Department of Bacteriology, University of Wisconsin - Madison, Madison, Wisconsin, USA.
Division of Biological Sciences, University of California San Diego, La Jolla, California, USA.
Geobiology. 2023 May;21(3):390-403. doi: 10.1111/gbi.12543. Epub 2023 Jan 5.
Carbon isotope biosignatures preserved in the Precambrian geologic record are primarily interpreted to reflect ancient cyanobacterial carbon fixation catalyzed by Form I RuBisCO enzymes. The average range of isotopic biosignatures generally follows that produced by extant cyanobacteria. However, this observation is difficult to reconcile with several environmental (e.g., temperature, pH, and CO concentrations), molecular, and physiological factors that likely would have differed during the Precambrian and can produce fractionation variability in contemporary organisms that meets or exceeds that observed in the geologic record. To test a specific range of genetic and environmental factors that may impact ancient carbon isotope biosignatures, we engineered a mutant strain of the model cyanobacterium Synechococcus elongatus PCC 7942 that overexpresses RuBisCO across varying atmospheric CO concentrations. We hypothesized that changes in RuBisCO expression would impact the net rates of intracellular CO fixation versus CO supply, and thus whole-cell carbon isotope discrimination. In particular, we investigated the impacts of RuBisCO overexpression under changing CO concentrations on both carbon isotope biosignatures and cyanobacterial physiology, including cell growth and oxygen evolution rates. We found that an increased pool of active RuBisCO does not significantly affect the C/ C isotopic discrimination (ε ) at all tested CO concentrations, yielding ε of ≈ 23‰ for both wild-type and mutant strains at elevated CO . We therefore suggest that expected variation in cyanobacterial RuBisCO expression patterns should not confound carbon isotope biosignature interpretation. A deeper understanding of environmental, evolutionary, and intracellular factors that impact cyanobacterial physiology and isotope discrimination is crucial for reconciling microbially driven carbon biosignatures with those preserved in the geologic record.
在地质记录中保存的碳同位素生物特征主要被解释为反映了古代蓝细菌通过 I 型 RuBisCO 酶催化的固碳作用。同位素生物特征的平均范围通常与现存蓝细菌产生的范围一致。然而，这一观察结果很难与几个环境（例如温度、pH 值和 CO 浓度）、分子和生理因素相协调，这些因素在地质历史时期可能存在差异，并可能导致现代生物中产生的分馏变化超过地质记录中观察到的变化。为了测试可能影响古代碳同位素生物特征的特定遗传和环境因素范围，我们对模式蓝细菌 Synechococcus elongatus PCC 7942 的突变株进行了工程改造，该突变株在不同大气 CO 浓度下过表达 RuBisCO。我们假设 RuBisCO 表达的变化会影响细胞内 CO 固定与 CO 供应的净速率，从而影响整个细胞的碳同位素分馏。特别是，我们研究了在 CO 浓度变化下 RuBisCO 过表达对碳同位素生物特征和蓝细菌生理学的影响，包括细胞生长和氧气演化速率。我们发现，活性 RuBisCO 池的增加并不会显著影响所有测试 CO 浓度下的 C/ C 同位素分馏（ε ），在高 CO 条件下，野生型和突变型菌株的 ε 均约为 23‰。因此，我们认为，预期的蓝细菌 RuBisCO 表达模式的变化不应混淆碳同位素生物特征的解释。深入了解影响蓝细菌生理学和同位素分馏的环境、进化和细胞内因素对于协调微生物驱动的碳生物特征与地质记录中保存的生物特征至关重要。
