Effect of CO₂ on growth and toxicity of Alexandrium tamarense from the East China Sea, a major producer of paralytic shellfish toxins

Highlights

• Strain of Alexandrium tamarense isolated from East China Seas, showed a significant response to elevated CO₂ levels in growth and toxicity.

• Strain ATDH grew faster and showed a larger density when exposed to elevated CO₂ concentration, especially in the exponential period.

• The concentration per cell of each PST derivate varied and eventually caused the cellular toxicity increased when exposed to higher pCO₂.

Abstract

In recent decades, the frequency and intensity of harmful algal blooms (HABs), as well as a profusion of toxic phytoplankton species, have significantly increased in coastal regions of China. Researchers attribute this to environmental changes such as rising atmospheric CO₂ levels. Such addition of carbon into the ocean ecosystem can lead to increased growth, enhanced metabolism, and altered toxicity of toxic phytoplankton communities resulting in serious human health concerns. In this study, the effects of elevated partial pressure of CO₂ (pCO₂) on the growth and toxicity of a strain of Alexandrium tamarense (ATDH) widespread in the East and South China Seas were investigated. Results of these studies showed a higher specific growth rate (0.31 ± 0.05 day⁻¹) when exposed to 1000 μatm CO₂, (experimental), with a corresponding density of (2.02 ± 0.19) × 10⁷ cells L⁻¹, that was significantly larger than cells under 395 μatm CO₂₍control). These data also revealed that elevated pCO₂ primarily affected the photosynthetic properties of cells in the exponential growth phase. Interestingly, measurement of the total toxin content per cell was reduced by half under elevated CO₂ conditions. The following individual toxins were measured in this study: C1, C2, GTX1, GTX2, GTX3, GTX4, GTX5, STX, dcGTX2, dcGTX3, and dcSTX. Cells grown in 1000 μatm CO₂ showed an overall decrease in the cellular concentrations of C1, C2, GTX2, GTX3, GTX5, STX, dcGTX2, dcGTX3, and dcSTX, but an increase in GTX1 and GTX4. Total cellular toxicity per cell was measured revealing an increase of nearly 60% toxicity in the presence of elevated CO₂ compared to controls. This unusual result was attributed to a significant increase in the cellular concentrations of the more toxic derivatives, GTX1 and GTX4.Taken together; these findings indicate that the A. tamarense strain ATDH isolated from the East China Sea significantly increased in growth and cellular toxicity under elevated pCO₂ levels. These data may provide vital information regarding future HABs and the corresponding harmful effects as a result of increasing atmospheric CO₂.

Introduction

Ocean acidification is a worldwide environmental problem with complex processes. Since the industrial revolution until as recently as 2011, the atmospheric partial pressure of CO₂ (pCO₂) has risen from 280 μatm to 430 ± 90 μatm due to the burning of fossil fuels, deforestation, industrialization, and cement production (IPCC 2014). Elevated CO₂ levels will likely increase even more, with some researchers predicting levels to double up to 750–1000 μatm by the year 2100. Such elevated CO₂ levels may cause ocean pH levels to decrease by as much as 0.3–0.4 units (IPCC 2014). Rising atmospheric CO₂ levels is increasing seawater CO₂ concentrations and is the main cause for ocean acidification. As human input of nutrients causes eutrophication, and then induce large CO₂ input into coastal water (Cai et al., 2011, Sunda and Cai, 2012), the increase in atmospheric pCO₂ can affect the carbon chemistry of the ocean ecosystem and may have significant implications for phytoplankton populations.

Among the many phytoplankton species that will be affected by acidification, species that cause harmful algal blooms (HABs), have received recent attention. These species, when grown outside of normal ecological levels, cause significant marine problems including mechanical damage to fish gills, toxic effects to filter-feeding invertebrates, and competition for resources (Hallegraeff, 1993, Anderson and Garrison, 1997). Recently, the frequency, scale, and distribution range of HABs are increasing worldwide (Borkman et al., 2014, van der Lingen et al., 2016, Wells et al., 2015). There are many reasons for this observation, including coastal eutrophication (Honjo, 1993, Anderson et al., 2002, Glibert et al., 2005, Hallegraeff, 2010), and major ocean biogeochemical changes such as acidification (Su et al., 2001, Cai et al., 2006, Moore et al., 2008, van der Lingen et al., 2016, Wells et al., 2015).

