南東海的季節性水文環境對細菌聯合群聚的影響

Abstract

細菌是否具有生物地理分布一直受到許多討論，傳統認為細菌族群量大、播遷能力高及世代交替時間短，因此其播遷應不受任何地理屏障的限制，當地的環境條件才是唯一的影響因子。但已有研究發現即便在海洋環境，細菌的群集分布仍受水團限制，顯示出海洋中的物理屏障會影響細菌的分布。而聯合群集理論是檢視環境因子與物種播遷對於聯合群集的相對影響力。因此本篇研究即以南東海海域的細菌聯合群集為對象，測試決定其聯合群集結構的影響因子是否受季節性水文變化的影響。南東海海域是一個具有季節性水文變化的環境，其環境的異質性及水團的混合程度皆隨季節而有所不同，我們將群集以整體為單位檢視春、夏、冬三個季節的聯合群集結構，並進一步將群集區分成四個主要的優勢群（包含Cyanobacteria, Alphaproteobacteria, Gammaproteobacteria and Actinobacteria）探討不同的分類群受環境因子的影響與其播遷能力是否有所不同。為了探討此議題，我們提出一個新的分析方法，此一分析方法將生物的系統演化資訊納入生物多樣性的考量，以distance-based redundancy analysis將生物多樣性的變異區分成可被環境以及空間解釋的兩個成分，並以環境及空間解釋的顯著力判定兩者的影響力。結果顯示，整體群集結構在五月(春天)顯著的被環境及空間因子所解釋，但在十二月(冬天)與八月(夏天)環境和空間對於群集結構皆沒有任何解釋力。當我們將群集區分成四個分類群以檢視不同分類群是否有不同的群集結構時，結果顯示五月(春天)所有分類群都受環境因子解釋，其中Alphaproteobacteria同時被環境與空間所解釋。而十二月(冬天)的各分類群則展現不同的聯合群集結構，其中以Gammaproteobacteria 受空間因子解釋；而Alphaproteobacteria 的分布則被環境因子所影響。在八月(夏天)的各分類群則都不被空間及環境因子解釋。本篇研究結果顯示聯合群集的決定因子會有季節性的變化，原因可能是洋流的季節性變化造成南東海海域的環境異質性及水團混合度不同所造成的。此外不同分類群所展現的不同聯合群集結構也顯示出分類群間具有不同的播遷能力。

Metacommunities theory examines the relative contribution of local environmental conditions versus dispersal processes in shaping community structure. We test the hypothesis that the determinism of bacterial metacommunities dynamics in the southern East China Sea (ECS) varies depending on seasonal hydrography, as the degree of environmental heterogeneity and connectivity among habitat patches is determined by circulation pattern. We analyzed metacommunities for the whole community as well as four dominant taxonomic groups (i.e. Cyanobacteria, Alphaproteobacteria, Gammaproteobacteria and Actinobacteria). We proposed a new analytical framework to include phylogenetic information into estimating β-diversity and used the distance-based redundancy analysis to partition the β-diversity into environmental and geographic components. When considering the whole community, environmental and spatial predictors were both significant in explaining β-diversity in May, during which the connectivity is intermediate. When connectivity is low (December) or too high (August), neither predictor was significant. Concerning the responses of different taxonomic groups, we found that all groups were explained by the environmental component in May, except for Alphaproteobacteria, which was influenced by both environmental and spatial effects. However in December, different groups exhibited different metacommunities structures; Alphaproteobacteria were influenced by environmental component, but Gammaproteobacteria were significantly explained by spatial component. In August, only Gammaproteobacteria was explained by environmental component. In conclusion, our results suggested that the determinism of metacommunities dynamics varied seasonally likely depending on the degree of the connectivity. Moreover, mechanisms vary among different taxonomic groups, suggesting that differential dispersal capacity among phylogenetic groups should be integrated into community assembly theory.