曲勝路, 楊茹君, 蘇 函, 劉 媛, 耿倩倩

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海洋中Fe(Ⅱ)的行為及微生物參與下的過程研究概述

曲勝路1, 2, 楊茹君1, 蘇 函1, 劉 媛1, 耿倩倩1

(1. 中國(guó)海洋大學(xué) 化學(xué)化工學(xué)院, 山東 青島 266100; 2. 青島海洋科學(xué)與技術(shù)國(guó)家實(shí)驗(yàn)室 海洋地質(zhì)過程與環(huán)境功能實(shí)驗(yàn)室, 山東 青島 266071)

Fe(Ⅱ); 微生物活動(dòng); N移除

1 海水中Fe(Ⅱ)的分布

海水中Fe(Ⅱ)的分布及含量受輸入、遷移轉(zhuǎn)化、遷出等過程的影響。從整體上看, Fe(Ⅱ)在垂直分布上主要集中在表層、OMZ區(qū)和近底層水中, 在水平分布上則呈現(xiàn)出近岸高, 遠(yuǎn)岸低的特點(diǎn)。

2 海洋中Fe(Ⅱ)的化學(xué)行為

海洋中Fe(Ⅱ)的循環(huán)過程包括Fe(Ⅱ)的輸入和輸出, 其中Fe(Ⅱ)的輸入分為外源輸入和海洋再生輸入, 外源輸入方式包括大氣沉降[23]、沉積物釋放[24]、地下水輸入[25]、冰川水釋放[26]等; 海洋再生方式包括光催化還原[7]、生物細(xì)胞裂解釋放[27]、微生物還原[28]等。海水中Fe(Ⅱ)的輸出則主要通過生物攝取[29]、化學(xué)氧化[30]、微生物氧化[31]、顆粒物吸附[32]等方式從海洋中移除。海水中Fe(Ⅱ)的化學(xué)反應(yīng)主要包括: 光催化Fe(Ⅲ)還原產(chǎn)生Fe(Ⅱ)、有機(jī)質(zhì)與Fe(Ⅱ)的配合, Fe(Ⅱ)的化學(xué)氧化移除等。

2.1 光還原過程產(chǎn)生Fe(Ⅱ)

在海洋透光層, 光催化還原是海水中Fe(Ⅱ)的主要來源。有機(jī)質(zhì)吸收光能, 釋放電子, 并將電子轉(zhuǎn)移給水合Fe(Ⅲ)氧化物, Fe(Ⅲ)還原為Fe(Ⅱ), 氧化物表面Fe(Ⅱ)-O的不穩(wěn)定, 發(fā)生解離將Fe(Ⅱ)釋放到水中(式1)。通常Fe(Ⅲ)氧化物的不定形程度越高, 表面有機(jī)絡(luò)合程度越大, 還原反應(yīng)效率就越高[9, 33-34]。

L(ligand)為與Fe(Ⅱ)絡(luò)合的有機(jī)配體, L′為有機(jī)配體發(fā)生光催化反應(yīng)后的氧化產(chǎn)物。

海水中大部分的溶解有機(jī)態(tài)Fe(Ⅲ)需要被還原為Fe(Ⅱ)才能被生物吸收利用, 有機(jī)態(tài)DFe(Ⅲ)的光還原機(jī)制為光誘導(dǎo)的有機(jī)配體向Fe(Ⅲ)的直接電子轉(zhuǎn)移的反應(yīng)過程(ligand-to-metal charge transfer, LMCT)(式2)[35-36]。

該還原反應(yīng)速率受配體種類、光照條件等因素影響較大[35, 37]。Kuma等[38]測(cè)得日本春季藻華期間的Funka灣表層水Fe(Ⅱ)含量為20~40 nmol/L, 光照模擬實(shí)驗(yàn)結(jié)果表明, 在羥基羧酸存在下, Fe(Ⅲ)能夠被還原為Fe(Ⅱ), 而在腐殖酸存在下, Fe(Ⅲ)沒有被光還原; Rijkenberg等[39]的研究表明, 脫鎂葉綠素、鐵色素、肌醇六磷酸等配體能夠有效增強(qiáng)Fe(Ⅲ)的光還原效率, 使Fe(Ⅱ)的濃度增加; 而去鐵敏B存在下卻抑制Fe(Ⅲ)的還原, 另外原卟啉IX能夠與Fe(Ⅱ)發(fā)生強(qiáng)絡(luò)合, 降低光產(chǎn)物Fe(Ⅱ)的濃度。此外研究發(fā)現(xiàn), 波長(zhǎng)在275~520 nm的光才能催化Fe(Ⅲ)的還原反應(yīng)發(fā)生, 短波長(zhǎng)的光催化效率較高[6, 40]。

