Demographic and mutualistic responses of stream nitrogen fixers to nutrients
Demographic and mutualistic responses of stream nitrogen fixers to nutrients
复制标题
河流固氮剂对养分的人口统计和互利反应
作者：
L. Kunza;R. Hall
摘要： 固氮菌将大气中的氮气转化为生物可利用的形式，但氮限制了许多生态系统的初级生产。尽管氮的来源丰富，但其他元素可能会限制底栖环境中通过固氮输入的氮。我们研究了氮、磷以及微量营养素铁和钼(固氮酶的辅助因子)对氮素固定的影响。我们使用短期营养添加来评估大量营养素对美国怀俄明州Ditch Creek完整的附着体生物膜固定氮气的影响。添加不同浓度的NH_4+、NO_3-−和PO43-−在4小时内没有改变氮的固定。我们通过6周的营养改良剂和营养扩散基质，评估了宏观和微量营养素对氮的固定、生物膜生物量和怀俄明州沟渠、斯普莱克和金鲍尔溪生物膜藻类组合的影响。添加Fe和Mo不影响生物膜生物量，但NO_3-−和PO43-−改变了生物膜生物量、藻类组合结构和固定N_2。N、P限制沟溪生物膜生物量，而P限制扩散溪生物膜生物量。NO_3-−抑制固氮作用，PO43-−促进固氮。NO3-−的抑制作用强于PO43-−的刺激作用。氮素固定剂的积累趋势与营养添加对氮素固定的影响相对应，主要是具有氮素固定内生菌的Epihemia Sorex。在对照和P处理中，藻类组合从以N_2固定剂为主转变为N和N+P处理中较少的N_2固定剂和增加了非N_2固定剂硅藻的生物量。N_2固定/单位E.Sorex生物量被NO_3-−抑制，而PO43-−对其有轻微的刺激作用。对营养物质的人口统计和互惠反应的组合通过固定氮素改变了氮素输入。
Abstract. N2-fixing bacteria convert atmospheric N2 gas to biologically available forms, yet N limits primary production in many ecosystems. Despite an abundant source of N, other elements may limit N input via N2-fixation in benthic environments. We examined how N, P, and the micronutrients Fe and Mo (cofactors of nitrogenase) might affect N2-fixation. We used short-term nutrient additions to assess macronutrient effects on N2-fixation of intact epilithic biofilm in Ditch Creek, Wyoming (USA). Additions of NH4+, NO3−, and PO43− at varying concentrations did not alter N2-fixation over 4 h. We assessed macro- and micronutrient effects on N2-fixation, biofilm biomass, and composition of the biofilm algal assemblage at Ditch, Spread, and Kimball Creeks, Wyoming, via 6-wk nutrient amendments with nutrient-diffusing substrata. Fe and Mo additions did not affect N2-fixation or biofilm biomass, but NO3− and PO43− altered the biofilm biomass, algal assemblage structure, and N2-fixation. N and P colimited biofilm biomass in Ditch Creek, and P limited biofilm biomass in Spread Creek. N2-fixation was inhibited by NO3− and stimulated by PO43−. Inhibition by NO3− was stronger than stimulation by PO43−. Trends in accumulation of N2-fixers, mostly Epithemia sorex with N2-fixing endosymbionts, corresponded to effects of nutrient additions on N2-fixation. The algal assemblage shifted from N2-fixer dominance in control and P treatments to fewer N2-fixers and increased biovolume of nonN2-fixing diatoms in the N and N+P treatments. N2-fixation/unit E. sorex biovolume was inhibited by NO3− and stimulated slightly by PO43−. A combination of demographic and mutualistic responses to nutrients altered N input via N2-fixation.