Mayuko Otsubo

Effects of temperature and irradiance on growth of strains belonging to seven Skeletonema species isolated from Dokai Bay, southern Japan

The effect of temperature on the growth of Skeletonema ardens, S. costatum sensu stricto, the S. marinoi–dohrnii complex, S. japonicum, S. menzelii, S. pseudocostatum and S. tropicum isolated from Dokai Bay in southern Japan were examined under five to seven different temperatures and an irradiance of 150 µmol m−2 s−1. The effect of irradiance on the growth of the seven Skeletonema species was also examined under a wide range of irradiances ranging from 7–700 µmol m−2 s−1 at 20°C. All Skeletonema species were able to grow at temperatures ranging from 15 to 25°C. Intra-species differences in specific growth rates of four strains for S. menzelii, and five strains for S. ardens, the S. marinoi–dohrnii complex, S. japonicum and S. tropicum were not significant (Kruskal–Wallis test, P > 0.05). Significant inter-species differences in specific growth rates were observed at 10, 15, 25 and 30°C (Kruskal–Wallis test, P < 0.01; Steel–Dwass test, P < 0.01). The S. marinoi–dohrnii complex and S. japonicum grew faster than other species at the lower temperatures of 10 and 15°C, and S. ardens and S. menzelii grew at the highest temperature of 35°C. The maximum specific growth rates (μ max) from growth–irradiance curves ranged from 1.50 to 3.44 d−1. Threshold values of irradiance (I 0) and saturation irradiance (S) for growth ranged from 3.9 to 7.6 µmol m−2 s−1, and from 250 to 740 µmol m−2 s−1, respectively. In Dokai Bay, our results suggested that the occurrence of Skeletonema species should be affected mainly by temperature and less by irradiance. In particular, only two species, the S. marinoi–dohrnii complex and S. japonicum could maintain their populations in the surface water during the cold season, whereas during other periods, all seven species could maintain their population under the strong influence of estuarine circulation, which rapidly flushed the surface water out of Dokai Bay. Temperature and irradiance dependent growth values were in good agreement with their geographical distributions. The S. marinoi–dohrnii complex and S. japonicum were capable of growing in cold regions, but our results suggested that S. ardens and S. menzelii will tend to prefer tropical regions.

Utility of mitochondrial-encoded cytochrome c oxidase I gene for phylogenetic analysis and species identification of the planktonic diatom genus Skeletonema

Small subunit (SSU) and large subunit (LSU) rDNA sequences have been commonly used to delineate the taxonomy and biogeography of the planktonic diatom genus Skeletonema, but the genes occur as multiple copies and are therefore not suitable for barcoding purposes. Here, we analyzed phylogenetic relationships of Skeletonema using the mitochondrial‐encoded cytochrome c oxidase I gene (cox1), as well as partial LSU rDNA (D1–D3) and SSU rDNA, to identify the factors that define species and to evaluate the utility of these three markers for this taxon. Twelve Skeletonema species were divided into six clades, I–VI, each of which comprised the same species by the three markers: clades I (S. japonicum, S. grethae, S. pseudocostatum, and S. tropicum), II (S. menzelii), III (S. dohrnii and S. marinoi), IV (S. costatum, S. potamos, and S. subsalsum), V (S. grevillei), and VI (S. ardens). However, the branching order among these clades was incongruent among the markers. In clade III, six S. marinoi strains had identical cox1 sequences. These S. marinoi strains branched along with S. dohrnii, except for strains from the Gulf of Naples, with high support in cox1. Species delimitation between S. dohrnii and S. marinoi was therefore not supported. In clade IV, S. costatum and S. subsalsum were robustly clustered, with S. potamos as a sister clade in the cox1 tree, not in the LSU and SSU trees. In clade II, cox1 also confirmed that S. menzelii includes three subclades potentially distinguishable from each other by morphological features. Cox1 proved to be the most useful marker for the identification of Skeletonema species because it gave a tree with highly supported clades, has sufficient variation within and among species, encodes a protein in a single copy, and requires relatively few primers.

Functional Role of Fibrillin5 in Acclimation to Photooxidative Stress

The functional role of a lipid-associated soluble protein, fibrillin5 (FBN5), was determined with the Arabidopsis thaliana homozygous fbn5-knockout mutant line (SALK_064597) that carries a T-DNA insertion within the FBN5 gene. The fbn5 mutant remained alive, displaying a slow growth and a severe dwarf phenotype. The mutant grown even under growth light conditions at 80 µmol m-2 s-1 showed a drastic decrease in electron transfer activities around PSII, with little change in electron transfer activities around PSI, a phenomenon which was exaggerated under high light stress. The accumulation of plastoquinone-9 (PQ-9) was suppressed in the mutant, and >90% of the PQ-9 pool was reduced under growth light conditions. Non-photochemical quenching (NPQ) in the mutant functioned less efficiently, resulting from little contribution by energy-dependent quenching (qE). The ultrastructure of thylakoids in the mutant revealed that their grana were unstacked and transformed into loose and disordered structures. Light-harvesting complex (LHC)-containing large photosystem complexes and photosystem core complexes in the mutant were less abundant than those in wild-type plants. These results suggest that the lack of FBN5 causes a decrease in PQ-9 and imbalance of the redox state of PQ-9, resulting in misconducting both short-term and long-term control of the input of light energy to photosynthetic reaction centers. Furthermore, in the fbn5 mutant, the expression of genes involved in jasmonic acid biosynthesis was suppressed to ≤10% of that in the wild type under both growth-light and high-light conditions, suggesting that FBN5 functions as a transmitter of 1O2 in the stroma.