Morpho-molecular description of a new HAB species, Pseudocochlodinium profundisulcus gen. et sp. nov., and its LSU rRNA gene based genetic diversity and geographical distribution

Highlights

• A species identified as Cochlodinium geminatum and Polykrikos geminatus reclassified.

• The species was re-described as Pseudocochlodinium profundisulcus gen. et sp. nov.

• Redescription based on characteristic ASC, cingulum, sulcus and molecular phylogeny.

• DNA metabarcoding found a wide distribution of the species in the coastal sediments of China.

• Analyzed on rRNA gene sequences suggest a possible S-N expansion or warm water preference.

Abstract

Harmful algal blooms (HABs) caused by an unknown dinoflagellate species have frequently occurred in the Pearl River Estuary, China Since 2006. These blooms were associated with severe water discoloration and economic losses, ranging from several km² to 300 km² with the maximum recorded cell density being 2.77 × 10⁷ cells⋅L⁻¹. This unknown dinoflagellate species was initially identified as Cochlodinium geminatum and subsequently reclassified as Polykrikos geminatus. However, after reviewing the original descriptions for Cochlodinium geminatum sensu Schütt (1895) and the genus Polykrikos, we considered this species is incongruent with their original descriptions. Further morphological examinations and particularly phylogenetic analyses based on the SSU and partial LSU rRNA genes of isolates and resting cysts from China and Japan prompted us to consider it a new species of a new genus. This new species was proposed to be Pseudocochlodinium profundisulcus gen. et sp. nov., based on its open comma-shaped apical structure complex (ASC), cingulum encircling the cell less than one and a half turns, a deep sulcus with a torsion of a half turn, either single cell or cell chain consisting of two cells with the same number of nuclei and zooids, the resting cyst bearing lobed ornaments, and the evolutionary distances from Polykrikos (and others) on the phylogenetic trees constructed using the concatenated SSU and partial LSU rRNA gene sequences. Metabarcoding investigation of surface sediment samples collected in China revealed that the species to be widely present along the entire Chinese coast with the highest abundance in the South China Sea. Further re-analysis of the Tara Oceans metabarcoding dataset targeting the SSU rRNA gene V9 domain suggested a global distribution of this new genus. Phylogenetic analyses on 46 OTUs (average length: ∼552 bases) of its LSU rRNA gene sequences (mainly D1-D2 domains) obtained from surface sediment samples revealed intraspecific genetic diversity of this species. Interestingly, based on the different distributions and the abundance of these OTUs along the coast of China, this species appeared to have expanded its distribution from the South China Sea to the northern Yellow Sea, or preferred a warm water habitat. We consider that the present work improves the taxonomy and provides important insights into the biogeography of Pseudocochlodinium profundisulcus.⋅

Introduction

Harmful algal blooms (HABs) have become a worldwide ecological problem, which have been threatening seafood safety, aquaculture, and ecosystem health (Zhou, 2010; Anderson et al., 2012; Dia et al., 2014; Glibert, 2014; Furuya et al., 2018; Sakamoto et al., 2021;). In the coastal waters of China, the frequency, intensity, duration, and particularly the economic losses of HABs have increasingly been reported over the past three decades (Wang and Wu, 2009; Zhou, 2010; Lu et al., 2014; Guo et al., 2015; Song et al., 2016; Sakamoto et al., 2021; Gu et al., 2021; Li et al., 2021; Tang et al., 2021). Specifically in the South China Sea, many HABs of different scales and species have been recorded in the Pearl River Estuary and its adjacent areas (Wei et al., 2012; Lu et al., 2014; Yu et al., 2018). In April of 2006, a bloom caused by an unknown dinoflagellate species occurred in the coastal water of Zhuhai, Guangdong province, and in September and October of the same year, two other blooms of the same species occurred in the same areas, with a cell density reaching 1.6 × 10⁶ cells⋅L⁻¹ (Ou et al., 2010). During September and October of 2009, a large-scale bloom of the same species broke out in the Pearl River Estuary, with a peak cell density of 4.13 × 10⁷ cells⋅L⁻¹ and a coverage of area about 300 km² (Ke et al., 2012; Pang et al., 2015). A small-scale bloom (89 km²) caused by this species in August, 2011 caused mortalities of larval and juvenile fish (round spadefish: Ephippus orbis and yellow grouper: Epinephelus awoara) and an economic loss of about 3.16 million Chinese Yuan (∼0.52 million US dollars; Bulletin of Marine Environment Status of Guangdong province, 2011; Guo et al., 2014). More blooms of this species, at relatively smaller scales, were subsequently recorded in the areas in 2012, 2013, and 2018 (Bulletin of Marine Environment Status of Guangdong province, 2012, 2013; Dong et al., 2020).

The organism responsible for the abovementioned HABs was initially identified as Cochlodinium geminatum (Schütt) Schütt based on morphological features including two-celled chains, the absence of an eyespot, a cingular displacement of about one and a half its width, and a deep sulcus (Qi et al., 2009). This same species was independently named as Cochlodinium sp. (Wang et al., 2011). It was subsequently transferred to the genus Polykrikos as Po. geminatus (Schütt) Qiu et Lin based on its shape of apical groove and molecular phylogeny (Qiu et al., 2013). Nevertheless, the name C. geminatum is still being used in literature (Liu et al., 2014; Pang et al., 2015; Yan et al., 2019; Dong et al., 2020).

This species has been regarded as a HABs species and drawn great attention from the Chinese and Japanese HAB research community during the past two decades. Lan and Gu (2014) demonstrated that Po. geminatus can produce resting cysts, through germination of cysts isolated from the South China Sea and the Bohai Sea, China. This organism has been reported to have adverse impacts on phytoplankton species and be toxic to Artemia salina and juvenile fish (Wu et al., 2013; Guo et al., 2014; Yan et al., 2019; Dong et al., 2020; Wu et al., 2021). In addition, it has been found to produce retinal, a potential teratogenic agent, during blooms, eliciting a potential risk to aquatic organisms (Dong et al., 2020, as C. geminatum).

Although C. geminatum has been reclassified as Po. geminatus, the dispute concerning its taxonomic status is still existing. Indeed, there are still significant discrepancies in morphology, diagnosis and phylogenetic positions between accepted Polykrikos species and the species of concern. In this study, we re-describe this organism as a new genus and species, Pseudocochlodinium profundisulcus gen. et sp. nov., based on detailed morphological observations and phylogenetic analyses using the concatenated SSU and partial LSU rRNA gene sequences of strains and cysts collected in China and Japan, and review of the original descriptions of C. geminatum sensu Schütt and Polykrikos. We also carried out metabarcoding investigation on surface sediment samples collected from the China Seas (South China Sea, East China Sea, Yellow Sea, and Bohai Sea), and analyzed the genetic diversity and geographical distribution of the species around China. Additionally, we constructed a global geographical distribution of this species (or Pseudocochlodinium sp.) using published records and re-analyzed the Tara Oceans metabarcoding dataset targeting the V9 domain of SSU rRNA gene of de Vargas et al. (2015). We consider that the present work improves the taxonomy and provides important insights into the biogeography of this species.