The Bloom-Forming Dinoflagellate Karenia mikimotoi Adopts Different Growth Modes When Exposed to Short or Long Period of Seawater Acidification.

Yuanyuan Li,Yanqun Wang,You Wang,Yijun Li,Zhengli Zhou,Bin Zhou,Keyu Lu,Mengxue Xu

doi:10.3390/toxins13090629

Abstract

Impacts of ocean acidification (OA) on noncalcifying organisms and the possibly responsible mechanism have aroused great research interests with the intensification of global warming. The present study focused on a noxious, noncalcifying, bloom-forming dinoflagellate, Karenia mikimotoi (K. mikimotoi), and its variation of growth patterns exposed to different periods of seawater acidification with stressing gradients was discussed. The dinoflagellates under short-time acidifying stress (2d) with different levels of CO2 presented significant growth inhibition (p < 0.05). The cell cycle was obviously inhibited at S phase, and the photosynthetic carbon fixation was also greatly suppressed (p < 0.05). Apoptosis was observed and the apoptotic rate increased with the increment of pCO2. Similar tendencies were observed in the key components of mitochondrial apoptotic pathway (the mitochondrial membrane potential (MMP), Caspase-3 and -9, and Bax/Bcl-2 ratio). However, under prolonged stressing time (8 d and 15 d), the growth of dinoflagellates was recovered or even stimulated, the photosynthetic carbon fixation was significantly increased (p < 0.05), the cell cycle of division presented little difference with those in the control, and no apoptosis was observed (p > 0.05). Besides, acidification adjusted by HCl addition and CO2 enrichment resulted in different growth performances, while the latter had a more negative impact. The results of present study indicated that (1) the short-time exposure to acidified seawater led to reduced growth performance via inducing apoptosis, blocking of cell cycle, and the alteration in photosynthetic carbon fixation. (2) K. mikimotoi had undergone adaptive changes under long-term exposure to CO2 induced seawater acidification. This further demonstrated that K. mikimotoi has strong adaptability in the face of seawater acidification, and this may be one of the reasons for the frequent outbreak of red tide. (3) Ions that dissociated by the dissolved CO2, instead of H+ itself, were more important for the impacts induced by the acidification. This work thus provides a new perspective and a possible explanation for the dominance of K. mikimotoi during the occurrence of HABs.

Highlights

Industrialization and fossil fuel combustion have increased the atmospheric CO2 concentration from a preindustrial level of approximately 280 ppmv to the current level of approximately 390 ppmv [1,2], and the excessive uptake of anthropogenicCO2 from the atmosphere subsequently results in ocean acidification (OA) [2,3]
The ecological impact of OA has aroused worldwide attention because it has been recognized as an important driver controlling the distribution, morphology, and biochemical performance of biological systems, which might result in changes in biodiversity, trophic interactions, and other ecosystem processes [6,7,8]
Cellular apoptosis of K. mikimotoi was induced by seawater acidification after 24 h of exposure, and the apoptotic rate greatly increased from 2.5% in the control to 28.65%

Summary

Introduction

Industrialization and fossil fuel combustion have increased the atmospheric CO2 concentration from a preindustrial level of approximately 280 ppmv (pH 8.2) to the current level of approximately 390 ppmv (pH 8.1) [1,2], and the excessive uptake of anthropogenicCO2 from the atmosphere subsequently results in ocean acidification (OA) [2,3]. The current prediction suggests the atmospheric CO2 concentrations would increase to 1000 ppmv and 2000 ppmv, respectively, by the years 2100 and 2300 if the present energy utilization structure persists [3], which is predicted to cause the decrease of ocean pH levels by as much as 0.3~0.4 units. Seawater acidification affects marine organisms through two pathways. It affects calcification rates by decreasing the calcium carbonate (CaCO3 ) saturation. This pathway usually occurs in calcifying organisms, and most studies have been devoted to these organisms [9,10,11,12,13]. Some reports showed that elevated CO2 levels inhibited the microalgal growth mainly by influencing inorganic carbon (Ci) acquisition in phytoplankton [17,18,19,20,21]

Methods

Discussion

Conclusion