中圖分類號:Q178.1 文獻(xiàn)標(biāo)志碼:A 文章編號:1674-3075(2025)04-0276-11
全球范圍內(nèi)藻類水華的發(fā)生頻率、規(guī)模和持續(xù)時(shí)間都在增加(Huismanetal,2018)。我國大部分富營養(yǎng)化湖泊頻繁暴發(fā)有害水華(Qinetal,2018),而藻類群體的形成和群體形態(tài)的維持是水華形成和暴發(fā)的關(guān)鍵(胥辰卉,2021)。在野外自然水體中,藻細(xì)胞往往以群體的形式存在(Wallace,2000),群體更容易漂浮到水體表面,與個(gè)體相比有更強(qiáng)的競爭優(yōu)勢,顯著促進(jìn)水華的暴發(fā)(張曉峰等,2005)。因此,有研究表明藻群體的形成是導(dǎo)致自然水體水華暴發(fā)的關(guān)鍵生物學(xué)因素(Hozumietal,2020)。簡言之,藻群體的形成極大地促進(jìn)了水華的暴發(fā),故在水華暴發(fā)的初期,抑制藻群體的形成是控制水華暴發(fā)的關(guān)鍵。
1藻類群體形成機(jī)制
在自然界中,藻類演化出一系列能夠抵御外界不良環(huán)境的防御性策略,其中群體形成是藻類最為重要的防御策略之一(Shen etal,2007;Wang etal,2013)。
在藻類群體的形成過程中,藻細(xì)胞會分泌胞外多糖(EPS),使單個(gè)藻細(xì)胞之間相互聚集形成藻群體(Otsu-kaetal,2000);另外,單個(gè)藻細(xì)胞還可以通過分裂增殖,與其產(chǎn)生的子細(xì)胞相互聚集形成藻群體。藻群體的形成受到多種機(jī)制的調(diào)控。其形成機(jī)制如表1所示。
1.1胞外聚合物機(jī)制
藻類在生長過程中會分泌大量胞外聚合物(EPS),將大量的單個(gè)藻細(xì)胞包裹成為藻細(xì)胞的集合體并使之相互聚集成團(tuán)、黏附形成穩(wěn)定的藻群體(邱東茹,2020;母銳敏等,2023)。藻細(xì)胞EPS的合成與分泌是藻群體形成的前提,在藻細(xì)胞的生長、聚集的過程中,與EPS合成相關(guān)的酶和載體蛋白的活性增加,EPS的合成進(jìn)程加快,直接影響藻群體的形成過程與藻群體形態(tài)結(jié)構(gòu)的穩(wěn)定(畢相東等,2014)。其形成機(jī)制的示意圖如圖1所示。
1.2細(xì)胞表面黏附機(jī)制
藻細(xì)胞表面之間的黏附受其表面的靜電相互作用(Zeta電位)和疏水性的影響(Liuetal,2016),進(jìn)而影響藻細(xì)胞的聚集和黏附。Zeta電位在維持藻細(xì)胞功能和調(diào)節(jié)過程中起到關(guān)鍵作用(Wilsonetal,2001),具有穩(wěn)定藻細(xì)胞表面的作用,促進(jìn)藻類群體的形成;另外,藻細(xì)胞表面的疏水性在藻細(xì)胞的黏附過程以及藻群體的形成過程中起到重要作用(Yangetal,2011),藻細(xì)胞表面的疏水性越強(qiáng),藻細(xì)胞更容易發(fā)生凝聚,聚集形成藻群體(Liuetal,2016)。
1.3 附生菌機(jī)制
藻群體的膠鞘黏液中常常包含大量細(xì)菌(Shietal,2009),它們也可促進(jìn)藻群體的形成。藻類釋放大量的溶解性有機(jī)碳,誘導(dǎo)細(xì)菌向藻類靠攏聚集(Christie-Olezaetal,2017),形成復(fù)雜的藻-菌群落,并釋放出特殊的促進(jìn)藻類和細(xì)菌互利共生的生物活性物質(zhì)(張圣潔等,2020;Zhuetal,2022),間接參與調(diào)節(jié)藻細(xì)胞的多種生理、生化活動(Zhangetal,2022),促進(jìn)藻群體的形成和穩(wěn)定(Liuetal,2022)。另外,附生菌分泌出的胞外DNA會促進(jìn)菌、藻細(xì)胞表面之間化學(xué)鍵的生成(孫明等,2018),由此促進(jìn)藻細(xì)胞與細(xì)菌之間形成生物膜,同時(shí)會促進(jìn)藻群體的形成(王昭藝等,2020)。其形成機(jī)制如圖2所示。
