evolved into multiple enzymatic forms with a range of kinetic properties, as well as co-evolving with CO(2)-concentrating mechanisms to cope with the different environmental contexts in which it must operate. The most dramatic evidence of this is the occurrence of multiple forms of Rubisco within autotrophic proteobacteria, where Forms II, IC, IBc, IAc, and IAq can be found either singly or in multiple combinations within a particular bacterial genome. Over the past few years there has been increasing availability of genomic sequence data for bacteria and this has allowed us to gain more extensive insights into the functional significance of this diversification. This paper is focused on summarizing what is known about the diversity of Rubisco forms, their kinetic properties, development of bacterial CO(2)-concentrating mechanisms, and correlations with metabolic flexibility and inorganic carbon environments in which proteobacteria perform various types of obligate and facultative chemo- and photoautotrophic CO(2) fixation.

New roads lead to Rubisco in archaebacteria.

The discovery of the CO(2)-fixing enzyme Rubisco in the Archaebacteria has presented a conundrum in that they apparently lack the gene for phosphoribulokinase, which is required to generate Rubisco's substrate ribulose 1,5-bisphosphate (RuBP). However, two groups have now demonstrated novel RuBP synthesis pathways, demystifying Rubisco's non-autotrophic and perhaps ancient role. A new CO(2) fixing role for Rubisco, which is distinct from the globally dominant Calvin cycle, is providing important clues furthering our understanding of the evolution of autotrophy. This perspective is strengthened by the additional recognition in this commentary that some Rubisco-containing Archaea do also contain PRK and may represent an interesting autotrophic evolutionary transition. Supplementary material for this article can be found on the BioEssays website (http://www.interscience.wiley.com/jpages/0265-9247/suppmat/index.html).