渔业科学进展  2021, Vol. 42 Issue (5): 113-123  DOI: 10.19663/j.issn2095-9869.20201107001
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引用本文 

杨金龙, 段志鸿, 丁文扬, 徐嘉康, 顾忠旗, 梁箫. 维生素B7和B12对细菌生物被膜形成及厚壳贻贝幼虫变态的影响[J]. 渔业科学进展, 2021, 42(5): 113-123. DOI: 10.19663/j.issn2095-9869.20201107001.
YANG Jinlong, DUAN Zhihong, DING Wenyang, XU Jiakang, GU Zhongqi, LIANG Xiao. Effects of VB7 and VB12 on Biofilm Formation and Larval Metamorphosis of the Mussel Mytilus coruscus[J]. Progress in Fishery Sciences, 2021, 42(5): 113-123. DOI: 10.19663/j.issn2095-9869.20201107001.

基金项目

国家自然科学基金项目(41876159)、上海市优秀学术带头人计划(20XD1421800)和南方海洋科学与工程广东省实验室人才团队引进重大专项(GML2019ZD0402)、国家重点研发计划项目(2020YFD0900804)共同资助

作者简介

杨金龙,教授,E-mail: jlyang@shou.edu.cn

通讯作者

梁箫,E-mail: x-liang@shou.edu.cn

文章历史

收稿日期:2020-11-07
收修改稿日期:2020-11-24
维生素B7和B12对细菌生物被膜形成及厚壳贻贝幼虫变态的影响
杨金龙 1,2,3, 段志鸿 1,2, 丁文扬 1,2, 徐嘉康 1,2, 顾忠旗 4, 梁箫 1,2     
1. 上海海洋大学 国家海洋生物科学国际联合研究中心 上海 201306;
2. 上海海洋大学 水产种质资源发掘与利用教育部重点实验室 上海 201306;
3. 南方海洋科学与工程广东省实验室 广东 广州 511458;
4. 浙江省嵊泗县海洋科技研究所 浙江 舟山 202450
摘要:为探究B族维生素对海洋细菌生物被膜形成、海洋贝类幼虫变态所产生的作用,本研究首先使用维生素B7 (VB7)和B12 (VB12)直接刺激厚壳贻贝(Mytilus coruscus)幼虫,观察其对变态的直接诱导活性;然后通过添加VB7和VB12,与海假交替单胞菌(Pseudoalteromonas marina)共同形成生物被膜,分析B族维生素对生物被膜形成及其生物学特性的影响;同时检测生物被膜变化对厚壳贻贝幼虫变态发育的影响。研究结果显示,0.02 mmol/L浓度VB7和VB12可以直接诱导厚壳贻贝幼虫的变态,且效果最为显著(P < 0.05);0.02 mmol/L浓度VB7和VB12处理后的海假交替单胞菌生物被膜对幼虫附着变态的诱导作用均显著提高(P < 0.05);进一步通过细菌密度计数、膜厚度分析、可拉酸染色和定量等方法,揭示VB7和VB12处理后生物被膜细菌密度、膜厚度以及胞外多糖、蛋白和脂质均显著增加(P < 0.05)。研究结果证实,VB7和VB12可能通过改变海洋细菌生物被膜的生物学特性,进而调控厚壳贻贝幼虫变态发育。本研究为探究厚壳贻贝幼虫附着变态的分子机制提供了新的理论依据和创新思路,同时为B族维生素在提高厚壳贻贝人工育苗技术、改善厚壳贻贝养殖产业问题和促进海洋牧场生态修复建设等方面的应用提供了理论基础。
关键词B族维生素    海洋细菌    厚壳贻贝    生物被膜    变态    
Effects of VB7 and VB12 on Biofilm Formation and Larval Metamorphosis of the Mussel Mytilus coruscus
YANG Jinlong 1,2,3, DUAN Zhihong 1,2, DING Wenyang 1,2, XU Jiakang 1,2, GU Zhongqi 4, LIANG Xiao 1,2     
1. International Research Center for Marine Biosciences, Ministry of Science and Technology, Shanghai Ocean University, Shanghai 201306, China;
2. Key Laboratory of Exploration and Utilization of Aquatic Genetic Resources, Ministry of Education, Shanghai Ocean University, Shanghai 201306, China;
3. Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory, Guangzhou, Guangdong 511458, China;
4. Shengsi Institute of Marine Science and Technology in Zhejiang Province, Zhoushan, Zhejiang 202450, China
Abstract: As a growth cofactor, vitamin B (VB) participates in the body's various physiological and biochemical processes in the form of prosthetic groups or coenzymes. Meanwhile, the vitamin B is an essential nutrient in ensuring the regular operation of most biological activities. In this study, pharmacological experiments, confocal microscopy observation, and biofilms staining with mordant were used to understand the effects of vitamins B on marine biofilm formation and metamorphosis of Mytilus coruscus, which is a marine bivalve. VB7 and VB12 were used to investigate its impact on biofilm formation of Pseudoalteromonas marina, which exhibits an induction activity on the metamorphosis of M. coruscus larvae. The direct induction activity of VB7 and VB12 on larval metamorphosis was also detected. Results showed that the VB7 and VB12 at 0.02 and 0.2 mmol/L could directly and significantly increase the larval metamorphosis, and the concentration of VB7 and VB12 at 0.02 mmol/L had the most potent induction. Moreover, VB7 and VB12 at 0.02 mmol/L can also accelerate the growth of P. marina. The biofilm of P. marina formed at 0.02 mmol/L VB7 or VB12 had significant inducting effects on larval metamorphosis. The biofilm thickness, bacterial density, and extracellular polymeric substances (EPS) such as polysaccharides (especially the colonic acid), proteins, and lipids increased significantly after VB7 or VB12 treatment. In summary, VB7 and VB12 can increase the mussel larval metamorphosis directly. At the same time, they can also induce the production of biofilm EPS and further enhance the mussel larval metamorphosis inducing ability of the biofilm, suggesting that VB7 and VB12 can promote the larval metamorphosis of M. coruscus by changing the biofilm EPS composition. The results of this study provide a new theoretical basis and innovative methods for exploring the molecular mechanism of larval metamorphosis. It also provides a solid theoretical basis for the application of vitamin B in improving the artificial breeding technology of M. coruscus, solving the problems of mussel aquaculture industry and ecological restoration.
Key words: B vitamins    Marine bacteria    Mytilus coruscus    Biofilm    Larval metamorphosis    