1.4其他機(jī)制
藻類群體的形成是一個(gè)復(fù)雜的生態(tài)學(xué)過程,涉及多種生物學(xué)機(jī)制的協(xié)同作用。除之前提到的3種機(jī)制外,還包括藻-藻相互作用、凝集素和胞外酶機(jī)制。藻-藻相互作用機(jī)制是指在藻類群體的形成過程中,藻類與藻類之間通過信息流的作用影響藻群體的形成,在藻類之間的相互作用下,誘導(dǎo)分泌信息化學(xué)活性物質(zhì),使藻細(xì)胞產(chǎn)生氧化應(yīng)激效應(yīng),改變藻細(xì)胞的形態(tài),抑制其生長,但是藻細(xì)胞通過相互聚集形成藻群體以抵御這種外界不利條件的影響(Melloetal,2012;Chen&Guo,2014)。凝集素機(jī)制是指藻細(xì)胞在特定環(huán)境會釋放出凝集素分子,并與其他藻細(xì)胞表面的凝集素受體(即藻細(xì)胞表面的糖分子)進(jìn)行專一性結(jié)合,藻細(xì)胞之間相互交聯(lián)黏附(鄭怡等,2008),促進(jìn)藻細(xì)胞聚集形成藻群體。胞外酶機(jī)制是指胞外酶通過影響藻細(xì)胞對營養(yǎng)元素的攝取和利用進(jìn)而影響藻細(xì)胞的分裂、增殖和聚集過程(Latouretal,2004;陳何舟等,2019)。在藻細(xì)胞的生長過程中,藻細(xì)胞分泌的胞外酶活性增加,可增強(qiáng)藻細(xì)胞對營養(yǎng)元素吸收、利用,從而促進(jìn)藻細(xì)胞的增殖。
綜上,藻群體的形成受到多種機(jī)制協(xié)同作用,其中EPS和附生菌起著關(guān)鍵性作用。大量研究表明EPS的分泌量增加可以促進(jìn)藻群體的形成(Xiaoetal,2019),附生異養(yǎng)細(xì)菌可以促進(jìn)單細(xì)胞形態(tài)藻聚集形成群體(Shenamp;Song,2011;Wang etal,2016),EPS含量、附生菌群落的變化都可能會導(dǎo)致藻群體的解散(Wangetal,2015)。然而,野外群體藻EPS分泌量遠(yuǎn)大于室內(nèi)培養(yǎng)單細(xì)胞藻EPS分泌量(Wangetal,2011),且附生菌群落更為豐富(Shietal,2009),藻群體經(jīng)室內(nèi)培養(yǎng)一段時(shí)間后,會解離成單細(xì)胞,難以維持群體形態(tài)(Bolchetal,1996;雷臘梅等,2007),這表明室內(nèi)培養(yǎng)藻的EPS機(jī)制和附生菌機(jī)制作用被大大削弱。因此,未來需深入探究室內(nèi)和野外藻群體形成機(jī)制的差異。
2影響藻群體形成的因素
2.1 非生物因素
2.1.1營養(yǎng)鹽氮和磷是藻類生長所需的重要營養(yǎng)鹽,通過影響藻細(xì)胞EPS的分泌,進(jìn)而影響藻群體的形成(Chenetal,2012)。在野外環(huán)境條件下,當(dāng)N、P處于一定濃度范圍時(shí),藻細(xì)胞EPS的分泌量會隨著N、P濃度的增加而增加,有利于藻群體的形成,但當(dāng)水體中的N、P濃度超過一定范圍時(shí),則不利于藻群體的形成(Maetal,2014)。但在實(shí)驗(yàn)室培養(yǎng)條件下,出于某種未知因素,藻類往往以單細(xì)胞狀態(tài)存在,很難形成藻群體(Zhangetal,2007;Caoamp; Zhou,2010)。
2.1.2生態(tài)因素光照強(qiáng)度對藻類的形態(tài)建成具有重要的影響(董靜和李根保,2016),在低光照強(qiáng)度下,藻細(xì)胞主要合成蛋白質(zhì),提高藻細(xì)胞的光合作用效率;在高光照強(qiáng)度下,促進(jìn)藻細(xì)胞碳水化合物的積累,刺激藻細(xì)胞EPS產(chǎn)量的增加(Lietal,2013;肖艷等,2014;張艷晴等,2014)。溫度直接影響藻類的光合作用和有機(jī)物碳水化合物的積累。藻細(xì)胞的生理結(jié)構(gòu)、形態(tài)以及酶(組成、濃度、活性)均受溫度的影響(董靜等,2023)。在適宜的水體擾動強(qiáng)度范圍,藻細(xì)胞的生理活性增強(qiáng),藻細(xì)胞EPS的分泌量增加,藻細(xì)胞的生長速率加快,但是,過強(qiáng)的水體擾動抑制藻細(xì)胞的生長,不利于藻群體的形成(楊桂軍等,2017;芮政等,2019)。
