Helical and linear morphotypes of <i>Arthrospira</i> sp. PCC 8005 display genomic differences and respond differently to ⁶⁰Co gamma irradiation

Arthrospira are multicellular cyanobacteria that typically reside in alkaline lakes of (sub)tropical regions and are mass cultivated around the globe in a variety of outdoor facilities and photobioreactors for their high nutritional, pharmaceutical and clinical value. Arthrospira sp. strain PCC 8005 was selected by the European Space Agency as an oxygen producer and nutritional end-product in a bioregenerative life support system for long-haul missions. Being highly resistant to ionizing radiation, it is also an ideal candidate for other space applications such as in situ resource utilization and terraformation. During long-term strain maintenance involving continuous subculturing we noted an irreversible morphological change in PCC 8005 subcultures i.e. from only helical to only straight trichomes. These morphotypes displayed differences in growth rate, buoyancy and resistance to gamma radiation. We also found marked differences in antioxidant capacity, pigment content and trehalose concentration, while whole-genome comparison revealed a difference of 168 SNPs, 48 indels and four large insertions affecting, in total, 41 coding regions across both genomes. Although nine of these regions encoded proteins with a known function, no conclusive genotype-phenotype associations could be determined. Nonetheless, genomic changes within the gvpC gene (encoding a gas vesicle protein) and within the regulatory region of the psbD gene (encoding the D2 protein of PSII) provided some clues for the observed differences in buoyancy and growth.节旋藻（Arthrospira）是一类多细胞蓝细菌，通常栖息于（亚）热带区域的碱性湖泊中。因其兼具极高的营养、药用与临床价值，目前已在全球范围内依托各类户外养殖设施与光生物反应器实现大规模培养。欧洲空间局（European Space Agency, ESA）将节旋藻菌株PCC 8005选定为长距离航天任务生物再生生命支持系统的氧气生产者与营养终产物。该菌株对电离辐射具有高度抗性，同时也是原位资源利用、行星地球化（terraformation）等其他航天应用的理想候选材料。在针对该菌株开展的长期连续传代保藏过程中，研究人员于PCC 8005的传代培养物中观察到不可逆的形态变化：藻丝由单一螺旋状转变为单一直丝状。两种形态型在生长速率、浮力及抗γ射线辐射能力方面均存在显著差异。此外，二者的抗氧化能力、色素含量与海藻糖浓度也存在明显差异；全基因组比对结果显示，两个基因组间存在168个单核苷酸多态性（Single Nucleotide Polymorphisms, SNPs）、48个插入缺失变异（insertions-deletions, indels）以及4个大片段插入事件，总计影响41个编码区域。尽管其中9个编码区域所编码的蛋白功能已明确，但目前尚未能确定明确的基因型-表型关联。不过，gvpC基因（编码气囊蛋白）与psbD基因（编码光系统II（Photosystem II）的D2蛋白）调控区的基因组变化，为观测到的浮力与生长差异提供了部分线索。