Parr L B, Perkins R G, Mason C F
Department of Biological Sciences, University of Essex, Colchester, UK.
Water Res. 2002 Apr;36(7):1735-42. doi: 10.1016/s0043-1354(01)00395-5.
The duckweeds Lemna minor L. and L. minuscula Herter reduced PSII quantum efficiency (F'q/F'm) of the filamentous green alga Cladophora glomerata Kützing by up to 42% over seven days when floating above mats of C. glomerata in containers. Dissolved oxygen (DO) increased by 23% at 30 degrees C in containers with C. glomerata over controls. But when the water surface in the containers was covered with Lemna spp. floating above C. glomerata, DO was 83% lower at 30 degrees C over seven days than in control samples with no duckweed or alga. Dissolved oxygen was lower beneath a thick mat (1 cm) of either Lemna spp. covering the surface than under a thin layer (single-frond canopy). PAM fluorimetry showed that maximum PSII efficiency (Fv/Fm) of C. glomerata in containers was reduced under a canopy of L. minor by 17% over seven days, and under L. minuscula by 22%. F'q/F'm of C. glomerata in containers exposed to 51 micromol m(-2) s(-1) PPFD decreased under a canopy of L. minor by 16% over seven days, and under L. minuscula by 19% compared to controls. When light response curves were compared, F'q/F'm was significantly reduced under canopies of L. minor at the highest temperatures tested (28 degrees C and 30 degrees C). L. minor significantly reduced relative electron transport rate (rel. ETR) of the controls by up to 71% at 30 degrees C. Relative electron transport rate did not reach light saturation point (Esat) except at 28 degrees and 30 degrees C under mats of L. minor. Whereas the highest rate of production (rel. ETRmax) and Esat increased with temperature in controls, under a canopy of Lemna, decreases were observed. It is suggested that, during periods of high summer temperature and irradiance, shading inhibits oxygenic photosynthesis in mats of C. glomerata beneath canopies of Lenma spp. This results in less oxygen being produced by the C. glomerata (oxygen produced by Lemna spp. is not released into the water), and this may further inhibit the C. glomerata by limiting oxygen-dependent electron transport and/or photorespiration. This feedback loop could lead to the eventual senescence of the C. glomerata. The combination of low oxygen, high temperature and stressed filamentous algae, particularly in slow or standing water, may help to explain sudden collapses in DO concentration, with detrimental effects on water quality downstream.
浮萍（小眼浮萍Lemna minor L.和微小浮萍L. minuscula Herter）在容器中漂浮于球状刚毛藻（Cladophora glomerata Kützing）垫层上方时，在七天内可使丝状绿藻球状刚毛藻的PSII量子效率（F'q/F'm）降低多达42%。与对照组相比，装有球状刚毛藻的容器在30摄氏度时溶解氧（DO）增加了23%。但是当容器中的水面被漂浮在球状刚毛藻上方的浮萍属植物覆盖时，在30摄氏度下，七天内的溶解氧比没有浮萍或藻类的对照样品低83%。在覆盖表面的厚浮萍垫层（1厘米）下方的溶解氧低于薄浮萍层（单叶冠层）下方。脉冲幅度调制荧光测定法显示，在七天内，容器中球状刚毛藻的最大PSII效率（Fv/Fm）在小眼浮萍冠层下降低了17%，在微小浮萍冠层下降低了22%。暴露于51微摩尔·米-2·秒-1光合有效辐射（PPFD）下的容器中，球状刚毛藻的F'q/F'm在七天内，在小眼浮萍冠层下比对照降低了16%，在微小浮萍冠层下降低了19%。当比较光响应曲线时，在测试的最高温度（28摄氏度和30摄氏度）下，小眼浮萍冠层下的F'q/F'm显著降低。在30摄氏度时，小眼浮萍使对照组的相对电子传递速率（rel. ETR）显著降低多达71%。除了在小眼浮萍垫层下28摄氏度和30摄氏度时，相对电子传递速率未达到光饱和点（Esat）。在对照组中，最高产量速率（rel. ETRmax）和Esat随温度升高而增加，而在浮萍冠层下则观察到下降。研究表明，在夏季高温和高光照期间，遮荫会抑制浮萍属植物冠层下球状刚毛藻垫层中的放氧光合作用。这导致球状刚毛藻产生的氧气减少（浮萍属植物产生的氧气不会释放到水中），这可能通过限制依赖氧气的电子传递和/或光呼吸进一步抑制球状刚毛藻。这种反馈回路可能导致球状刚毛藻最终衰老。低氧、高温和受胁迫的丝状藻类的组合，特别是在缓慢流动或静止的水中，可能有助于解释溶解氧浓度的突然下降，对下游水质产生不利影响。