2.1.3金屬離子微量營養(yǎng)元素是構(gòu)成細(xì)胞內(nèi)重要結(jié)構(gòu)的必需物質(zhì)(李威等,2008;Wangetal,2011),在藻細(xì)胞的生長繁殖過程中,添加適量微量營養(yǎng)元素,可以誘導(dǎo)藻細(xì)胞產(chǎn)生更多的EPS(Satoetal,2017),促進(jìn)藻群體形成。重金屬離子會對藻細(xì)胞產(chǎn)生脅迫作用(Alidoustetal,2016),當(dāng)藻細(xì)胞受到重金屬離子的刺激,會加快分泌大量的EPS與其相互結(jié)合,減輕毒害作用,而分泌的EPS則易使得藻細(xì)胞集聚形成藻群體(楊芳等,2007;孫秀麗等,2021;Bietal,2013),當(dāng)濃度超過一定范圍后,藻細(xì)胞的生長反而會受到抑制(張少斌等,2012)。
2.1.4其他因素藻細(xì)胞分泌的藻毒素(MC)可以刺激藻細(xì)胞EPS的分泌,有利于藻細(xì)胞集聚形成藻群體,并促進(jìn)藻群體大小的增加(Ganetal,2012)。腐殖酸(HA)可以與水體中有害物質(zhì)進(jìn)行絡(luò)合,降低對藻的毒害作用(Dingetal,2018),但同樣腐殖酸與微量營養(yǎng)元素絡(luò)合,會降低其生物利用率,不利于藻群體的形成(Maetal,2021)。當(dāng)抗生素處于低濃度范圍時(shí),有利于藻細(xì)胞相互聚集,隨著抗生素濃度的升高,生態(tài)毒性效應(yīng)越大,越不利于藻群體的形成(Wanetal,2015;Xinetal,2022)。陰離子表面活性劑對藻群體的形成具有促進(jìn)作用,而陽離子表面活性劑和非離子表面活性劑對藻群體的形成則產(chǎn)生抑制作用(鄭子英和劉雷,2011;Limaetal,2022)。
綜上,藻群體的形成受到多種非生物因素的調(diào)控,不同的因素對藻群體形成的作用不同(表2),外部非生物因素是促使藻群體形成的主要因素(圖3)。
2.2生物因素
2.2.1水生動物藻類在浮游動物的牧食壓力的刺激下可以誘導(dǎo)形成藻群體(楊州等,2005;Yangetal,2012)。浮游動物主要以小型藻類為食,藻細(xì)胞通過分泌EPS,形成藻群體,有效降低被浮游動物捕食的風(fēng)險(xiǎn)(Burkert etal,2001;Jang etal,2003)。水生生物分泌的化學(xué)信息物質(zhì)同樣也可以誘發(fā)藻群體的形成,當(dāng)藻細(xì)胞暴露于水生生物釋放的化學(xué)物質(zhì)中,可發(fā)現(xiàn)藻細(xì)胞聚集形成藻群體(Kimetal,2008)。
2.2.2水生植物水生植物對藻類的生長存在一定的克制作用。一方面體現(xiàn)在它們與藻類之間會搶奪生存必需的營養(yǎng)物質(zhì),另一方面體現(xiàn)在它們能夠向外部環(huán)境釋放出一種特殊的化感物質(zhì)來抑制藻細(xì)胞的生長,包括破壞藻類的光合系統(tǒng)、影響藻類細(xì)胞膜的結(jié)構(gòu)、降低藻細(xì)胞內(nèi)酶的活性(楊小杰等,2016;Yuanetal,2020)。從這些方面來看,水生植物的化感作用對藻群體形成的影響似乎是不利的。
