Rapid recovery of coral communities from a mass bleaching event in the summer of 2016, observed in Amitori Bay, Iriomote Island, Japan.

Devastating bleaching of coral communities at Amitori Bay, Iriomote Island, Japan, occurred in 2016 during the third global mass bleaching event in 2014-2017. The present study documented changes in coral communities in Amitori Bay from just before until after the 2016 bleaching event (2016-2020), by measuring coral cover and recruitment at nine sites (with two additional sites in 2018) in the bay. Spawning rates of acroporid corals were also monitored from 2017 to 2019 by visual observation and using bundle collectors to observe how long the effect of bleaching persisted. Reductions of 64.7 and 89.5% from 2016 to 2017 were observed in cover and recruitment of all coral families, respectively. Coral cover of all coral families recovered to pre-bleaching levels by 2020 and recruitment in 2020 was about two times greater than the pre-bleaching level. These results mirrored those of acroporids. Spawning rates of Acropora corals increased significantly from 40.6% in 2017 to 90.0% in 2019. Recovery of coral cover 4 years after the severe bleaching event was likely related to regrowth of remnants and of surviving juveniles of < 5 cm. The sudden increase in recruitment was likely driven by a combination of larval supply from other populations, increased numbers of reproductive adults, increases in spawning rates, and increased larval retention in the bay due to wind conditions in 2020. This study suggests that coral communities as in Amitori Bay will be critical for local-scale community persistence, serving as both source and sink populations. Supplementary Information: The online version contains supplementary material available at 10.1007/s00227-022-04091-2.

Biosynthesis and transformation of homoanatoxin-a in the cyanobacterium Raphidiopsis mediterranea Skuja and structures of three new homologues.

The biosynthetic origin of the C-12 methyl group in homoanatoxin-a (1) was identified by the feeding experiment of L-[methyl-13C]-methionine in the culture of the cyanobacterium Raphidiopsis mediterranea Skuja strain LBRI 48. Remarkably high incorporation (80%) of 13C was observed at C-12. The in vivo enzymatic transformation of 1 was also examined by the prolonged culture of strain LBRI 48. The cells harvested at the stationary phase (15 days of incubation) gave higher contents of 4S-hydroxyhomoanatoxin-a (2), 4R-hydroxyhomoanatoxin-a (3), 2,3-epoxyhomoanatoxin-a (4), and 4-ketohomoanatoxin-a (5) than those from the cells collected at the late logarithmic growth phase (5 days). Compounds 2-5 would be transformed from 1 in the cells. The ratio of anatoxin-a and 1 was not significantly changed between two phases. Compound 5 was generated from 1 by air oxidation during storage even under dry and cool (-30 degrees C) conditions, but the oxidation was prevented in a water solution at both room temperature and -30 degrees C (frozen stock). Homoanatoxin-a (1) gave 2,3-dihydro-3-methoxyhomoanatoxin-a (6) during the separation procedures probably by the Michael reaction of methanol used as solvent. It should be noted that 4 was isolated for the first time from a cyanobacterium as the natural product. Compounds 3 and 5 were new members of the anatoxins.

Simultaneous production of homoanatoxin-a, anatoxin-a, and a new non-toxic 4-hydroxyhomoanatoxin-a by the cyanobacterium Raphidiopsis mediterranea Skuja.

A neurotoxin, homoanatoxin-a, was identified from a toxic strain of the cyanobacterium Raphidiopsis mediterranea Skuja (strain LBRI 48) isolated from Lake Biwa, Japan, as the major toxin component (0.57% of dry cell-weight). This cyanobacterium produced anatoxin-a and a new homoanatoxin-a derivative as minor components (0.04 and 0.06%, respectively). The structure of a new compound was assigned based on the spectral data as 4-hydroxyhomoanatoxin-a, which was not toxic to mice up to 2.0 mg/kg by intraperitoneal injection. The isolation of minor components was accomplished by improved extraction and separation procedures: (1) extraction with methanol-water (4:1) from dried cells, (2) adsorption of aqueous residue on ODS column (or cartridge) after evaporation of methanol, (3) cleaning up of the column by successive elution with water and 50% methanol/water, (4) elution of a toxic fraction by 20% methanol/water containing 0.1% TFA and (5) HPLC (ODS) purification with methanol/water containing 0.05% TFA. The procedures were effective in removing impurities and concentrating alkaloidal neurotoxins. It should be noted that this is the first report demonstrating the simultaneous production of anatoxin-a and homoanatoxin-a by the same strain of cyanobacterium.