2.2 有機(jī)質(zhì)與Fe(Ⅱ)的配合

Fe(Ⅱ)能夠與海水中的有機(jī)質(zhì)配合, 較穩(wěn)定地存在于海水中。Millero等[30]測(cè)定了有機(jī)質(zhì)含量不同的Biscayne Bay和Gulf Stream 樣品中Fe(Ⅱ)的氧化速率, 結(jié)果表明有機(jī)質(zhì)含量較高的Biscayne Bay水中Fe(Ⅱ)的氧化半衰期為2.4~6.2 min, 是低含量有機(jī)質(zhì)Gulf Stream水的2~5倍, 有機(jī)質(zhì)成分分析表明, 分子質(zhì)量≤500 g/mol的有機(jī)質(zhì)能有效阻止Fe(Ⅱ)的氧化; 而Roy等[41]對(duì)亞北極太平洋西部表層水的研究再次驗(yàn)證了有機(jī)質(zhì)與Fe(Ⅱ)的絡(luò)合作用能夠延長(zhǎng)Fe(Ⅱ)的穩(wěn)定時(shí)間。

濕沉降是海水中Fe(Ⅱ)配體的重要來源。Kieber等[23]研究發(fā)現(xiàn)雨水來源的Fe(Ⅱ)的氧化速率遠(yuǎn)低于其他來源。這主要是由于雨水中的有機(jī)質(zhì)與Fe(Ⅱ)發(fā)生絡(luò)合, 抑制了Fe(Ⅱ)的氧化, 而紫外光消解可以破壞配合效應(yīng)[42]。Willey等[43]比較了雨水、河水等來源的有機(jī)配體與Fe(Ⅱ)的絡(luò)合能力, 發(fā)現(xiàn)雨水中疏水可萃取溶解有機(jī)質(zhì)能夠與Fe(Ⅱ)發(fā)生絡(luò)合, 使氧化半衰期由理論值的幾分鐘延長(zhǎng)至4 h, 其絡(luò)合能力與菲咯嗪(ferrozine, FZ)近似, 而河水來源的有機(jī)配體則對(duì)Fe(Ⅱ)的絡(luò)合作用不明顯。

2.3 海水中Fe(Ⅱ)的化學(xué)氧化

海水中Fe(Ⅱ)的氧化劑除了O2外, 還包括一些氧化中間體(如H2O2、O2–、OH·)。這些中間體主要由Fe(Ⅱ)的化學(xué)氧化、光催化反應(yīng)產(chǎn)生。King等[47]的研究表明, O2和H2O2對(duì)水體中Fe(Ⅱ)的氧化存在競(jìng)爭(zhēng)關(guān)系, 當(dāng)水體中H2O2的濃度高于一定值時(shí)(10–7mol/L),則可作為水體中Fe(Ⅱ)的主要氧化劑[48]。

目前, 國(guó)際上主要通過流動(dòng)注射化學(xué)發(fā)光法(flow injection- chemiluminescence, FI-CL)現(xiàn)場(chǎng)測(cè)定Fe(Ⅱ), 并在采樣和分析過程中采取盡量縮短測(cè)定時(shí)間、保持海水溫度、氛圍二級(jí)分樣、加入Fe(Ⅱ)固定劑等措施來降低Fe(Ⅱ)的氧化損失[8, 50-52]。由于發(fā)光信號(hào)與Fe(Ⅱ)濃度之間呈一元二次函數(shù)關(guān)系, 即y=kx2+bx+c, 因此在計(jì)算的過程中, 一般采用作標(biāo)準(zhǔn)曲線的方法求海水中Fe(Ⅱ)濃度。首先采樣后立即測(cè)定未知海水發(fā)光信號(hào)y′, 然后將同時(shí)采集的海水靜置10 h后作為空白海水, 并在空白海水中添加Fe(Ⅱ)標(biāo)準(zhǔn), 測(cè)定發(fā)光信號(hào)y與Fe(Ⅱ)濃度之間的函數(shù)式, 將y′代入方程計(jì)算得x′值即為未知海水中Fe(Ⅱ)的濃度[50](圖1)。之前Moffett等[51]采用不同的作圖方法, 在采樣后立即加Fe(Ⅱ)作標(biāo)準(zhǔn)加入曲線, 得到一元二次函數(shù)方程, 但該曲線在x軸上的截距大小是海水中Fe(Ⅱ)與試劑中干擾物質(zhì)濃度之和, 因此需減去試劑中干擾物質(zhì)的濃度, 才是海水中實(shí)際的Fe(Ⅱ)濃度。

3 生物過程對(duì)Fe(Ⅱ)的生成及遷移轉(zhuǎn)化的影響

海洋浮游植物在生長(zhǎng)和消亡的過程中吸收、釋放Fe(Ⅱ)造成其在海洋生物圈中的循環(huán), 海洋中Fe(Ⅱ)參與的生物過程主要包括浮游生物對(duì)Fe(Ⅱ)的攝取、微生物還原產(chǎn)生Fe(Ⅱ)以及Fe(Ⅱ)的生物氧化等過程。與Fe(Ⅲ)的吸收相比, Fe(Ⅱ)可以直接穿透生物細(xì)胞膜或被細(xì)胞膜上配體蛋白奪取至細(xì)胞內(nèi)[55-56], 不需要進(jìn)行與有機(jī)質(zhì)的絡(luò)合或還原過程[57-58], 是海洋生物所需Fe的重要來源。在Fe(Ⅱ)含量較低的海域, 一些藻類、酵母、細(xì)菌也能夠?qū)e(Ⅲ)化合物還原為Fe(Ⅱ)后再進(jìn)行吸收。