厚壳贻贝(Mytilus coruscus)为一种具备较高营养的经济贝类(Wang et al, 2012; 徐嘉康等, 2017),常见于东海、黄海和渤海沿岸海域,主产区为浙江舟山东海海域,其养殖生产已发展了十几年,年度平均养殖数目高达50~60亿粒以上(王如才等, 1998; 罗友声等, 2003)。但近年来,由于贻贝养殖区域划分不规范、各级管理存在缺陷、贻贝灾害频发等问题,造成厚壳贻贝资源衰退(王靖陶等, 2010)。为了恢复和发展厚壳贻贝的野生资源,陆地工厂化人工育苗、海区增殖放流、贻贝海洋牧场养殖示范区建设等技术与产业迅速发展起来(张义浩等, 2003; 常抗美, 2007; 罗海忠等, 2016)。厚壳贻贝在其生活史中必须经历从浮游生活阶段过渡到附着生活阶段的变态发育过程,才可长成成贝(Wang et al, 2012)。附着变态后的贻贝也可通过切断其足丝的方式,寻找新的适宜环境进行二次附着(李太武, 2013)。因此,如何提高厚壳贻贝幼虫的附着变态率就成为其增养殖相关产业发展的核心技术问题。

研究表明,由海洋细菌所形成的生物被膜可以实现对大部分海洋无脊椎动物幼虫附着变态情况的调节(杨金龙等, 2012)。例如,假交替单胞菌属(Pseudoalteromonas)的某些细菌(Uhlinger et al, 1983; Holmström et al, 1999)可形成对厚壳贻贝(Yang et al, 2013)和华美盘管虫(Hydroides elegans) (Shikuma et al, 2014)等幼虫附着变态具有显著诱导作用的生物被膜。生物被膜的形成受许多因素的影响(Yang et al, 20142016a2016b)。其中,对生物机体生理生化有影响的钙、铁离子等营养素会导致海洋细菌形成的生物被膜的形态结构、分布和蛋白含量具有差异,并影响厚壳贻贝的附着(孙俊杰等, 2016; 常睿珩等, 2020)。

