Cyanobacterial harmful algal blooms (cyanoHABs) continue to increase in frequency and magnitude, threatening global freshwater ecosystems and services. In north-temperate lakes cyanobacteria appear in early summer, succeeding green algae as the dominant phytoplankton group, a pattern thought to be mediated by changes in temperature and bioavailable nutrients. To understand additional drivers of this successional pattern our study used reciprocal invasion experiments to examine the competitive interaction between Microcystis aeruginosa, a dominant contributor to cyanoHABs, and the green alga Chlorella sorokiniana. We considered two factors that may impact these interactions: (1) strain variation, with a specific emphasis on the presence or absence of the gene for the hepatotoxin microcystin, and (2) host-associated bacteria. We used toxic M. aeruginosa PCC 7806 (microcystin producing strain), a non-toxic mutant of PCC 7806, non-toxic M. aeruginosa PCC 9701 (non-microcystin producing strain), and C. sorokiniana. Each organism was available free of all bacteria (i.e., axenic) and with a re-introduced defined bacterial community to generate their xenic counterparts. Competitive interactions were assessed with reciprocal invasion experiments between paired xenic and paired axenic populations of C. sorokiniana and one of the two Microcystis strains, each assessed separately. Flow cytometry and random forest models were used to rapidly discriminate and quantify phytoplankton population densities with 99% accuracy. We found that M. aeruginosa PCC 7806, but not strain PCC 9701, could proliferate from low abundance in a steady-state population of C. sorokiniana. Further, the presence of bacteria allowed M. aeruginosa PCC 7806 to grow to a higher population density into an established C. sorokiniana population than when grown axenic. Conversely, when M. aeruginosa was dominant, C. sorokiniana was only able to proliferate from low density into the PCC 9701 strain, and only when axenic. The mutant of PCC 7806 lacking the ability to produce microcystin behaved similarly to the toxic wild-type, implying microcystin is not responsible for the difference in competitive abilities observed between the two wild-type strains. Quantification of microcystins (MCs) when PCC 7806 M. aeruginosa was introduced into the C. sorokiniana culture showed two-fold more MCs per cell when host-associated bacteria were absent compared to present in both species cultures. Our results show that the ability of M. aeruginosa to compete with C. sorokiniana is determined by genomic differences beyond genes involved in microcystin toxin generation and indicate an important role of host-associated bacteria in mediating phytoplankton interspecies interactions. These results expand our understanding of the key drivers of phytoplankton succession and the establishment and persistence of freshwater harmful cyanobacterial blooms.

蓝藻有害藻华（cyanoHAB）的频率和数量不断增加，威胁着全球淡水生态系统和服务。在北温带湖泊中，蓝细菌在初夏出现，其次是绿藻，成为主要的浮游植物群，这种模式被认为是由温度和生物有效性养分的变化介导的。为了了解这种连续模式的其他驱动因素，我们的研究使用了相互入侵实验来检验铜绿微囊藻（cyanoHAB的主要贡献者）与绿藻小球藻之间的竞争性相互作用。。我们考虑了可能影响这些相互作用的两个因素：（1）菌株变异，特别着重于肝毒素微囊藻毒素基因的存在与否，以及（2）宿主相关细菌。我们使用了有毒的铜绿假单胞菌PCC 7806（产生微囊藻毒素的菌株），无毒的PCC 7806突变体，无毒的铜绿假单胞菌PCC 9701（产生微囊藻毒素的菌株）和C. sorokiniana。每个生物体都不含所有细菌（例如，轴生细菌），并且具有重新引入的特定细菌群落以产生其对应的细菌。有竞争力的相互作用用的配对xenic和成对的无菌群体之间相互侵入实验评估C. sorokiniana和两个中的一个微囊藻菌株，分别进行评估。流式细胞仪和随机森林模型用于快速区分和定量浮游植物种群密度，准确率达99％。我们发现铜绿假单胞菌PCC 7806，而不是PCC 9701菌株，可以在稳定的C. sorokiniana种群中从低丰度繁殖。此外，细菌的存在允许铜绿假单胞菌PCC 7806生长到比建立的树莓菌更高的种群密度，以建立成熟的C. sorokiniana种群。相反，当铜绿假单胞菌占优势时，梭状芽胞杆菌只能从低密度增殖到PCC 9701菌株中，并且只有在出现焦虑时才如此。缺乏产生微囊藻毒素能力的PCC 7806突变体的行为与毒性野生型相似，这表明微囊藻毒素对两种野生型菌株之间观察到的竞争能力的差异不负责任。当将PCC 7806铜绿假单胞菌引入到C. sorokiniana培养物中时，微囊藻毒素（MC）的定量 显示，当缺少宿主相关细菌时，每个细胞中的MC含量是两种物种培养物中的两倍。我们的结果表明铜绿假单胞菌与索氏梭菌竞争的能力它是由微囊藻毒素毒素产生所涉及的基因以外的基因组差异决定的，表明宿主相关细菌在介导浮游植物物种间相互作用中起着重要作用。这些结果扩大了我们对浮游植物演替的主要驱动力以及淡水有害蓝藻水华的建立和持久性的理解。