2.2.3異養(yǎng)細(xì)菌細(xì)菌與藻細(xì)胞之間存在著特異性相互關(guān)聯(lián),對藻細(xì)胞的生長既可以表現(xiàn)為促進(jìn)作用,又可以表現(xiàn)為抑制作用(張佳等,2019)。附生細(xì)菌可以通過釋放生長因子促進(jìn)藻類生長(Daoetal,2020)。細(xì)菌對藻類生長的消極作用主要體現(xiàn)在溶藻細(xì)菌可以通過分泌一些胞外活性溶藻物質(zhì)殺死藻細(xì)胞,對藻細(xì)胞的生長產(chǎn)生抑制作用,從而抑制藻群體的形成(Yang etal,2017)。
2.2.4藻類藻類的化感作用在一定程度上影響藻群體的形成(Mello etal,2012;Chenamp;Guo,2014)。當(dāng)多種藻類混合培養(yǎng)時(shí),一種藻細(xì)胞分泌的活性化感物質(zhì)會誘導(dǎo)另一種藻細(xì)胞產(chǎn)生氧化應(yīng)激效應(yīng)并對其生長產(chǎn)生抑制作用(Chiaetal,2018)。當(dāng)藻細(xì)胞處于低競爭壓力時(shí),混合藻培養(yǎng)的分泌物會誘導(dǎo)藻細(xì)胞群落的形成,而暴露于高比例藻混合培養(yǎng)物的分泌物中,才會觀察到明顯的生長抑制作用(Melloetal,2012;Chenamp;Guo,2014)。
綜上,生物因素包括水生動物、水生植物、細(xì)菌和藻類,是影響藻群體形成的重要因素。其中,水生動物對藻群體的形成主要為促進(jìn)作用,水生植物對藻群體的形成主要為抑制作用,而細(xì)菌和藻類對藻群體的形成既可起到促進(jìn)作用又可起到抑制作用,具體作用效果如表3所示。
3藻群體的競爭優(yōu)勢
自然水體的藻類大多數(shù)是以群體的形式存在,可以說群體的形成是藻類形成水華的主要前提之一。與單細(xì)胞的藻類相比,群體形態(tài)的藻類具有更多的生存競爭優(yōu)勢,具體表現(xiàn)在以下幾個(gè)方面。
(1)抵御浮游動物的牧食及魚類的濾食。藻群體通常是由成百上千甚至上萬個(gè)藻細(xì)胞相互黏結(jié)組成的團(tuán)聚體,藻群體的直徑通??梢猿^ 0.1mm ,最大的甚至能超過 1mm ,由于藻細(xì)胞黏結(jié)形成藻群體后,體積會有所增加,并且在藻群體的表面形成了厚實(shí)的膠被,可以更大程度地降低浮游動物對藻類的牧食作用以及濾食性魚類的濾食效率,提高藻類在水體中的生存競爭優(yōu)勢(Xieamp;Liu,2001;Vanetal,2011)。
(2)藻毒素的分泌增加。藻毒素是一種藻細(xì)胞內(nèi)次級代謝產(chǎn)物,在藻群體的形成過程中,藻毒素的分泌量也會隨之增加、有關(guān)藻毒素的表達(dá)基因比例增加,藻類通過分泌藻毒素有效抵御攝食者的攝食(Kurmayeretal,2003),對藻種間的競爭起到了重要的作用(楊佳等,2012),為自身提供生存競爭優(yōu)勢。
(3)完善自身的浮力調(diào)節(jié)機(jī)制。與藻群體的浮力調(diào)節(jié)機(jī)制相比,單個(gè)藻細(xì)胞的浮力調(diào)節(jié)機(jī)制更加完善。張永生等(2011)研究發(fā)現(xiàn),單個(gè)藻細(xì)胞通過改變藻細(xì)胞內(nèi)的偽空胞數(shù)量的方式使藻細(xì)胞能夠在水體中自由垂向遷移,而藻群體還能通過分泌出的EPS黏著作用,使藻群體的細(xì)胞間產(chǎn)生大量的空隙,由此增加藻細(xì)胞浮力,使藻類上浮,可提高藻細(xì)胞光合作用效率,使之更具生存競爭優(yōu)勢。
(4)抗脅迫能力增強(qiáng)。藻群體中的藻細(xì)胞相比于單個(gè)藻細(xì)胞的抗脅迫能力更強(qiáng)(Maetal,2014;Lietal,2015)。當(dāng)藻細(xì)胞形成藻群體后,EPS分泌量增加,減弱細(xì)胞損傷,更有利于藻細(xì)胞提高自身在環(huán)境中的生存競爭優(yōu)勢(Wuetal,2011),并且藻群體具有更高效的光合電子傳遞系統(tǒng)、更高的對低濃度磷的親和力以及更強(qiáng)的對溶藻細(xì)菌的抗性(Shenetal,2007;Wang et al,2013)
4展望