HAB species have the same physiological response to ocean acidification as other phytoplankton species, including altered growth and carbon fixation rates, shifts in nutrient uptake, changes in elemental ratios, and increased sensitivity to ultraviolet radiation (Riebesell, 2004, Fu et al., 2007, Fu et al., 2008a, Fu et al., 2008b, Fu et al., 2010, Fu et al., 2012, Feng et al., 2008, Feng et al., 2009, Feng et al., 2010, Hutchins et al., 2007, Hutchins et al., 2009, Riebesell et al., 2007, Riebesell et al., 2008, Rost et al., 2008, Beardall et al., 2009, Gao et al., 2012a, Gao et al., 2012b). Since ocean acidification is associated with increasing availability of dissolved inorganic carbon (DIC), increasing pCO₂ concentrations are a major cause of toxic HABs. Studies have shown that under increasing pCO₂ concentrations, the diatom Pseudo-nitzschia spp. demonstrated increased cellular growth rates and elevated domoic acid toxin concentrations causing amnesic shellfish poisoning (Sun et al., 2011, Tatters et al., 2012). Karlodinium veneficum, an ichthyotoxic dinoflagellate, has been shown likewise to increase growth rates and produce more carbon-based karlotoxin under elevated levels of pCO2 (Fu et al., 2010). Although the dinoflagellate Karenia brevis, which can produce brevetoxins, maintains the same toxin production per cell after rising atmospheric CO₂ levels, it grows much faster and increases the likelihood of blooms. Thus, ocean acidification is expected to increase the toxicity of K. brevis blooms due to potential increases in bloom biomass yield (Errera et al., 2014, Hardison et al., 2014).

Toxic dinoflagellates within the genus Alexandrium, which cause the most widespread and dangerous HABs globally, can produce paralytic shellfish toxins (PSTs). These PSTs typically include various analogues of the following categories: (1) the carbamate compounds, including saxitoxin (STX), neosaxitoxin (NEO) and the C-11 O-sulfated gonyautoxin1–4 (GTX1, GTX2, GTX3 and GTX4); (2) N-sulfocarbamoyl analogues, which include the N- sulfocarbamoyl-II-hydroxy sulfate C toxins (C1, C2, C3 and C4), gonyautoxin 5 and 6 (GTX5 and GTX6); (3) the decarbamoyl derivates, such as decarbamoylsaxitoxin (dcSTX), decarbamoyl- neosaxitoxin (dcNEO), decarbamoylgonyautoxin 1–4 (dcGTX1, dcGTX2, dcGTX3 and dcGTX4). These compounds vary in toxic potency (Oshima, 1995) and can cause severe harm to humans consuming shellfish in polluted waters resulting in paralytic shellfish poisoning (Anderson et al., 2012). Thus, HABs of species from this genus pose a significant threat to ecological, economic, and public health issues in temperate and subarctic coastal areas worldwide (Shumway, 1990, Anderson et al., 2012). Therefore, understanding the physiological and ecological factors that control the timing and biogeography of toxic Alexandrium blooms is very important for ensuring seafood safety.

Members of the genus Alexandrium are particularly sensitive to pCO₂ concentrations (Flores-Moya et al., 2012, Tatters et al., 2013, Van De Waal et al., 2014). Several toxic Alexandrium species such as A. minutum (Flores-Moya et al., 2012), A. ostenfeldii (Kremp et al., 2012), A. catenella (Fu et al., 2012, Tatters et al., 2013) and A. fundyense (Hattenrath-Lehmann et al., 2015), have shown strain specific increases in growth and toxicity when exposed to different elevated pCO₂ concentrations.

A. tamarense is one of the most important species of toxic Alexandrium genus due to its global distribution. This species is also widespread in the East and South China Seas (Wang et al., 2006). A. tamarense has also been shown to have a high sensitivity to atmospheric CO₂ concentrations. Van De Waal et al. (2014) investigated the impact of elevated pCO₂ on PST content and composition from two strains of A. tamarense, called Alex 2 and Alex 5, isolated from the North Sea off the east coast of Scotland. These studies showed that elevated pCO₂ had minor consequences for growth and toxin production in both Alex 2 and Alex 5.

In the present study, the impact of elevated pCO₂ on PST production in one strain of A. tamarense, ATDH, isolated from the East China Sea was investigated. Results of these studies showed a significant response in growth and toxicity in response to elevated CO₂ levels. These studies may help researchers move toward a better understanding of how this harmful bloom species may respond to the predicted future elevation of atmospheric CO₂.