Cuvelier Marie L, Guo Jian, Ortiz Alejandra C, van Baren Marijke J, Tariq Muhammad Akram, Partensky Frédéric, Worden Alexandra Z
Monterey Bay Aquarium Research Institute (MBARI), Moss Landing, CA, United States of America.
Department of Biomolecular Engineering, University of California Santa Cruz, Santa Cruz, CA, United States of America.
PLoS One. 2017 Mar 9;12(3):e0172135. doi: 10.1371/journal.pone.0172135. eCollection 2017.
Micromonas is a unicellular marine green alga that thrives from tropical to polar ecosystems. We investigated the growth and cellular characteristics of acclimated mid-exponential phase Micromonas commoda RCC299 over multiple light levels and over the diel cycle (14:10 hour light:dark). We also exposed the light:dark acclimated M. commoda to experimental shifts from moderate to high light (HL), and to HL plus ultraviolet radiation (HL+UV), 4.5 hours into the light period. Cellular responses of this prasinophyte were quantified by flow cytometry and changes in gene expression by qPCR and RNA-seq. While proxies for chlorophyll a content and cell size exhibited similar diel variations in HL and controls, with progressive increases during day and decreases at night, both parameters sharply decreased after the HL+UV shift. Two distinct transcriptional responses were observed among chloroplast genes in the light shift experiments: i) expression of transcription and translation-related genes decreased over the time course, and this transition occurred earlier in treatments than controls; ii) expression of several photosystem I and II genes increased in HL relative to controls, as did the growth rate within the same diel period. However, expression of these genes decreased in HL+UV, likely as a photoprotective mechanism. RNA-seq also revealed two genes in the chloroplast genome, ycf2-like and ycf1-like, that had not previously been reported. The latter encodes the second largest chloroplast protein in Micromonas and has weak homology to plant Ycf1, an essential component of the plant protein translocon. Analysis of several nuclear genes showed that the expression of LHCSR2, which is involved in non-photochemical quenching, and five light-harvesting-like genes, increased 30 to >50-fold in HL+UV, but was largely unchanged in HL and controls. Under HL alone, a gene encoding a novel nitrite reductase fusion protein (NIRFU) increased, possibly reflecting enhanced N-assimilation under the 625 μmol photons m-2 s-1 supplied in the HL treatment. NIRFU's domain structure suggests it may have more efficient electron transfer than plant NIR proteins. Our analyses indicate that Micromonas can readily respond to abrupt environmental changes, such that strong photoinhibition was provoked by combined exposure to HL and UV, but a ca. 6-fold increase in light was stimulatory.
扁藻是一种单细胞海洋绿藻，在从热带到极地的生态系统中都能茁壮生长。我们研究了处于适应状态的指数中期扁藻（扁藻RCC299）在多个光照水平以及日周期（14小时光照：10小时黑暗）下的生长和细胞特征。我们还在光照期4.5小时时，将适应了光暗周期的扁藻暴露于从适度光照到高光（HL）以及高光加紫外线辐射（HL + UV）的实验性转变中。通过流式细胞术对这种绿藻的细胞反应进行了定量分析，并通过qPCR和RNA测序对基因表达的变化进行了分析。虽然叶绿素a含量和细胞大小的指标在高光和对照中表现出相似的日变化，白天逐渐增加，夜晚减少，但在高光加紫外线辐射转变后，这两个参数都急剧下降。在光照转变实验中，叶绿体基因中观察到两种不同的转录反应：i）转录和翻译相关基因的表达在整个时间过程中下降，并且这种转变在处理组中比对照组更早发生；ii）相对于对照组，高光下几个光系统I和II基因的表达增加，在相同的日周期内生长速率也增加。然而，在高光加紫外线辐射条件下，这些基因的表达下降，这可能是一种光保护机制。RNA测序还揭示了叶绿体基因组中的两个基因，类ycf2和类ycf1，以前未曾报道过。后者编码扁藻中第二大的叶绿体蛋白，与植物Ycf1具有较弱的同源性，植物Ycf1是植物蛋白转运体的重要组成部分。对几个核基因的分析表明，参与非光化学猝灭的LHCSR2以及五个类捕光基因的表达在高光加紫外线辐射条件下增加了30至>50倍，但在高光和对照条件下基本不变。仅在高光条件下，一个编码新型亚硝酸还原酶融合蛋白（NIRFU）的基因增加，这可能反映了在高光处理提供的625 μmol光子 m-2 s-1条件下氮同化的增强。NIRFU的结构域结构表明它可能比植物亚硝酸还原酶蛋白具有更有效的电子转移。我们的分析表明，扁藻能够很容易地对突然的环境变化做出反应，因此，同时暴露于高光和紫外线会引发强烈的光抑制，但光照增加约6倍则具有刺激作用。