富營養(yǎng)化水體頻繁暴發(fā)水華是目前全球水生態(tài)環(huán)境所面臨的一大挑戰(zhàn),藻類群體的形成是其水華發(fā)生的關(guān)鍵過程,揭示群體的形成過程和影響因素是明確水華發(fā)生機(jī)理的重要途徑,有利于更全面揭示水華暴發(fā)的驅(qū)動機(jī)制,為水華有效防控提供理論依據(jù)。近年來大量研究廣泛探討了群體形成機(jī)制、影響因子以及獲得的生存競爭優(yōu)勢,但仍有很多問題需要進(jìn)一步明確。
(1)氮磷營養(yǎng)鹽是影響藻細(xì)胞生長和EPS分泌的重要因素。在氮磷營養(yǎng)限制條件下,群體藻和單細(xì)胞藻的生長均受到抑制,但實(shí)驗(yàn)室培養(yǎng)藻的比生長速率高于自然條件下的生長速率(Lietal,2013),并且群體藻EPS產(chǎn)量遠(yuǎn)大于單細(xì)胞藻EPS產(chǎn)量,氮磷營養(yǎng)鹽、EPS產(chǎn)量以及藻生長速率三者之間變動關(guān)系可能是影響藻群體形成的重要因素。未來,應(yīng)當(dāng)更全面地探究藻群體形成過程中三者之間變化關(guān)系,為藻群體的形成提供參考依據(jù)。
(2)EPS的物化特性是影響藻細(xì)胞表面膠被形成的決定性因素,而膠被對于藻類形成群體形態(tài)至關(guān)重要。相同的藻類在實(shí)驗(yàn)室或自然環(huán)境條件下其膠被特性可能存在差異,這可能是影響藻群體形成的重要因素。在野外自然水體有限的資源條件下,形成藻群體膠被或是最優(yōu)的生存策略,但其科學(xué)的認(rèn)識仍顯不足,尚需深入探索。未來,可考慮依靠基因和代謝組學(xué)方法,揭示藻群體膠被形成的機(jī)制,解析膠被對環(huán)境變化響應(yīng)的機(jī)制,更全面地探究藻群體膠被合成和分泌過程中具體的響應(yīng)信號體系。
(3)目前大多數(shù)相關(guān)研究都基于實(shí)驗(yàn)室模擬,室內(nèi)條件下往往N、P營養(yǎng)鹽充足,并且缺乏底泥基質(zhì),造成細(xì)菌來源的缺失,在室內(nèi)條件下培養(yǎng)的藻細(xì)胞很難聚集形成藻群體或者所形成的藻群體較小,基于此進(jìn)行的相關(guān)研究與野外藻群體形成的實(shí)際情況存在一定差異,如何在野外條件下研究藻類水華形成過程中單細(xì)胞與群體之間形態(tài)的轉(zhuǎn)變規(guī)律,將是水華暴發(fā)機(jī)理研究的熱點(diǎn)與難點(diǎn)。在未來應(yīng)當(dāng)模擬野外自然環(huán)境條件,并在此基礎(chǔ)上探究藻類從單細(xì)胞到群體的變化,為藻類水華成群機(jī)理研究積累重要數(shù)據(jù)。
(4)藻類群體的形成是由多種因素共同作用產(chǎn)生的,單個(gè)因素對藻群體形成的影響作用大都已經(jīng)很明確,然而多因素的耦合作用下對藻群體形成的影響及在自然環(huán)境中各種因素對藻群體形成的貢獻(xiàn)度仍不清晰。在未來應(yīng)當(dāng)根據(jù)實(shí)際自然水環(huán)境條件開展相關(guān)實(shí)驗(yàn)研究,探究基于自然環(huán)境水平條件下,多因素耦合對藻群體形成的影響及其貢獻(xiàn)度,將有利于藻群體解散的因素加以利用,對尋求更具針對性的藻類水華防控措施具有重要指導(dǎo)意義。
綜上,通過對水華形成機(jī)制途徑的分析,從本質(zhì)上了解并掌握藻群體的形成機(jī)理以及藻群體的競爭優(yōu)勢,對加強(qiáng)自前水華暴發(fā)的防治工作具有指導(dǎo)意義,為水華的有效防治提供必要基礎(chǔ)。
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Formation, Influencing Factors and Competitive Advantages of an Algae Bloom Colony