B族维生素作为一类有机物质,广泛存在于自然环境中,是维持生物体生命活动的重要活性物质(Degnan et al, 2014)。它能够调控大多数细菌的生理和生化反应,例如糖代谢、脂代谢、蛋白代谢和DNA合成等,从而影响生物体生长发育(Woods et al, 1953; 王春华等, 2011; 王雅平等, 2019; 周莹等, 2020)。研究表明,B族维生素通过影响布鲁杆菌属(Brucella)和沙门菌属(Salmonella)细菌的主要营养物质代谢等生理生化反应进而调控细菌生物量(Matras, 1973);B族维生素可显著促进海蠕虫(Capitella teleta)幼虫附着变态(Burns et al, 2018)。其中,维生素B7(VB7)和B12(VB12)以辅基或辅酶的形式参与机体的生理生化反应,调节新陈代谢并维持细胞、器官和组织结构和功能的完整,确保生命活动正常运行(Brown, 1989; Knowles et al, 1989; Ekhard et al, 1998)。但B族维生素对海洋细菌生物被膜形成及海洋贝类幼虫变态的影响尚不清楚。

本研究通过将厚壳贻贝眼点幼虫直接暴露于VB7和VB12中,以及在对厚壳贻贝幼虫附着变态具有高诱导活性的海假交替单胞菌(Pseudoalteromonas marina)生物被膜形成过程中添加VB7和VB12,明确其对厚壳贻贝幼虫附着变态的影响,探索B族维生素、海洋细菌生物被膜形成和海洋贝类幼虫变态三者间的相互关系,为解析厚壳贻贝附着变态分子机制提供新思路和理论依据。同时,也为B族维生素应用于海洋牧场建设中人工鱼礁礁体构造和厚壳贻贝人工养殖行业技术的完善提供科学依据。

1 材料与方法 1.1 实验菌株

实验所用的海假交替单胞菌分离自浙江省舟山市嵊泗县海域(30°72′N、122°76′E)挂板形成的自然生物被膜,并储存于–80℃超低温冰箱中(Peng et al, 2020)。

1.2 实验材料

实验所用的VB7和VB12购于西格玛奥德里奇(上海)贸易有限公司。实验中使用的厚壳贻贝幼虫在2019年中旬在浙江省某县(30°69′N、122°46′E)采集。在实验室环境中,以10 ind./mL密度于盐度30的自然海水中暂养,每2 d换水,每天投喂1×104 cells/mL湛江等鞭金藻(Isochrysis zhanjiangensis)藻液,避光18℃充气培养7 d后进行实验(梁箫等, 2020a)。

1.2.1 VB7和VB12直接诱导

将VB7、VB12和通过杀菌处理过的海水相溶(autoclaved filtered seawater, AFSW),并在此基础上形成pH=7.6的母液,将母液加入无菌培养皿(规格为64 mm×19 mm)中,并加适量AFSW定容至20 mL,设立空白对照组和B族维生素终浓度为0.02、0.2、2和20 mmol/L的不同实验组,每组设置9个生物学重复。将20只眼点幼虫添加到无菌培养皿中,置于18℃避光环境中培养96 h。记录12、24、48及96 h的幼虫附着变态数量,通过运算获得幼虫附着变态率。

1.2.2 VB7和VB12处理后海假交替单胞菌生长曲线绘制

参照朱艳蕾等(2016)方法测定处理后海假交替单胞菌生长曲线,取菌液(OD600 nm约为1),按1%接种量转接至盛有300 mL 2216E液体培养基的圆底烧瓶内连续培养,封瓶膜封口,不同培养时间取培养液,立即测定OD600 nm值,每个时间点设置3个生物学重复,以OD值为纵坐标、培养时间为横坐标,绘制生长曲线。

1.2.3 VB7和VB12处理后海假交替单胞菌生物被膜制备

参照杨金龙等(2015)方法制备生物被膜,取储存的海假交替单胞菌划线于2216E平板上,25℃培养12 h后,挑取单菌落接种到2216E液体培养基中,置于25℃避光条件下扩大培养。以3500 r/min离心15 min,去除上清液,用AFSW洗涤细菌沉淀3次,最后定容至50 mL制成细菌悬浊液,取1 mL稀释(细菌悬浊液∶AFSW=1∶99)后的菌液过滤至0.22 μm滤膜上,0.1%吖啶橙染色5 min,然后在荧光倒置显微镜(Olympus BX51)下观察,计算细菌浓度。在装有无菌载玻片的无菌培养皿中分别加入适量的菌液与VB7和VB12的混合物,加适量AFSW定容至20 mL,并使细菌的初始浓度为5×108 cells/mL,VB7和VB12最终浓度为0 (对照)和0.02 mmol/L,每组设9个生物学重复,避光18℃下培养48 h,以制备生物被膜。

