The ADA clade of Nostocales cyanobacteria, a group that is prominent in current harmful algal bloom events, now includes over 40 genome sequences with the recent addition of sixteen novel sequenced genomes (Dreher et al., Harmful Algae, 2021). Fourteen genomes are complete (closed), enabling highly detailed assessments of gene content and genome architecture. ADA genomes contain 5 rRNA operons, genes expected to support a photoautotrophic and diazotrophic lifestyle, and a varied array of genes for the synthesis of bioactive secondary metabolites. Genes for the production of the taste-and-odor compound geosmin and the four major classes of cyanotoxins — anatoxin-a, cylindrospermopsin, microcystin and saxitoxin — are represented in members of the ADA clade. Notably, the gene array for the synthesis of cylindrospermopsin by Dolichospermum sp. DET69 was located on a plasmid, raising the possibility of facile horizontal transmission. However, genes supporting independent conjugative transfer of this plasmid are lacking. Further, analysis of genomic loci containing this and other cyanotoxin gene arrays shows evidence that these arrays have long-term stability and do not appear to be genomic islands easily capable of horizontal transmission to other cells. There is considerable diversity in the gene complements of individual ADA genomes, including the variable presence of physiologically important genes: genomes in three species-level subclades lack the gas vesicle genes that facilitate a planktonic lifestyle, and, surprisingly, the genome of Cuspidothrix issatschenkoi CHARLIE-1, a reported diazotroph, lacks the genes for nitrogen fixation. Notably, phylogenetically related genomes possess limited synteny, indicating a prominent role for chromosome rearrangements during ADA strain evolution. The genomes contain abundant insertion sequences and repetitive transposase genes, which could be the main drivers of genome rearrangement through active transposition and homologous recombination. No prophages were found, and no evidence of viral infection was observed in the bloom population samples from which the genomes discussed here were derived. Phages thus seem to have a limited influence on ADA evolution.

在当前的有害藻华事件中非常重要的一个群体是Nostocales cyanobacteria ADA进化枝，现在包括40多个基因组序列，最近又增加了16个新的测序基因组（Dreher等人，Harmful Algae，2021年）。14个基因组是完整的（封闭的），可以对基因含量和基因组结构进行高度详细的评估。ADA基因组包含5个rRNA操纵子，有望支持光合自养和重氮养生生活方式的基因，以及用于合成生物活性次生代谢物的各种基因。在ADA进化枝的成员中代表了产生味觉和气味的复合土臭素和四大类氰毒素的基因-Anatoxin-a，cylindrospermopsin，microcystin和saxitoxin。值得注意的是，通过Dolichospermum SP。DET69位于质粒上，从而提高了水平传输的可能性。但是，缺少支持该质粒的独立结合转移的基因。此外，对包含该和其他氰毒素基因阵列的基因组位点的分析表明，这些阵列具有长期稳定性，并且似乎不是容易向其他细胞水平传播的基因组岛。单个ADA基因组的基因补体存在相当大的多样性，包括生理上重要的基因的不定存在：三个物种级亚群中的基因组缺乏促进浮游生活方式的气泡基因，并且令人惊讶的是，库氏孢子虫（Caspidothrix issatschenkoi）的基因组CHARLIE-1是一种重氮营养菌，它缺乏固氮基因。值得注意的是，系统发育相关的基因组具有有限的同义性，表明在ADA菌株进化过程中染色体重排起着重要作用。基因组包含丰富的插入序列和重复的转座酶基因，这可能是通过主动转座和同源重组进行基因组重排的主要驱动力。没有发现前噬菌体，也没有在开花种群样本中观察到病毒感染的证据，此处讨论的基因组是从该样本中获得的。因此，噬菌体似乎对ADA的发展影响有限。