ZHOU Shiyingl,2, FENG Bing2, WANG Qiang2, ZHU Dongliang2,3, XIA Wei2, YAO Na2 ,ZHANG Mengl,: (1.School ofResources and Environmental Engineering, Jiangxi University of Science and Technology, Ganzhou 3410o0,P.R. China; 2. Jiangxi Academy of Ecological Environment Science Research amp; Planning, Nanchang 330039, P.R. China; 3. Environmental and Chemical Engineering College of Nanchang Hangkong University, Nanchang 330063,P.R.China)
Abstract: Algal blooms caused by eutrophication is a serious ecological environmental problem at home and abroad. The formation of algal colonies is a critical step in the occurrence of algal blooms.In this study,we systematically reviewed the formation mechanisms of algal colonies,elaborated on the role of algal colonization in algal bloom formation and described the competitive advantages of colonization. Our aim was to support efective mitigation of algal blooms.The formation mechanisms of algal colonies are diverse and typically depend on secretion of extracellular polymeric substances (EPS),cell surface adhesion,epiphytic bacteria,and algae-algae interactions.The influencing factors of algal colony formation have been classified as abiotic and biotic.Abiotic factors include nutrients,light intensity,temperature, disturbances,trace elements,microcystins,antibiotics and surface active agents.Biotic factors include zooplankton,aquatic plants and the heterotrophic bacteria that play a critical role in the formation of algal aggregates. Compared to a single algal cell algal colonies have competitive advantages that include enhanced nutrient acquisition, improved light and temperature adaptation,and strengthened defenses against chemical stressors and predators.
Key words: algalbloom; algalcolony; abiotic factors; biotic factors; competitiveadvantage