1.2.4 幼虫附着变态实验

将附有生物被膜的载玻片转移至20 mL AFSW无菌培养皿中,并向培养皿中添加20只眼点幼虫,该实验由空白(Blank, 无菌玻片)、肾上腺素(Epinephrine, EPI)、自然生物被膜(Bacterial biofilm, BF)这3种对照组和实验组(生物被膜)组成(Satuito et al, 1999; Yang et al, 2008)。每组设置9个生物学重复,避光18℃条件下记录12、24、48和96 h幼虫附着变态率(梁箫等, 2020b)。

1.2.5 生物被膜细菌密度计数

借鉴杨娜等(2017)提出的方法,记录细菌密度值,并把生物被膜在5%的福尔马林试剂内浸泡48 h,0.1%吖啶橙染色5 min,之后放到荧光显微镜(Olympus BX51)下计数细菌密度,每片生物被膜中随机选择10个视野,每组设置3个生物学重复。

1.2.6 生物被膜膜厚度分析

参照杨娜等(2017)的方法分析膜厚度,将生物被膜在5%的甲醛溶液中固定24 h,避光条件下5 μg/mL碘化丙啶(PI)浸染处理20 min,之后通过1×PBS进行多次清洗。并借助激光共聚焦显微镜(confocal laser scanning microscopy, CLSM)展开详细观察,针对每组设置3个生物学重复,并自由挑选出10个视野进行成像分析,以确定生物被膜膜厚度。

1.2.7 生物被膜胞外产物分析

参照González-Machado等(2018)方法分析胞外产物。用0.9%生理盐水清洗培养好的生物被膜3次,通过相关试剂(表 1)进行染色处理,该步骤需要在没有光线的环境下操作20 min。之后通过0.9%的盐水进行漂洗,并放在同样没有光线的环境中,待其干燥之后通过CLSM显微镜详细观察,并自由挑选出10个视野对其成像效果予以分析。每组设置3个生物学重复。

表 1 生物被膜胞外产物染色试剂 Tab.1 Staining reagent of biofilm EPS composition
1.2.8 生物被膜可拉酸染色

参照杨金龙等(2015)的方法制备生物被膜,从载玻片刮下生物被膜,涂布在玻璃片上,然后风干、染色。可拉酸染色参考Ren等(2016)的方法,将0.3 g碱性品红、90 mL含5%苯酚和10 mL 95%乙醇混合制成品红溶液,将玻璃片染色3 min。用媒染剂溶液[2.0 g/L KAl(SO4)2∶0.3 g/L FeCl3∶1.5 g/L丹宁酸=5∶2∶2]染色3 min。最后,1%亚甲基蓝溶液染色30 s。在每次染色之前,蒸馏水冲洗生物被膜以去除上一种染色液。细菌细胞染成红色,可拉酸染成蓝色。每组设置3个生物学重复。

1.2.9 可拉酸定量

参考Obadia等(2007)的方法定量可拉酸。从12个生物被膜中收集细菌细胞,并将其悬浮在1 mL蒸馏水中。100℃煮沸细菌10 min,13, 000 g离心15 min,弃去沉淀,收集上清液,定量可拉酸。加入4.5 mL H2SO4/H2O溶液(6∶1),于1 mL上清液中制成混合液,100℃反应20 min。冷却至25℃,将2 mL混合液用于检测396 nm (A396co)和427 nm (A427co)吸光度。将100 μL的3%(m/v)半胱氨酸盐酸溶液与剩余混合液在25℃下避光孵育1 h后,在396 nm (A396cy)和427 nm (A427cy)测吸光度。将(A396cy–A396co)–(A427cy–A427co)的值代入L-岩藻糖浓度标准曲线(10~100 μg/mL)计算可拉酸浓度。

1.3 数据统计与分析

使用JMP软件(ver.10.0.0)进行统计分析和相关性检验(Liang et al, 2020)。将幼虫的变态率百分比转化为反正弦以观察正态分布情况,如果符合该现象,则需要通过单因素方法对其中的差异进行分析。反之,便需要进行Kruskal-Wallis检验分析。Spearman多元分析方法用于幼虫变态率与B族维生素浓度及细菌密度之间的相关性分析,采用P为检验值,r为相关系数,显著性水平设置为0.05。生物被膜胞外产物含量分析使用image软件。

