太湖藻源性颗粒物分解过程中氨基酸的变化特征
2017-01-20汤祥明邵克强
刘 颢,汤祥明,高 光,冯 胜,邵克强,胡 洋
(1:常州大学环境与安全工程学院,常州 213164)(2:中国科学院南京地理与湖泊研究所湖泊与环境国家重点实验室,南京 210008)
太湖藻源性颗粒物分解过程中氨基酸的变化特征
刘 颢1,2,汤祥明2,高 光2,冯 胜1,邵克强2,胡 洋2
(1:常州大学环境与安全工程学院,常州 213164)(2:中国科学院南京地理与湖泊研究所湖泊与环境国家重点实验室,南京 210008)
氨基酸;分解速率;氮形态;光照;蓝藻水华;太湖
本文对比了在自然光照和无光条件下,高密度太湖蓝藻分解过程中PAA的分解量、分解速率和组成,DAA含量和组成以及分解过程中氮形态的变化,并探讨了氨基酸态氮在富营养化水体氮循环中的作用,为阐明水华过程中氮素的迁移和转化规律及夏季水华现象发生的机理提供理论依据.
1 材料与方法
1.1 实验设置和样品采集
1.1.1 实验设置 实验在中国科学院太湖湖泊生态系统研究站生态室水槽内进行. 以6只用纯水洗净晾干的100 L有盖塑料水桶为培养容器,在各个水桶底部中央固定一造浪泵(功率为24 W),用以模拟水动力条件,保持整个系统内有一定的混合度. 从太湖湖泊生态系统研究站栈桥打捞发生水华时的蓝藻和湖水,蓝藻藻浆用64 μm尼龙网浓缩,湖水用64 μm尼龙网过滤. 将处理后的湖水加入上述水桶内,并添加适量蓝藻藻浆,混匀后测定水体中叶绿素a(Chl.a)浓度,保持水桶内Chl.a初始浓度为800 μg/L左右,与太湖蓝藻水华堆积时水体中的Chl.a浓度相当. 实验时3只水桶暴露在自然光照之下(自然光照组);其余3只水桶外壁和上盖用铝箔包好,创造无光条件(无光分解组),使得每个处理组有3个平行. 水桶以随机顺序悬挂放在有机玻璃房内的大型水槽内,每个水桶内放置一根加热棒,保持温度为28±2℃,连续培养21 d(实验时间为2014年8月1日-21日).
1.2 理化因子分析
PAA的降解速率采用Olson的指数模型[18]计算:
Ct/C0=e-kt
(1)
式中,C0为PAA初始浓度(mmol/L);Ct为降解时间t的PAA浓度(mmol/L);t为分解时间(d);k为降解速率常数(d-1).
1.3 氨基酸分析
每次采集的水样经GF/F滤膜过滤(GF/F滤膜预先经500℃处理4 h),记录过滤体积,将膜对折,包于铝箔中,-20℃冷冻保存;另取25 ml上述用GF/F滤膜过滤后的水置于洗净烘干的塑料瓶中,-20℃冷冻保存,待分析. 其中,滤膜上的为PAA,而塑料瓶中的用来测量DAA.
PAA浓度分析:将滤膜置于真空冷冻干燥机中冻干后放入水解管中,用5 ml 6 mol/L HCl溶解,氮吹1 min,封瓶,在110℃下水解22 h[19]. 水解结束后,打开水解管,吸取上层水解液离心(8000转/min)后,取2 ml上清液置于旋转蒸发仪中蒸发至干,用1 ml样品稀释液(pH=2.2)溶解,并用0.2 μm滤膜过滤,收集滤液放入2 ml离心管中,然后用氨基酸分析仪(Sykam)进行分析.
DAA浓度分析:在冻干的塑料瓶中加5 ml 6 mol/L HCl溶解后将全部液体转移入水解管中,并加入微量0.2%(w/v)抗坏血酸,氮吹1 min,封瓶,在110℃下水解22 h. 冷却后打开水解管,将水解液中的液体完全取出,测量其体积并至于旋转蒸发仪中蒸发至干,用1 ml样品稀释液(pH=2.2)溶解,并用0.2 μm滤膜头过滤,收集滤液放入2 ml离心管中,然后用氨基酸分析仪(Sykam)进行分析.
1.4 数据分析
实验数据通过Microsoft Excel 2013录入,分别采用SPSS 19.0和Origin 8.6软件分析及作图. 采用独立样本t检验分析不同组别之间是否存在显著性差异.
2 结果
2.1 蓝藻分解过程中理化指标的变化
实验时光照强度的变化因天气而异,出现了较为明显的波动. 在实验初期(1~4 d),光照强度比较大,在第2 d达到最大值116.44 W/m2. 随后出现两次波动,第1次波动的最小值为29.27 W/m2(第6 d),最大值为78.56 W/m2(第9 d);第2次波动在第15 d出现最小值15.75 W/m2,随后上升至82.69 W/m2(第21 d)(图1).
