微囊藻藻團粒徑分布動態之研究

Abstract

近年來台灣許多水庫因優養化現象，導致水質的惡化，其中又以微囊藻(Microcystis)為常見的藻華優勢物種。水庫發生藻華時期，不僅會使淨水成本增加，同時造成許多環境問題。微囊藻具有在水中垂直移動之能力，克服自然水體光線與營養鹽上下分離之情形，幫助藻團取得生長優勢。而過去研究亦發現自然水體中微囊藻團的粒徑，並非均為單一大小，而是具有某特定之粒徑分布。微囊藻許多生理、生態的表現皆受粒徑之影響，如比生長速率、水體中垂直移動之速度與距離。微囊藻維持特定粒徑分布的能力，可能為其適應環境得到生長優勢的關鍵因素 微囊藻團在生長的過程中，在藻團中不同位置藻細胞之異速生長，造成藻團形狀不規則性，機率性發生部分結構鬆散，藻團因此分裂。本研究雖無法追蹤同一藻團的分裂情形，但以實驗方法透過篩選不同粒徑範圍的藻團加以培養，找出藻團分裂與粒徑間的關聯性，結果顯示各粒徑範圍藻團在經過培養後，粒徑分佈均有擴展，向更大及更小粒徑延伸。考慮比生長速率與分裂機率受藻團粒徑的影響，本研究建立數學模式以描述藻團在水體中粒徑分布之動態。不同初始粒徑分布藻團模擬的結果顯示，微囊藻依此假設之生長與分裂機制，粒徑分布可快速達到動態平衡，維持一穩定狀態之分布，並可能利用此穩定粒徑分布，適應環境，取得生長的優勢。

Cyanobacterial blooms have been reported in freshwater environments world-wide. Some of these algae lead to serious environmental problems, such as producing toxins and foul odors. Microcystis, one of the most common species occuring during cyanobacterial blooms, is able to form aggregates. Colony formation may be crucial to the dominance of blooms of Microcystis. Forming large colonies are helpful to their vertical migration, nutrient storage, and to defend themselves against grazing pressure. But there is negative relationship between colony size and the specific growth rate. The previous study in this laboratory also found that the particle size of Microcystis in Hsin-Shan Reservoir is not uniform, but in a specific distribution. In this study we hypothesized that colony growth and disaggregation can lead Microcystis to a specific particle size distribution. Experiments were conducted with Microcystis colonies collected from Hsin-Shan Reservoir. To understand the mechanism of colony disaggregation we need to select colonies in different size ranges and analyze the relationship between colony size and probability of colony disaggregation. The results indicated that particle size distribution of Microcystis extended from original distribution to larger or smaller sizes after incubation. We developed a model considering the specific growth rate and the probability of colony disaggregation relationship with colony size. The results of the simulation indicate that Microcystis can maintain a specific size distribution. That is to say, Microcystis may preserve a dynamic equilibrium of colonial size distribution by following growth and the colony disaggregation pattern we proposed.