2 结果 2.1 VB7和VB12对幼虫变态的直接影响

图 1显示,0.02、0.2 mmol/L VB7诱导幼虫变态率分别为(28.33±0.83)%和(11.67±0.83)%,VB12诱导幼虫变态率分别为(13.89±1.39)%和(1.67±0.83)%,与空白对照组相比,均可显著提高诱导厚壳贻贝幼虫变态(P < 0.05),其中以0.02 mmol/L的诱导效果最好。而2、20 mmol/L VB7的变态率分别为(5.56±1.30)%和(1.67±0.83)%,VB12的变态率分别为(1.67±0.83)%和0%,与空白对照组相比,均出现显著抑制作用(P < 0.05),其中以20 mmol/L浓度的抑制作用最强。

图 1 VB7和VB12对厚壳贻贝幼虫变态的影响(72 h) Fig.1 Effects of VB7 and VB12 on larval metamorphosis in the mussel M. coruscus at 72 h 不同字母表示差异显著(P < 0.05)。下同 Different letters show significant difference (P < 0.05). The same as below
2.2 VB7和VB12对海假交替单胞菌生长的影响

对照组和处理组海假交替单胞菌在培养1 h后进入生长对数期,6 h后进入稳定期(图 2)。培养6 h时,OD600 nm值分别为OD(P. marina)=0.9543、OD(P. marina+ 0.02 mmol/L VB7)=1.0607和OD(P. marina+0.02 mmol/L VB12)=0.9923,相比于单一细菌的对照组,0.02 mmol/L的VB7和VB12在生长对数期对细菌生长均具有显著影响(P < 0.05)。其中,VB7对细菌生长的促进作用持续至稳定期,而VB12在细菌生长9 h后无明显作用。

图 2 VB7和VB12处理后的海假交替单胞菌生长曲线 Fig.2 Growth curve of P. marina after treatment with VB7 or VB12
2.3 VB7和VB12处理后海假交替单胞菌生物被膜对幼虫变态的影响

图 3显示,由VB7和VB12与海假交替单胞菌形成的生物被膜诱导的幼虫附着变态率显著高于细菌单一生物被膜(P < 0.05)。0.02 mmol/L浓度VB7处理后,海假交替单胞菌生物被膜诱导幼虫附着变态率为(71.0±3.9)%;0.02 mmol/L浓度VB12处理后的幼虫附着变态率为(60.1±3.3)%,二者均显著高于细菌单一生物被膜附着变态率(38.8±3.09)% (P < 0.05)。

图 3 VB7和VB12处理后的海假交替单胞菌生物被膜对厚壳贻贝幼虫变态的影响(48 h) Fig.3 Effects on metamorphosis of post-larvae on the P. marina biofilms after treatment with VB7 or VB12 at 48 h 1. 空白;2. 肾上腺素;3. 自然微生物被膜;4. 细菌;5. 细菌+VB7;6. 细菌+VB12 1. Blank; 2. EPI; 3. BF; 4. P. marina; 5. P. marina+VB7; 6. P. marina+VB12
2.4 VB7和VB12处理后海假交替单胞菌生物被膜生物量的变化

图 4显示,初始浓度为5×108 cells/mL的海假交替单胞菌分别与VB7和VB12共同形成生物被膜,其细菌密度明显高于细菌单一生物被膜(P < 0.05),VB7和VB12处理后的生物被细菌聚集性增强(图 5),且膜厚度显著提高(P < 0.05)(图 6)。相关分析结果显示,生物被膜所具备的细菌密度和膜厚度均与诱导活性具有极显著相关性(P < 0.05)(表 2)。

图 4 VB7和VB12处理后的海假交替单胞菌生物被膜细菌密度 Fig.4 Density of P. marina biofilms after treatment with VB7 or VB12 数据为9个生物学重复的平均值±标准误。下同 Data were Mean±SE of 9 duplicates. The same as below
图 5 激光共聚焦扫描电镜下VB7和VB12处理后的海假交替单胞菌生物被膜形态 Fig.5 CLSM reveals morphology of P. marina biofilms after VB7 or VB12 treatment 1. P. marina; 2. P. marina+VB7; 3. P. marina+VB12
图 6 激光共聚焦扫描电镜下VB7和VB12处理后的海假交替单胞菌生物被膜厚度 Fig.6 CLSM reveals thickness of P. marina biofilms after treatment with VB7 or VB12
表 2 生物被膜厚度和细菌密度与诱导活性的相关性分析 Tab.2 Correlation analysis between biofilm thickness, bacterial density and inducing activity
2.5 海假交替单胞菌生物被膜胞外产物的变化

