Macroalgal Morphogenesis Induced by Waterborne Compounds and Bacteria in Coastal Seawater

Axenic gametes of the marine green macroalga Ulva mutabilis Føyn (Ria Formosa, locus typicus) exhibit abnormal development into slow-growing callus-like colonies with aberrant cell walls. Under laboratory conditions, it was previously demonstrated that all defects in growth and thallus development can be completely abolished when axenic gametes are inoculated with a combination of two specific bacterial strains originally identified as Roseobacter sp. strain MS2 and Cytophaga sp. strain MS6. These bacteria release diffusible morphogenetic compounds (= morphogens), which act similar to cytokinin and auxin. To investigate the ecological relevance of the waterborne bacterial morphogens, seawater samples were collected in the Ria Formosa lagoon (Algarve, Southern Portugal) at 20 sampling sites and tidal pools to assess their morphogenetic effects on the axenic gametes of U. mutabilis. Specifically the survey revealed that sterile-filtered seawater samples can completely recover growth and morphogenesis of U. mutabilis under axenic conditions. Morphogenetic activities of free-living and epiphytic bacteria isolated from the locally very abundant Ulva species (i.e., U. rigida) were screened using a multiwell-based testing system. The most represented genera isolated from U. rigida were Alteromonas, Pseudoalteromonas and Sulfitobacter followed by Psychrobacter and Polaribacter. Several naturally occurring bacterial species could emulate MS2 activity (= induction of cell divisions) regardless of taxonomic affiliation, whereas the MS6 activity (= induction of cell differentiation and cell wall formation) was species-specific and is probably a feature of difficult-to-culture bacteria. Interestingly, isolated bacteroidetes such as Algoriphagus sp. and Polaribacter sp. could individually trigger complete Ulva morphogenesis and thus provide a novel mode of action for bacterial-induced algal development. This study also highlights that the accumulation of algal growth factors in a shallow water body separated from the open ocean by barrier islands might have strong implications to, for example, the wide usage of natural coastal seawater in algal (land based) aquacultures of Ulva.

海洋大型绿藻突变礁膜（Ulva mutabilis Føyn，模式产地：里亚福莫萨潟湖）的无菌配子（axenic gametes）会异常发育为生长缓慢的愈伤组织样菌落，且细胞壁存在缺陷。既往研究证实，在实验室条件下，当将无菌配子接种于最初鉴定为玫瑰杆菌属（Roseobacter）菌株MS2和噬纤维菌属（Cytophaga）菌株MS6的两种特定细菌的组合体系中时，其生长与叶状体发育的所有缺陷均可完全消除。这类细菌可释放可扩散的形态发生化合物（即形态发生素），其作用模式类似细胞分裂素（cytokinin）与生长素（auxin）。为探究水传细菌形态发生素的生态相关性，本研究在葡萄牙南部阿尔加维地区的里亚福莫萨潟湖的20个采样点及潮池采集海水样本，以评估这些样本对突变礁膜无菌配子的形态发生效应。调查结果显示，无菌过滤的海水样本可在无菌培养条件下完全恢复突变礁膜的生长与形态发生过程。本研究采用多孔板测试系统（multiwell-based testing system），对从当地优势绿藻——硬礁膜（U. rigida）中分离得到的自由生活细菌与附生细菌的形态发生活性进行了筛选。从硬礁膜分离得到的优势菌属依次为交替单胞菌属（Alteromonas）、假交替单胞菌属（Pseudoalteromonas）与硫化杆菌属（Sulfitobacter），其次为嗜冷杆菌属（Psychrobacter）与极地杆菌属（Polaribacter）。多种天然细菌物种可不依赖分类学归属，模拟MS2菌株的活性（即诱导细胞分裂）；而MS6菌株的活性（即诱导细胞分化与细胞壁形成）则具有物种特异性，这一特性可能为难培养细菌所特有。有趣的是，分离得到的拟杆菌如艾格农杆菌属（Algoriphagus）菌株与极地杆菌属（Polaribacter）菌株可单独触发完整的礁膜形态发生过程，为细菌诱导的藻类发育提供了全新的作用模式。本研究同时指出，在被障壁岛与外海隔绝的浅水区中，藻类生长因子的积累可能对绿藻陆基规模化养殖中天然沿海海水的广泛应用等场景具有重要参考意义。