Cyanobacterial blooms often consist of numerous co-existing cyanobacterial species, with predominant taxa dynamically varying intra-annually. Parasitism by fungi (chytrids) has come into focus as an important factor driving short-term bloom dynamics. Using microscopic analysis, Illumina sequencing and cyanobacterial toxin analyses, we monitored the seasonal succession of Dolichospermum blooms in a reservoir along with environmental parameters. We identified two consecutive Dolichospermum blooms that were characterized by a straight and a coiled morphotype, separated by a complete bloom collapse. Phylotyping provided evidence for three putative Dolichospermum amplicon sequence variants (ASVs); i.e. Dolichospermum1 & 2 in the first bloom (straight filaments) and Dolichospermum3 in the second bloom (coiled filaments). Morphotype succession as well as total filament concentration did not correlate with any of the measured environmental parameters. Fungal parasitism by the chytrid Rhizosiphon crassum occurred in straight Dolichospermum filaments only. Coiled filaments showed no infection despite ambient presence of chytrids, deduced from fungal ASVs, throughout the entire observation period. Toxin concentrations (microcystins (MCs) and anabaenopeptins) correlated significantly with the abundance of the straight Dolichospermum morphotype. Enhanced cyanotoxin biosynthesis in the straight Dolichospermum morphotype, interpreted as a defensive reaction to fungal parasitism, appeared to come at the expense of lowered competitiveness with the co-occurring coiled morphotype. Our findings support the hypothesis that selective parasitism by chytrids is an important factor driving short-term morphotype and toxin dynamics within cyanobacterial blooms.

蓝藻水华通常由许多并存的蓝细菌物种组成，主要的类群在年内动态变化。真菌（chytrids）的寄生虫已成为引起短期开花动态的重要因素。使用显微镜分析，Illumina测序和蓝细菌毒素分析，我们监测了水库中Dolichospermum盛开的季节演替以及环境参数。我们确定了两个连续的Dolichospermum花朵，它们的特征是笔直的和盘绕的形态型，被完全的花朵塌陷分开。Phylotyping提供了三个推定的Dolichospermum扩增子序列变体（ASV）的证据。即Dolichospermum1＆2第一朵花（直丝）中的Dolichospermum3和第二朵花（卷丝）中的Dolichospermum3。形态型演替以及总丝浓度与任何测得的环境参数均不相关。乳糜根瘤菌（Rhizosiphon crassum）发霉的真菌寄生虫仅在直的Dolichospermum细丝中发生。在整个观察期间，尽管从真菌ASV推断出存在糜蛋白酶，但丝状丝都没有感染。毒素浓度（微囊藻毒素（microcystins，MCs）和anaenopepteptins）与直链杜鹃（Dolichospermum）形态型的丰度显着相关。直纹紫草中增强的氰毒素生物合成形态型，被解释为对真菌寄生性的防御反应，似乎是以同时出现的盘绕形态降低竞争力为代价的。我们的发现支持以下假说：chytrids的选择性寄生是驱动蓝藻水华内短期形态和毒素动态的重要因素。