荧光染色显示(图 7),海假交替单胞菌单一生物被膜细菌呈颗粒状分布,胞外产物分布较为均匀,而VB7和VB12处理后的生物被膜细菌多聚集状态,胞外产物含量明显升高,且多呈块状分布。

图 7 激光共聚焦扫描电镜下VB7和VB12处理后的海假交替单胞菌生物被膜胞外产物 Fig.7 CLSM reveals extracellular products of P. marina bacterial biofilms after VB7 or VB12 treatment 1. P. marina; 2. P. marina+VB7; 3. P. marina+VB12
a:α多糖;b:β多糖;c:蛋白;d:脂质
a: α-Polysaccharide; b: β-Polysaccharide; c: Protein; d: Lipids

3组生物被膜α多糖、β多糖、蛋白质和脂质含量见图 8。与单一生物被膜相比,VB7处理后具有更高含量的α多糖、β多糖、蛋白质和脂质。其中,单一生物被膜的α多糖含量仅为(1536.50±57.36) μm3,而VB7处理后生物被膜的α多糖含量为(800, 590.25± 46, 499.65) μm3,上调了520倍(P < 0.05)(图 8)。

图 8 激光共聚焦扫描电镜下VB7和VB12处理后的P. marina生物被膜胞外产物含量 Fig.8 CLSM reveals extracellular products of P. marina biofilms after VB7 or VB12 treatment 浓度数据为9个重复的平均值±标准误 Data for concentration were Mean±SE of nine replicates
2.6 处理后海假交替单胞菌生物被膜的可拉酸含量

染色结果显示,相比于单一细菌生物被膜,VB7和VB12处理后生物被膜可拉酸分布更密集(图 8),且含量明显提高(图 9)。单一细菌生物被膜可拉酸含量为(30.46±6.03) μg/mL,而VB7和VB12处理后含量分别为(215.78±24.18)和(158.27±23.27) μg/mL,升高了7.08倍和5.20倍(P < 0.05) (图 9)。

图 9 媒染剂染色生物被膜光学显微镜的观察 Fig.9 Light microscopic observation of mordant-stained biofilms 1. P. marina; 2. P. marina+VB7; 3. P. marina+VB12
细菌被染成红色,可拉酸被染成蓝色
The strains were stained red and the colonic acid was stained blue
图 10 VB7和VB12处理后的P. marina生物被膜可拉酸含量 Fig.10 Colanic acid production in P. marina biofilms after VB7 or VB12 treatment 浓度数据为4个重复的平均值±标准误 Data for concentration were Mean±SE of four replicates
3 讨论 3.1 B族维生素对生物被膜形成的影响

B族维生素作为生长辅助因子,以辅基或辅酶的形式参与机体的生理生化反应,调节新陈代谢,并维持细胞、器官和组织结构和功能的完整,是确保所有生命活动正常运行的重要营养物质(Brown et al, 1989; Knowles et al, 1989; Ekhard et al, 1998)。本研究的结果显示,0.02 mmol/L浓度下VB7和VB12与初始细菌浓度为5×108 cells/mL的P. marina共同形成生物被膜,与P. marina单一生物被膜相比,细菌密度和膜厚度显著增加,细菌成聚集状态,并产生大量的多糖等胞外产物。Matras(1973)研究表明,B族维生素可通过影响布鲁杆菌属和沙门菌属细菌的主要营养物质代谢等生理生化反应来调节细菌生物量。因此,推测B族维生素的加入可以提高生物被膜的生物量和胞外产物含量,进而促进生物被膜的形成。

3.2 B族维生素对幼虫变态的影响 3.2.1 B族维生素对幼虫变态的直接影响

许多海洋无脊椎动物在发育为成体前处于浮游幼虫阶段。幼虫的浮游阶段可以短则几分钟,也可以长达几个月。通常,海洋无脊椎动物幼虫的附着变态主要受外界环境和内源性因素控制(Crisp, 1974; Pawlik, 1992),特别是外界环境因子对于幼虫的附着变态至关重要(Morse, 1990; McClintock et al, 2001)。B族维生素广泛存在于自然环境中,并作为营养物质参与幼虫的附着变态过程。Burns等(2018)研究表明,B族维生素可通过影响海蠕虫幼虫的主要营养物质代谢水平进而提高幼虫附着变态率。然而,关于B族维生素与海洋贝类附着二者间的相互作用关系仍有待研究。

