title: Responses of the picoprasinophyte Micromonas commoda to light and ultraviolet stress.
abstract: 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.
authors.affiliation: Monterey Bay Aquarium Research Institute (MBARI), Moss Landing, CA, United States of America.
authors.forename: Marie L
authors.initials: ML
authors.lastname: Cuvelier
authors.affiliation: Monterey Bay Aquarium Research Institute (MBARI), Moss Landing, CA, United States of America.
authors.forename: Jian
authors.initials: J
authors.lastname: Guo
authors.affiliation: Monterey Bay Aquarium Research Institute (MBARI), Moss Landing, CA, United States of America.
authors.forename: Alejandra C
authors.initials: AC
authors.lastname: Ortiz
authors.affiliation: Monterey Bay Aquarium Research Institute (MBARI), Moss Landing, CA, United States of America.
authors.forename: Marijke J
authors.initials: MJ
authors.lastname: van Baren
authors.affiliation: Department of Biomolecular Engineering, University of California Santa Cruz, Santa Cruz, CA, United States of America.
authors.forename: Muhammad Akram
authors.initials: MA
authors.lastname: Tariq
authors.affiliation: Sorbonne Universités-UPMC Université Paris 06, CNRS UMR, Station Biologique, CS, Roscoff, France.
authors.forename: Frédéric
authors.initials: F
authors.lastname: Partensky
authors.affiliation: Monterey Bay Aquarium Research Institute (MBARI), Moss Landing, CA, United States of America.|Department of Ocean Sciences, University of California Santa Cruz, Santa Cruz, CA, United States of America.|Integrated Microbial Biodiversity Program, Canadian Institute for Advanced Research, Toronto, Canada.
authors.forename: Alexandra Z
authors.identifier: http://orcid.org/0000-0002-9888-9324
authors.initials: AZ
authors.lastname: Worden
mesh_terms: D020418:Algal Proteins / Q000235:genetics*; D000460:Chlorophyta / Q000235:genetics* / Q000528:radiation effects; D018506:Gene Expression Regulation, Plant / Q000528:radiation effects*; D059014:High-Throughput Nucleotide Sequencing; D008027:Light; D048030:RNA, Algal / Q000235:genetics; D017423:Sequence Analysis, RNA; D014466:Ultraviolet Rays
chemical_list: D020418:Algal Proteins; D048030:RNA, Algal