Abstract
Salinity is an important factor for regulating metabolic processes in aquatic organisms; however, its effects on toxicity and STX biosynthesis gene responses in dinoflagellates require further elucidation. Herein, we evaluated the physiological responses, toxin production, and expression levels of two STX synthesis core genes, sxtA4 and sxtG, in the dinoflagellate Alexandrium pacificum Alex05 under different salinities (20, 25, 30, 35, and 40 psu). Optimal growth was observed at 30 psu (0.12 cell division/d), but cell growth significantly decreased at 20 psu and was irregular at 25 and 40 psu. The cell size increased at lower salinities, with the highest size of 31.5 µm at 20 psu. STXs eq was highest (35.8 fmol/cell) in the exponential phase at 30 psu. GTX4 and C2 were predominant at that time but were replaced by GTX1 and NeoSTX in the stationary phase. However, sxtA4 and sxtG mRNAs were induced, and their patterns were similar in all tested conditions. PCA showed that gene transcriptional levels were not correlated with toxin contents and salinity. These results suggest that A. pacificum may produce the highest amount of toxins at optimal salinity, but sxtA4 and sxtG may be only minimally affected by salinity, even under high salinity stress.
Highlights
The dinoflagellate Alexandrium is an armoured photosynthetic microeukaryote that is distributed in coastal waters [1,2]
The Korean A. pacificum Alex05 was successively adapted to the different salinity conditions, and its growth was observed over 40 days
We revealed the correlation between growth rate, cell size, and toxin production in toxic A. pacificum under different salinities
Summary
The dinoflagellate Alexandrium is an armoured photosynthetic microeukaryote that is distributed in coastal waters [1,2]. Some grow rapidly when growth conditions are optimal, causing harmful algal blooms or red tides [4]. They can produce potent biotoxins, such as saxitoxin analogues (STXs), spiroimines, goniodomins, and lytic compounds [4,5,6]. These toxins can cause paralytic shellfish poisoning (PSP) when toxin-contaminated shellfish are consumed [7]. More than 2000 cases of PSP were recorded around the world in the 2000s, and this issue poses a global threat to human health [8,9]
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