Kanako Naito

Biosynthesis and release of methylarsenic compounds during the growth of freshwater algae.

Arsenic transformations by freshwater algae have been studied under laboratory conditions. By the use of a new analytical method, we identified methylarsenic(III) species in the growth medium of green-alga Closterium aciculare incubated under axenic conditions. The arsenate concentration in the experimental medium began to decrease just after inoculation, and the levels of arsenite and methylarsenicals increased with the growth of C. aciculare. Initially, most of the arsenate was converted into arsenite, which peaked in concentration during the exponential phase. Methylarsenicals accumulated rapidly in the stationary phase. DMAA(V) production was enhanced when the ratio of phosphate to arsenate decreased in the culture medium. The levels of DMAA(V) increased continuously toward the end of the experiment. On the other hand, methylarsenic(III) species remained relatively steady during the stationary phase. Methylarsenic(III) species accounted for 0-35% of methylarsenicals. These results suggest that arsenite and methylarsenicals (containing methylarsenic(III) species) are supplied by phytoplankton, and serve as evidence of the origin of methylarsenic(III) species in natural waters.

Complexation of iron by microbial siderophores and effects of iron chelates on the growth of marine microalgae causing red tides

The growth rates of 13 species of abundant red tide algae in media with different iron species complexed with microbial siderophores (Ferrichrome and Ferrioxamine) and Fe‐Catechol were investigated. Our study demonstrated that the Fe‐chelates (at molar ratios = 1:1) were bioavailable to some red tide species. In Fe‐Catechol medium, growth was observed for the raphidophyte Heterosigma akashiwo, the dinoflagellates Heterocapsa circularisquama and Heterocapsa triquetra, the diatom Ditylum brightwellii, the cryptophyte Rhodomonas ovalis, the chlorophyte Oltmannsiellopsis viridis, and the haptophyte Cricosphaera roscoffensis. In Ferrioxamine medium, we found the growth of the dinoflagellate Karenia mikimotoi, the diatom Ditylum brightwellii, and the cryptophyte Rhodomonas ovalis. But, the existence of higher ligand concentrations (molar ratios ≥ 1:10) decreased the growth rates of most red tide species that were examined. Furthermore, all red tide species examined were not able to grow in Ferrichrome medium. In particular, the Chattonella species examined did not grow in the presence of Fe‐chelates. These results suggest that bioavailability of iron depends not only on ligand species, but also on the concentration of the ligands; moreover, microbial siderophores may play an important role in controlling the uptake of iron complexed with organic materials that exist in coastal water and the formation of red tides in coastal areas.

Secretion of iron-complexing ligands from Closterium aciculare (Charophyceae, Chlorophyta) under iron-deficient conditions

Abstract Siderophores, ferric iron-specific ligands, are produced by many species of prokaryotes, fungi and higher plants under iron-limited conditions. We report the possibility of siderophore production by a freshwater eukaryotic microalga Closterium aciculare under iron-deficient conditions. Closterium aciculare secreted a substance that is reactive in the chrome azurol S (CAS) assay in an iron-deficient, chemically defined medium. The final concentrations of the CAS-reactive substance were 0.17 and 0.35 μM for some 0.1 g 1−1 of C. aciculare in the medium with noniron and 10 nM iron addition, respectively. The amount of the CAS-reactive substance increased along with the increase in cell density of C. aciculare, whereas the addition of iron with nonlimiting concentration (100 nM) to the medium completely repressed the production of the CAS-reactive substance. Because C. aciculare did not produce the CAS-reactive substance in the iron-replete medium, the CAS-reactive substance was considered to be a siderophore.

Effects of organic iron complexes on the growth of red tide causative phytoplankton

Iron (Fe) is a crucially essential element for the growth of phytoplankton. Various mechanisms for the biological acquisition of iron require that the dissolved inorganic or free ion species [Fe(II) and Fe(III)] are ultimate iron species taken up by phytoplankton. But, iron mostly dissolves as complexes with the organic ligands and the concentration of dissolved iron is extremely low in natural seawater. The growths of 3 species of red tide causative phytoplankton (Chattonella antiqua, Heterosigma akashiwo and Heterocapsa circularisquama) were examined using a newly developed artificial synthetic medium in the presence of different iron species complexed with organic ligands. Our study demonstrated that the organic iron of different ligands was bioavailable to red tide phytoplankton. In iron salicylate and citrate chelates media, the growths were confirmed for the raphidophyte H. akashiwo and the dinoflagellate H. circularisquama. And all red tide species examined could grow in iron ethylenediaminetetraacetate (EDTA) chelates medium. These results suggest that bioavailable Fe depends not only on dissolved inorganic iron (soluble and colloidal phase) but also on chelates with organic ligands. And further, the natural orga nic matter such as salicylic acid and citric acid may play an important role in forming red tides in coastal area as well as siderophore-like organic compounds and regeneration of iron through bacterial activity.