3.1 異化Fe(Ⅲ)還原過程

圖2 細(xì)菌異化還原Fe(Ⅲ)機(jī)制

目前研究最多的是sp.和sp.等還原菌, 它們主要通過一系列電子轉(zhuǎn)移蛋白的協(xié)同作用, 將細(xì)胞質(zhì)內(nèi)電子逐步由細(xì)胞內(nèi)膜、細(xì)胞質(zhì)向細(xì)胞外膜轉(zhuǎn)移。進(jìn)入細(xì)胞質(zhì)的溶解態(tài)Fe(Ⅲ)被還原酶所攜帶的電子被還原, 不溶態(tài)Fe(Ⅲ)氧化物則在胞外得到細(xì)胞外膜上的電子發(fā)生還原[65]。White等[66]還對(duì)細(xì)胞外膜中轉(zhuǎn)移蛋白結(jié)構(gòu)及其對(duì)電子轉(zhuǎn)移速率的影響進(jìn)行了研究, 結(jié)果表明細(xì)菌細(xì)胞膜上存在三種色素蛋白組合體。由于組合體的存在, 電子向胞外轉(zhuǎn)移的速率增加至單個(gè)轉(zhuǎn)移蛋白的103倍。這種高效的電子轉(zhuǎn)移機(jī)制為環(huán)境中Fe(Ⅲ)的快速還原提供了重要保證。

圖3 微生物還原Fe(Ⅲ)的三種電子轉(zhuǎn)移途徑

3.2 鐵氨氧化過程產(chǎn)生Fe(Ⅱ)

3.3 Fe(Ⅱ)的微生物氧化

圖4 三類海洋細(xì)菌催化的Fe(Ⅱ)氧化反應(yīng)

參與厭氧光氧化Fe(Ⅱ)反應(yīng)的細(xì)菌種類包括綠硫細(xì)菌、紫色非硫細(xì)菌和紫色硫細(xì)菌等[78], 由位于細(xì)菌細(xì)胞膜上的專一性氧化酶催化氧化反應(yīng)的發(fā)生[79]。其他研究表明細(xì)菌的厭氧光合作用比有氧光合作用出現(xiàn)時(shí)間更早, 而地球早期地層中鐵礦帶(banded iron formations, BIFs)的形成也與光養(yǎng)細(xì)菌的厭氧Fe(Ⅱ)氧化過程緊密相關(guān)[31, 80]。

細(xì)菌氧化Fe(Ⅱ)產(chǎn)生的Fe(Ⅲ)會(huì)吸附在細(xì)胞表面形成結(jié)殼, 嚴(yán)重阻礙細(xì)胞生命活動(dòng)。研究發(fā)現(xiàn)一些Fe(Ⅱ)氧化細(xì)菌可通過產(chǎn)生Fe(Ⅲ)有機(jī)配體[85]、降低pH[86]等方式來避免Fe(Ⅲ)的結(jié)殼, 因此具有更強(qiáng)的環(huán)境適應(yīng)能力。

4 Fe(Ⅱ)的氧化過程與N移除之間的關(guān)系

Fe(Ⅱ)主要通過參與透光層中浮游植物硝酸還原酶的合成來影響N的吸收。而海洋缺氧沉積物和OMZ作為海洋脫氮的主要區(qū)域, 每年通過反硝化或氨氧化過程移除的氮可達(dá)海洋總移除量的70%[87-89], 且沉積物中分布著豐富的鐵礦和還原微生物, 厭氧間隙水和OMZ水體中DFe(Ⅱ)含量普遍較高[18, 90], 因此推測(cè)海洋缺氧環(huán)境中存在著微生物活躍Fe-N遷移轉(zhuǎn)化活動(dòng)。

圖5 沉積物中的Fe-N循環(huán)[91]

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(本文編輯: 康亦兼)

A review of the behavior and microbial activity of Fe(Ⅱ) in seawater

QU Sheng-lu1, 2, YANG Ru-jun1, SU Han1, LIU Yuan1, GENG Qian-qian1

(1. College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, China; 2. Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071, China)

Fe(Ⅱ); microbialactivities; nitrogen removal

Apr. 28, 2017

P734.2

A

1000-3096(2017)10-0139-10

10.11759/hykx20170428001

2017-04-28;

2017-10-25

青島海洋科學(xué)與技術(shù)國(guó)家實(shí)驗(yàn)室海洋地質(zhì)過程與環(huán)境功能實(shí)驗(yàn)室開放基金資助項(xiàng)目(MGQNLM-KF201701)

[Supported by the Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, grant No. MGQNLM-KF201701]

曲勝路(1993-), 女, 河北衡水人, 碩士研究生, 主要從事海洋痕量元素生物地球化學(xué)研究, 電話: 17864277355, E-mail: 1175198588@qq.com; 楊茹君, 通信作者, 博士, 副教授, 電話: 0532-66781815, E-mail: yangrj@ouc.edu.cn