Laboratorio de Limnología (Lab. 44, 4to piso), Dpto. de Ecología, Genética y Evolución, Facultad de Ciencias Exactas y Naturales (Pab. II), Univ. de Buenos aires (UBA), IEGEBA (CONICET-UBA), Intendente Güiraldes 2620, C1428EHA, Buenos Aires, Argentina.
Department of Integrative Biology, Ecology, Evolutionary Biology and Behavior Program, W.K. Kellogg Biological Station, Michigan State Univ., 3700 East Gull Lake Dr., Hickory Corners, MI 49060, USA.
Harmful Algae. 2016 Dec;60:131-138. doi: 10.1016/j.hal.2016.11.007. Epub 2016 Nov 18.
Dolichospermum flos-aquae and Cylindrospermopsis raciborskii are two cyanobacteria species which cause harmful blooms around the world. Both these species share the capacity to fix atmospheric nitrogen in heterocytes (cell where fixation occurs). While Dolichospermum can express heterocytes at rather regular intervals across the filament, Cylindrospermopsis can only express heterocytes at the end of the filament. The aim of this study was to experimentally assess the role of heterocyte position in the eco-physiological responses of these bloom forming cyanobacteria. Replicated monocultures of each species were grown at different eutrophication scenarios (limiting and sufficient nitrogen and phosphorus concentrations, in factorial design). Dolichospermum reached high biomass regardless of the nitrogen (and phosphorus) provided, suggesting that this species could bloom in situations with and without nitrogen limitation. In contrast, Cylindrospermopsis reached high biomass only when nitrogen supply was high; its biomass was 15-20 times lower when relying on nitrogen fixation. Hence, despite its ability to fix nitrogen, blooms of Cylindrospermopsis would be expected only under high total nitrogen availability. In Dolichospermum heterocytes occurred only in the scenarios without supplied nitrogen while in Cylindrospermopsis heterocytes occurred regardless of nitrogen availability. Yet, in both species nitrogen fixation occurred (heterocytes were functional) only when nitrogen was limiting, and nitrogen fixation increased significantly at higher phosphorus concentration. Finally, in the absence of supplied nitrogen, filament length in Dolichospermum was the longest, while filaments in Cylindrospermopsis were the shortest (up to 13 times shorter than at nitrogen sufficiency). Therefore, heterocyte expression in Dolichospermum, and filament length in Cylindrospermopsis seem good proxies of nitrogen fixation. The eco-physiological responses recorded here help understand the distribution of these species along nutrient gradients in nature.
水华束丝藻和节旋藻是两种能在全世界范围内引发有害水华的蓝藻。这两种蓝藻都具有在异形胞（固定氮发生的细胞）中固定大气氮的能力。虽然水华束丝藻可以在丝状体上相当规律的间隔表达异形胞，但节旋藻只能在丝状体的末端表达异形胞。本研究的目的是从实验上评估异形胞位置在这些形成水华的蓝藻的生态生理响应中的作用。每种蓝藻的重复单细胞培养物在不同富营养化条件下（限制和充足的氮磷浓度，析因设计）生长。水华束丝藻在无论氮（和磷）供应如何的情况下都能达到高生物量，这表明该物种可以在有或没有氮限制的情况下发生水华。相比之下，节旋藻只有在氮供应充足时才能达到高生物量；当依赖氮固定时，其生物量要低 15-20 倍。因此，尽管节旋藻具有固氮能力，但只有在总氮供应充足的情况下才会出现节旋藻水华。在水华束丝藻中，异形胞仅在没有供应氮的情况下发生，而在节旋藻中，异形胞的发生与氮的可用性无关。然而，在这两种蓝藻中，只有在氮限制时才会发生氮固定（异形胞是功能的），并且在较高的磷浓度下，氮固定显著增加。最后，在没有供应氮的情况下，水华束丝藻的丝状体长度最长，而节旋藻的丝状体最短（比氮充足时短 13 倍）。因此，水华束丝藻中的异形胞表达和节旋藻中的丝状体长度似乎是氮固定的良好替代指标。这里记录的生态生理响应有助于了解这些物种在自然中沿着营养梯度的分布。
