The unicellular alga Chlamydomonas reinhardtii and the bacterium Pseudomonas protegens serve as a model to study the interactions between photosynthetic and heterotrophic microorganisms. P. protegens secretes the cyclic lipopeptide orfamide A that interferes with cytosolic Ca²⁺ homeostasis in C. reinhardtii resulting in deflagellation of the algal cells. Here, we studied the roles of additional secondary metabolites secreted by P. protegens using individual compounds and co-cultivation of algae with bacterial mutants. Rhizoxin S2, pyrrolnitrin, pyoluteorin, 2,4-diacetylphloroglucinol (DAPG) and orfamide A all induce changes in cell morphology and inhibit the growth of C. reinhardtii. Rhizoxin S2 exerts the strongest growth inhibition, and its action depends on the spatial structure of the environment (agar versus liquid culture). Algal motility is unaffected by rhizoxin S2 and is most potently inhibited by orfamide A (IC₅₀ = 4.1 μM). Pyrrolnitrin and pyoluteorin both interfere with algal cytosolic Ca²⁺ homeostasis and motility whereas high concentrations of DAPG immobilize C. reinhardtii without deflagellation or disturbance of Ca²⁺ homeostasis. Co-cultivation with a regulatory mutant of bacterial secondary metabolism (ΔgacA) promotes algal growth under spatially structured conditions. Our results reveal how a single soil bacterium uses an arsenal of secreted antialgal compounds with complementary and partially overlapping activities.

中文翻译：

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细菌 Pseudomonas protegens 使用毒素混合物对抗微藻莱茵衣藻

单细胞藻类莱茵衣藻和细菌Pseudomonas protegens可作为研究光合微生物和异养微生物之间相互作用的模型。P。protegens分泌环状脂肽 orfamide A，它干扰C 中的细胞溶质 Ca ²⁺稳态。reinhardtii导致藻细胞脱鞭毛。在这里，我们研究了P分泌的其他次生代谢物的作用。保护素使用单独的化合物和藻类与细菌突变体的共同培养。Rhizoxin S2、pyrrolnitrin、pyoluteorin、2,4-diacetylphloroglucinol (DAPG) 和 orfamide A 都诱导细胞形态的变化并抑制C的生长。莱茵衣藻。Rhizoxin S2 发挥最强的生长抑制作用，其作用取决于环境的空间结构（琼脂与液体培养）。藻类运动不受根瘤菌素 S2 的影响，受 orfamide A 的抑制作用最强 (IC ₅₀  = 4.1 μM)。硝吡咯和藤黄绿菌素两者干扰藻类细胞内的Ca ²⁺内稳态和运动性，而高浓度的DAPG固定的Ç。莱茵哈特没有鞭毛或 Ca ²⁺稳态的干扰。与细菌次级代谢调节突变体 (Δ gacA ) 的共培养促进了空间结构条件下的藻类生长。我们的结果揭示了单个土壤细菌如何使用具有互补和部分重叠活性的分泌性抗藻化合物库。