Seawater carbonate chemistry and toxicity of Pseudo-nitzschia fraudulenta in a laboratory experiment

Anthropogenic CO2 is progressively acidifying the ocean, but the responses of harmful algal bloom species that produce toxins that can bioaccumulate remain virtually unknown. The neurotoxin domoic acid is produced by the globally-distributed diatom genus Pseudo-nitzschia. This toxin is responsible for amnesic shellfish poisoning, which can result in illness or death in humans and regularly causes mass mortalities of marine mammals and birds. Domoic acid production by Pseudo-nitzschia cells is known to be regulated by nutrient availability, but potential interactions with increasing seawater CO2 concentrations are poorly understood. Here we present experiments measuring domoic acid production by acclimatized cultures of Pseudo-nitzschia fraudulenta that demonstrate a strong synergism between projected future CO2 levels (765 ppm) and silicate-limited growth, which greatly increases cellular toxicity relative to growth under modern atmospheric (360 ppm) or pre-industrial (200 ppm) CO2 conditions. Cellular Si:C ratios decrease with increasing CO2, in a trend opposite to that seen for domoic acid production. The coastal California upwelling system where this species was isolated currently exhibits rapidly increasing levels of anthropogenic acidification, as well as widespread episodic silicate limitation of diatom growth. Our results suggest that the current ecosystem and human health impacts of toxic Pseudo-nitzschia blooms could be greatly exacerbated by future ocean acidification and 'carbon fertilization' of the coastal ocean.人为源二氧化碳（anthropogenic CO2）正持续使海洋酸化，但可产生可生物富集毒素的有害藻华（harmful algal bloom）物种的响应几乎仍未被探明。软骨藻酸（domoic acid）是由全球分布的硅藻伪菱形藻属（Pseudo-nitzschia）产生的神经毒素。该毒素可引发失忆性贝毒（amnesic shellfish poisoning），该病症可导致人类患病甚至死亡，且时常造成海洋哺乳动物与鸟类的大规模死亡。已知伪菱形藻细胞的软骨藻酸生成受营养盐可用性调控，但海洋二氧化碳浓度升高与其之间的潜在相互作用仍鲜有研究。本研究通过对欺诈伪菱形藻（Pseudo-nitzschia fraudulenta）的驯化培养物开展软骨藻酸生成量测定实验，发现未来预估的二氧化碳浓度水平（765 ppm）与硅限制生长之间存在显著协同效应，相较于现代大气浓度（360 ppm）或前工业化时期（200 ppm）的二氧化碳条件，该协同效应大幅提升了细胞毒性。细胞硅碳比随二氧化碳浓度升高而降低，这一变化趋势与软骨藻酸生成的趋势恰好相反。该菌株分离自加利福尼亚沿岸上升流系统，该区域目前正经历人为源海洋酸化的快速加剧，同时普遍存在硅藻生长受间歇性硅限制的现象。本研究结果表明，未来海洋酸化与近岸海洋“碳施肥（carbon fertilization）”效应可能会大幅加剧有毒伪菱形藻藻华对生态系统与人类健康造成的现有影响。