实验开始时,在自然光照组和无光分解组DO浓度分别为6.16和5.76 mg/L,随后两种不同处理组中DO浓度均明显下降至最低,分别为3.18和1.61 mg/L. 随后上升,到第21 d上升至实验初始水平(5.76和6.43 mg/L)(图1). 不同处理方式之间具有显著差异(P<0.05).
实验初期,有光和无光条件下Chl.a浓度分别为804.61和793.96 μg/L,在0~5 d内均下降很快,分别降至463.80和114.23 μg/L,之后下降速率放缓,到21 d分别降至369.24和12.21 μg/L,自然光照组和无光分解组之间差异显著(P<0.05). 在8~13 d,自然光照组中Chl.a浓度甚至还有上升的趋势(图1).
TN浓度在实验后期略有增加,整体变化不明显. 有光和无光条件下从实验前的9.75和9.70 mg/L分别增加到实验后的10.54和11.72 mg/L. 无光分解组的TN浓度总是稍小于自然光照组. 不同处理方式之间不具有显著差异(图1).
图1 藻源性颗粒物降解过程中理化因子的变化Fig.1 Variation of physical and chemical factors during the process of algae-originated particles decomposition
2.2 蓝藻分解过程中氨基酸浓度的变化
实验期间,自然光照组和无光分解组中PAA浓度均随时间明显下降(图2),在0~5 d,无光分解组和自然降解组中PAA浓度分别从0.44和0.46 mmol/L降至0.15和0.31 mmol/L,随后两种不同的处理中PAA浓度变化不明显,到实验结束时分别为0.06和0.30 mmol/L,无光分解组仍有一定的降解. 通过分析可以看出,实验期间,自然光照组中PAA的降解率为34.8%,而无光分解组中PAA的降解率高达86.0%,无光分解组中PAA分解量较自然分解组更大,两组之间差异显著(P<0.05).
DAA浓度的变化与PAA不同,呈现出先增加后降低的趋势(图2). 实验前期0~6 d,自然光照组和无光分解组DAA浓度均显著增加,第6~8 d,分别上升到最大值(10.94和7.94 μmol/L). 这时,水体中的DAA处于相对稳定的状态,DAA的生成与降解相对平衡. 第8 d之后,均呈现一个下降的过程,在第21 d分别降至3.13和0.66 μmol/L. 无光分解组DAA最终浓度低于初始浓度. 将DAA随时间的变化曲线分为上升期(0~5 d)和降解期(10~21 d). 可以发现在上升期,无光分解组DAA生成速率小于自然降解组;而在降解期,两组的分解速率并无显著性差异(P<0.05). 说明光照对上升期的DAA变化影响比较大.
图2 藻源性颗粒物降解过程中PAA和DAA浓度的变化Fig.2 Variations of PAA and DAA during the process of algae-originated particles decomposition
无光分解组中PAA的降解比较符合Olson指数衰减模型(R2=0.9494),降解速率常数为0.17424 d-1;自然光照组的降解则不太符合,降解速率常数为0.03916 d-1(图3). 从PAA降解速率曲线来看,自然光照组的降解速率常数不足无光分解组的1/4,表明了光照对PAA降解的影响很大.
图3 无光分解组和自然降解组中PAA的Olson指数衰减拟合曲线Fig.3 Olson exponent fitting curves of PAA in aphotic decomposition group and natural light group
图4 藻源性颗粒物分解过程中酸性、中性和碱性氨基酸的百分含量Fig.4 Percentage composition of acidic, neutral and basic amino acids during the process of algae-originated particles decomposition
2.3 蓝藻分解过程中氨基酸组成的变化
选择实验初期(第0 d)、实验中期(第7 d)和实验末期(第21 d),在两种不同的处理条件下PAA和DAA中酸性、中性和碱性氨基酸的百分比含量的三角图(图4). 可以发现,在所有实验条件和时间点,PAA和DAA中中性氨基酸百分含量总是最大的,占总氨基酸含量的60%~90%,酸性氨基酸次之,碱性氨基酸最小.
随着实验的进行,PAA中氨基酸组分变化不明显,而DAA中则酸性氨基酸比重逐渐降低,从初始条件下的18%~20%降低到11%左右,而中性和碱性氨基酸比重则逐渐升高(中性氨基酸从70%~72%增加到77%左右、碱性氨基酸从8.8%增加到10%左右). 其中,在第21 d无光分解组中DAA几乎检测不出酸性氨基酸,比重几乎为零. 但是,在相同的时间点,不同处理方式对PAA和DAA的组成改变不大(图4).
