To examine the impact of heterotrophic bacteria on microalgal physiology, we co-cultured the diatom Phaeodactylum tricornutum with six bacterial strains to quantify bacteria-mediated differences in algal biomass, total intracellular lipids, and for a subset, extracellular metabolite accumulation. A Marinobacter isolate significantly increased algal cell concentrations, dry biomass, and lipid content compared to axenic algal cultures. Two other bacterial strains from the Bacteroidetes order, of the genera Algoriphagus and Muricauda, significantly lowered P. tricornutum biomass, leading to overall decreased lipid accumulation. These three bacterial co-cultures (one mutualistic, two competitive) were analyzed for extracellular metabolites via untargeted liquid chromatography mass spectrometry to compare against bacteria-free cultures. Over 80% of the extracellular metabolites differentially abundant in at least one treatment were in higher concentrations in the axenic cultures, in agreement with the hypothesis that the co-cultured bacteria incorporated algal-derived organic compounds for growth. Furthermore, the extracellular metabolite profiles of the two growth-inhibiting cultures were more similar to one another than the growth-promoting co-culture, linking metabolite patterns to ecological role. Our results show that algal–bacterial interactions can influence the accumulation of intracellular lipids and extracellular metabolites, and suggest that utilization and accumulation of compounds outside the cell play a role in regulating microbial interactions.

中文翻译：

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细菌对三角褐指藻（芽孢杆菌科）生长、脂质产生和细胞外代谢物积累的影响

为了检查异养细菌对微藻生理学的影响，我们将硅藻三角褐指藻与六种细菌菌株共培养，以量化细菌介导的藻类生物量、细胞内总脂质和细胞外代谢物积累的差异。甲Marinobacter分离物显著增加藻类细胞浓度，干燥生物质，并与无菌藻类培养脂质含量。来自拟杆菌目的另外两种细菌菌株，即Algoriphagus和Muricauda属，显着降低了P. tricornutum生物量，导致脂质积累总体减少。通过非靶向液相色谱质谱法分析这三种细菌共培养物（一种互惠，两种竞争）的细胞外代谢物，以与无细菌培养物进行比较。在至少一种处理中，超过 80% 差异丰富的细胞外代谢物在无菌培养物中浓度较高，这与共培养细菌掺入藻类衍生有机化合物以促进生长的假设一致。此外，与促进生长的共培养物相比，两种生长抑制培养物的细胞外代谢物谱彼此更相似，将代谢物模式与生态作用联系起来。