Although the green alga Chlamydomonas reinhardtii has long served as a reference organism, few studies have interrogated its role as a primary producer in microbial interactions. Here, we quantitatively investigated C. reinhardtii’s capacity to support a heterotrophic microbe using the established coculture system with Mesorhizobium japonicum, a vitamin B12-producing α-proteobacterium. Using stable isotope probing and nanoscale secondary ion mass spectrometry (nanoSIMS), we tracked the flow of photosynthetic fixed carbon and consequent bacterial biomass synthesis under continuous and diurnal light with single-cell resolution. We found that more 13C fixed by the alga was taken up by bacterial cells under continuous light, invalidating the hypothesis that the alga’s fermentative degradation of starch reserves during the night would boost M. japonicum heterotrophy. 15NH4 assimilation rates and changes in cell size revealed that M. japonicum cells reduced new biomass synthesis in coculture with the alga but continued to divide – a hallmark of nutrient limitation often referred to as reductive division. Despite this sign of starvation, the bacterium still synthesized vitamin B12 and supported the growth of a B12-dependent C. reinhardtii mutant. Finally, we showed that bacterial proliferation could be supported solely by the algal lysis that occurred in coculture, highlighting the role of necromass in carbon cycling. Collectively, these results reveal the scarcity of fixed carbon in this microbial trophic relationship (particularly under environmentally relevant light regimes), demonstrate B12 exchange even during bacterial starvation, and underscore the importance of quantitative approaches for assessing metabolic coupling in algal-bacterial interactions.

中文翻译：

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微生物光养-异养相互作用模型中固定碳转移的稀缺


尽管绿藻莱茵衣藻长期以来一直被用作参考生物，但很少有研究质疑其作为微生物相互作用中初级生产者的作用。在这里，我们使用已建立的与 Mesorhizobium japonicum（一种产生维生素 B12 的 α-变形菌）共培养系统，定量研究了莱茵衣藻支持异养微生物的能力。使用稳定同位素探测和纳米级二次离子质谱（nanoSIMS），我们以单细胞分辨率跟踪了连续日光下光合作用固定碳的流动以及随后的细菌生物量合成。我们发现，在连续光照下，更多的藻类固定的 13C 被细菌细胞吸收，这使得藻类在夜间发酵降解淀粉储备会促进日本海藻异养的假设无效。 15NH4 同化率和细胞大小的变化表明，日本海藻细胞在与藻类共培养中减少了新的生物量合成，但继续分裂——营养限制的标志，通常称为还原分裂。尽管存在这种饥饿的迹象，该细菌仍然合成维生素 B12 并支持依赖 B12 的莱茵衣藻突变体的生长。最后，我们表明细菌增殖可以仅由共培养中发生的藻类裂解来支持，这突出了坏死物在碳循环中的作用。总的来说，这些结果揭示了这种微生物营养关系中固定碳的稀缺性（特别是在环境相关的光照条件下），证明了即使在细菌饥饿期间也存在 B12 交换，并强调了定量方法评估藻类-细菌相互作用中代谢耦合的重要性。