2.4 蓝藻分解过程中氮素百分含量的变化
图5 藻源性颗粒物分解过程中不同N素和其他N)的百分含量during the process of algae-originated particles decomposition
3 讨论
3.1 光照对藻源性颗粒物中氨基酸分解量、分解速率和组成的影响
PAA的分解量和分解速率会受到光照条件的制约;同样,作为PAA分解的中间产物,DAA也间接地受到光的作用. 一般认为,光照对水华生消影响最大,是蓝藻生长和水华暴发的主导性因子[10],对PAA分解有着显著的影响. 在有光照的条件下,PAA的分解会比无光条件下小得多. 这是由两方面因素造成的:一方面,在光照条件下,浮游植物会通过光合作用进行有机质的合成[20-21],而PAA是合成的产物之一. 这使得在自然光照条件下,堆积在上部的蓝藻可以通过光合作用合成PAA,宏观上降低了PAA的降解速度. 一定条件下,甚至会趋于PAA的降解和合成的平衡状态,导致在往后的一段时间内Chl.a保持一个稳定的高值. 另一方面,无光条件会使浮游植物细胞中进行氨基酸合成的碳骨架无效化,导致氨基酸的合成受到抑制[22-23],并且在光照强度很小的条件下,蓝藻的光合作用变弱,而呼吸作用变强,迫使系统内的DO浓度水平降低,形成兼性厌氧的环境,加快了藻源性颗粒物中PAA的降解[24]. 因此,在光照较弱的甚至是无光的环境中,藻源性颗粒物中PAA分解很快,提高了藻源性颗粒物转化为无机氮的能力[25-26].
而水体中DAA的现存量取决于PAA转化为DAA和DAA转化为其他物质这两步反应. 首先,PAA转化为DAA会受到光的影响. 吴丰昌等[27]认为有机质从高分子量到低分子量降解途径可能主要是光化学降解. 因此光照条件下DAA的生成量会大于无光条件. 尽管在无光条件以及其造成的低氧环境可以促进有机质的分解[26],但是DAA的转化主要是依靠细菌的吸收[28],而光照与否引起水体中细菌的数量和结构以及功能的变化还需进一步深入探讨.
3.2 藻源性颗粒物中氨基酸分解转化对蓝藻水华维持的意义
图6 藻源性颗粒物分解过程中N素的可能转化途径(实线表示无论是否光照,均会发生的转化途径;虚线表示主要发生在无光条件下的转化途径)Fig.6 Possible transformation pathways of N forms during the process of algae-originated particles decomposition
4 结论
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Characteristics of amino acids during the process of algae-originated particles decomposition in Lake Taihu
LIU Hao1,2, TANG Xiangming2**, GAO Guang2, FENG Sheng1, SHAO Keqiang2& HU Yang2
(1:SchoolofEnvironmentandSafetyEngineering,ChangzhouUniversity,Changzhou213164,P.R.China)(2:StateKeyLaboratoryofLakeScienceandEnvironment,NanjingInstituteofGeographyandLimnology,ChineseAcademyofSciences,Nanjing210008,P.R.China)
The characteristics of decomposition rate and quantity of amino acids, as well as the change of nitrogen forms, were explored during the process of decomposition of high-density cyanobacterial particles in this study. The results showed that the initial concentrations of particulate amino acids (PAA) were 0.46 mmol/L in natural light group and 0.44 mmol/L in the aphotic group. At the end of the experiment, the PAA was much stable with a concentration of 0.30 mmol/L in natural light group, while decreased dramatically to 0.06 mmol/L in the aphotic group. The degradation rate constants were 0.03916 and 0.17424 d-1, respectively. The concentration of dissolved amino acids (DAA) was much lower in comparison to PAA in the process of decomposition. In detail, the DAA increased gradually and peaked at 10.94 and 7.94 μmol/L for the two groups, respectively, and then declined to around the initial value in the end of the 21st day. At the beginning of the experiment, PAA accounted for 74%-80% of the total amino acids, then PAA was transformed to DAA and ammonia (NH+4-N) quickly, and finally the NH+4-N was transformed gradually to NO-3-N by nitrification. Compared with the natural light group, the decomposition of particles in aphotic group was more complete. Algal photosynthesis inhibited the decomposition of cyanobacterial particles in the natural group. Our results demonstrated that amino acids are potential nitrogen sources of phytoplankton and could be demineralized to NH+4-N to support phytoplankton growth during cyanobacterial blooms. Therefore, the decomposition of high-density algal particles plays a key role in the maintenance of cyanobacterial blooms.
Amino acids; degradation rate; nitrogen form; illumination; cyanobacterial blooms; Lake Taihu
*中国科学院南京地理与湖泊研究所“一三五”战略发展项目(NIGLAS2012135002)、国家自然科学基金项目(41471040,31100342,41501101)、南京水利科学研究院水利部水科学与水工程重点实验室开放研究基金项目(YK914006)和江苏省自然科学基金项目(BK20151059)联合资助. 2015-12-04收稿; 2016-05-27收修改稿. 刘颢(1991~),男,硕士研究生; E-mail: mizar_alcor@qq.com.
*通信作者; E-mail: xmtang@niglas.ac.cn.
J.LakeSci.(湖泊科学), 2017, 29(1): 95-104
DOI 10.18307/2017.0111
©2017 byJournalofLakeSciences