Comparative transcriptomics elucidates the cellular responses of an aeroterrestrial zygnematophyte to UV radiation

The zygnematophytes are the closest relatives of land plants and comprise several lineages that adapted to a life on land. Species of the genus Serritaenia form colorful, mucilaginous capsules, which surround the cells and block harmful solar radiation, one of the major terrestrial stressors. In eukaryotic algae, this 'sunscreen mucilage' represents a unique photoprotective strategy, whose induction and chemical background are unknown. We generated a de novo transcriptome of Serritaenia testaceovaginata and studied its gene regulation under moderate UV radiation (UVR) that triggers sunscreen mucilage under experimental conditions. UVR induced the repair of DNA and the photosynthetic apparatus as well as the synthesis of aromatic specialized metabolites. Specifically, we observed pronounced expressional changes in the production of aromatic amino acids, phenylpropanoid biosynthesis genes, potential cross-membrane transporters of phenolics, and extracellular, oxidative enzymes. Interestingly, the most up-regulated enzyme was a secreted class III peroxidase, whose embryophyte homologs are involved in apoplastic lignin formation. Overall, our findings reveal a conserved, plant-like UVR perception system (UVR8 and downstream factors) in zygnematophyte algae and point to a polyphenolic origin of the sunscreen pigment of Serritaenia, whose synthesis might be extracellular and oxidative, resembling that of plant lignins.

UV radiation and freshwater zooplankton: damage, protection and recovery

While many laboratory and field studies show that zooplankton are negatively affected when exposed to high intensities of ultraviolet radiation (UVR), most studies also indicate that zooplankton are well adapted to cope with large variations in their UVR exposure in the pelagic zone of lakes. The response mechanisms of zooplankton are diverse and efficient and may explain the success and richness of freshwater zooplankton in optically variable waters. While no single behavioural or physiological protection mechanism seems to be superior, and while several unexplained and contradictory patterns exist in zooplankton UVR ecology, recent increases in our understanding are consistent with UVR playing an important role for zooplankton. This review examines the variability in freshwater zooplankton responses to UVR, with a focus on crustacean zooplankton (Cladocera and Copepoda). We present an overview of UVR-induced damages, and the protection and recovery mechanisms freshwater zooplankton use when exposed to UVR. We review the current knowledge of UVR impact on freshwater zooplankton at species and community levels, and discuss briefly how global change over the last three decades has influenced the UVR milieu in lakes.

Sublethal stress: Impact of solar UV radiation on protein synthesis in the copepod Acartia tonsa

Aquatic organisms respond to environmental challenges such as thermal stress with the rapid induction of highly conserved polypeptides known as stress proteins or heat shock proteins (Hsps). Solar ultraviolet radiation (UVR, 280-400 nm) is an important environmental stressor in marine ecosystems. Here, we present results of experiments conducted with the marine copepod Acartia tonsa to follow the de novo protein synthesis and measure the level of constitutive and inducible isoforms of the Hsp70 gene family of stress proteins after UV exposure. Animals were collected from Tampa Bay, Florida (USA), and exposed to solar radiation (full spectrum), UV-A (320-400 nm) and PAR (400-700 nm), or PAR only, for periods of 0.5-4 h. Controls were kept in the dark. Protein synthesis was robust under all treatments when the copepods were exposed to low solar radiation intensities. Conversely, high solar radiation intensities (both UV-B and UV-A) caused an overall suppression in the protein synthesis of the copepods with no detectable induction of stress-inducible isoforms of Hsps. Immunochemical assays (western blotting) showed that UVR increased levels (3.5-4-fold increase compared to the dark control) of the constitutively expressed 70 kDa heat-shock (Hsc70) protein in A. tonsa, without indication of inducible isoform upregulation.

A novel heat stress-responsive gene in the marine diatom Chaetoceros compressum encoding two types of transcripts, a trypsin-like protease and its related protein, by alternative RNA splicing

We investigated the change of mRNA expression patterns in the laboratory-grown diatom Chaetoceros compressum under heat-stress conditions by mRNA arbitrarily primed (RAP) RT-PCR. Cells grown at 20 degrees C were subjected to heat treatment at 30 degrees C for 15 min and subsequently maintained at 20 degrees C for 8 h. Four genes including HI-5 were detected as heat stress-responsive genes by fingerprint analysis of RAP RT-PCR. Cloning for full-length cDNA sequences of HI-5 transcripts and related genomic DNA analysis revealed that two types of mRNA, HI-5a and HI-5b, were transcribed from the single HI-5 gene. While the HI-5a protein contained a catalytic domain characteristic to trypsin-like proteases, the HI-5b protein lacked this domain due to an insertion in the associated mRNA of 112 nucleotides; this insertion sequence contained a stop codon near the central region. Quantitative RT-PCR was performed to investigate the changes in expression levels of the two types of mRNA following heat treatment. The HI-5b transcripts were constitutively expressed in both unstressed and heat-stressed cells. In contrast, the number of HI-5a transcripts markedly increased in cells immediately after heat stress, reaching levels 19-fold higher at 8 h after heat stress than that in unstressed cells. These results suggest that RNA splicing plays a key role in heat stress-dependent expression of the HI-5a and HI-5b transcripts from the single HI-5 gene in the diatom.