研究结果显示,0.02、0.2 mmol/L浓度下VB7和VB12的孵育均可以显著诱导厚壳贻贝幼虫变态。其中以0.02 mmol/L浓度下的诱导作用较强,但2、20 mmol/L浓度下VB7和VB12对幼虫变态起抑制作用。因此,推测在一定浓度的条件下,B族维生素可能通过增强幼虫糖类、脂类和蛋白质代谢及免疫系统功能进而促进厚壳贻贝等海洋贝类幼虫的附着变态。

3.2.2 生物被膜与幼虫变态的关系

生物被膜生物量和胞外产物(胞外多糖、蛋白、脂质等)含量是影响海洋无脊椎动物附着的关键因素(Kirchman et al, 1981; Hadfield et al, 2011)。研究表明,厚壳贻贝幼虫附着变态和稚贝附着都受到细菌密度的影响(Wang et al, 2012; 杨金龙等, 2013; Yang et al, 2014)。厚壳贻贝与生物被膜相关性研究证实,细菌密度与附着变态呈显著相关;自然生物被膜和胞外产物的群落结构对合浦珠母贝(Pinctada fucata)幼虫附着具有十分关键的诱导功能(Yu et al, 2010);海洋假单胞菌Strain S9所对应的生物被膜胞外多糖大幅度加快了海鞘(Ciona intestinalis)幼虫附着变态(Szewzyk et al, 1991)。因此,推测厚壳贻贝幼虫附着变态除了与B族维生素的直接诱导作用有关外,还可能与生物被膜的生物量和胞外产物含量有关。研究结果显示,VB7和VB12处理后的生物被膜显著提高了厚壳贻贝幼虫的附着变态率,其生物被膜的生物量和胞外产物含量显著高于P. marina单一生物被膜,其中,胞外多糖含量升高520倍。前期研究发现,多糖能够诱导许多海洋无脊椎动物幼虫附着变态(Kirchman et al, 1982; Szewzyk et al, 1991; Matsumura et al, 1998; Khandeparker et al, 2003; Bao et al, 2007; Zeng et al, 2015),其中,最具代表性的是由生物学重复单元(葡萄糖、岩藻糖、半乳糖和葡萄糖醛酸)与α键和β键相连所组成的可拉酸(Whitfield, 2006; Schmid et al, 2015),可拉酸能在细菌周围产生高度负电荷胶囊(Goebel, 1963; Allen et al, 1987),并将吸附细菌周围的矿物质和营养物质(Ophir et al, 1994),促进具有诱导海洋无脊椎动物幼虫附着变态活性的附着基的形成。实验证明,P. marina细菌多糖相关基因缺失后,可拉酸的产生量增多,细胞内c-di-GMP水平升高,c-di-GMP水平的升高导致细菌运动能力降低,进而促进细菌粘附和生物被膜的形成,从而提高厚壳贻贝幼虫的附着变态率(Whiteley et al, 2015; Pérez-Mendoza et al, 2016; Peng et al, 2020)。本研究结果显示,VB7和VB12处理后生物被膜的生物量和胞外产物(可拉酸)含量均显著升高,并均对厚壳贻贝幼虫的附着变态存在显著诱导作用。因此,推测B族维生素可能通过协同c-di-GMP调节可拉酸的产生,从而正调控生物被膜的形成和厚壳贻贝幼虫的附着变态,但B族维生素对c-di-GMP水平的影响尚需进一步研究。这一发现为阐明生物被膜与幼虫之间的相互作用提供了新的视角。

本研究首次发现,VB7、VB12两种B族维生素对厚壳贻贝幼虫变态具有直接诱导作用。同时,VB7和VB12通过促进P. marina生物被膜的形成,增加生物被膜的细菌密度和膜厚度,提高生物被膜胞外产物含量,从而间接促进厚壳贻贝幼虫的附着变态。本研究为探究厚壳贻贝幼虫附着变态的分子机制提供了新的理论依据和创新思路,同时,为B族维生素在提高厚壳贻贝人工育苗技术、改善厚壳贻贝养殖产业问题和海洋牧场建设等生态修复方面的应用提供了理论基础。

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