Molecular and Biochemical Basis for Stable Carbon Isotope Ratios in Hydrothermal Vent Ecosystems

对稳定同位素的研究使我们能够深入了解碳循环的各个方面。例如，这种稳定的碳同位素数据已被用来支持一些热液喷口动物依赖于化学合成内共生生物产生的有机物的假设。不幸的是，在各种生态系统中观测到的同位素分馏并不总是很容易理解，导致对给定生态系统中物种之间的热带关系得出模棱两可的结论。热液喷口生态系统再次提供了一个例子。共生双壳类的del13C值（del13C = -27%至-35%）与自由生活的趋化自养生物相似，表明共生生物的营养作用。相比之下，管虫-化学自养共生的食物来源就不太清楚了，因为这些组织表现出惊人的重稳定同位素值（del13Cs = -9至-35%）。虽然提出了许多理论，但共生的较重同位素值从未得到充分解释。对不同Rubisco表达形式的识别可能具有深远的意义，而不仅仅是对热液喷口生态系统碳循环的解释。最近在鞭毛藻中发现了II型Rubisco，这表明这种酶并不局限于细菌自养生物，并且可以很好地解释从开放海洋中回收的有机物的同位素重del13C值。准确地解决各种形式的固定碳酶Rubisco的影响，可能对生态和古生物研究中del13C数据的解释产生巨大影响。＊＊＊

9504257 Cavanaugh Studies of stable isotopes allow valuable insight into various aspects of carbon cycling. For example, such stable carbon isotope data have been used to lend support to the putative dependence of several hydrothermal vent animals on organic matter produced by chemosynthetic endosymbionts. Unfortunately the observed isotope fractionations in various ecosystems have not always been readily understood, leading to equivocal conclusions regarding tropic relations between species in a given ecosystem. Again, the hydrothermal vent ecosystems provide an example of this. del13C values measured for symbiotic bivalves (del13C = -27% to -35%) mimic those of free-living chemoautotrophs, indicating the symbionts' nutritional role. In contrast, the food sources are less clear for tubeworm-chemoautotroph symbioses since these tissues exhibit surprisingly heavy stable isotope values (del13Cs = -9 to -35%). While a number of theories have been presented, the heavier isotope values for the symbioses have never been adequately explained. The recognition of different Rubisco forms being expressed may have far reaching implications, beyond interpretations of carbon cycling at hydrothermal vent ecosystems. The recent discovery of a form II Rubisco in a dinoflagellate indicates that this enzyme is not confined to the bacterial autotrophs, and may well explain the isotopically heavy del13C values of organic matter recovered from the open ocean. Accurately resolving the effects of various forms of the carbon fixation enzyme, Rubisco, may have a tremendous impact on interpretation of del13C data in ecological and paleontological studies. ***