Sterols of Testudodinium testudo (formerly Amphidinium testudo): Production of the Δ⁸⁽¹⁴⁾ sterol gymnodinosterol and chemotaxonomic relationship to the Kareniaceae

Abstract

Testudodinium testudo is a peridinin‐containing dinoflagellate recently renamed from Amphidinium testudo. While T. testudo has been shown via phylogenetic analysis of small subunit ribosomal RNA genes to reside in a clade separate from the genus Amphidinium, it does possess morphological features similar to Amphidinium sensu stricto. Previous studies of Amphidinium carterae and Amphidinium corpulentum have found the sterols to be enriched in Δ8(14) sterols, such as 4α‐methyl‐5α‐ergosta‐8(14),24(28)‐dien‐3β‐ol (amphisterol), uncommon to most other dinoflagellate taxa and thus considered possible biomarkers for the genus Amphidinium. Here, we provide an examination of the sterols of T. testudo and show they are dominated not by amphisterol, but rather by a different Δ8(14) sterol, (24R)‐4α‐methyl‐5α‐ergosta‐8(14),22‐dien‐3β‐ol (gymnodinosterol), previously thought to be a major sterol only within the Kareniaceae genera Karenia, Karlodinium, and Takayama. Also found to be present at low levels were 4α‐methyl‐5α‐ergosta‐8,14,22‐trien‐3β‐ol, a sterol previously observed in Karenia brevis to be an intermediate in the production of gymnodinosterol, and cholesterol, a sterol common to many other dinoflagellates. The presence of gymnodinosterol in T. testudo is the first report of this sterol as the sole major sterol in a dinoflagellate outside of the Kareniaceae. The implication of this chemotaxonomic relationship to the Kareniaceae is discussed.

Sterols of morphologically distinct, okadaic acid‐producing Prorocentrum texanum var. texanum and var. cuspidatum isolated from the Gulf of Mexico

“…The balance of evidence of several studies on dinoflagellate sterol compositions over the last couple of decades indicates that, in general, dinoflagellates with similar phylogenies produce similar sets of sterols, thus reinforcing the conclusions of Leblond et al (2010). In some cases this has yielded surprises, such as the dinoflagellate T. testudo , which is phylogenetically close to members of the Kareniacae (Horiguchi et al, 2012), being found to produce one of the major sterols common to these dinoflagellates (Leblond et al, 2022). This being said, dinoflagellates are evolutionarily complex organisms with unknown twists and turns in their evolution, and there will no doubt be surprises in sterol composition and/or other phenotypic traits currently considered to be consistent with phylogenetic placements.…”

“…As a class, many dinoflagellates have been observed to produce dozens of sterols, including 4‐desmethyl sterols like cholesterol (cholest‐5‐en‐3β‐ol) and 4α‐methyl‐substituted sterols such as dinosterol (4α,23,24‐trimethyl‐5α‐cholest‐22E‐en‐3β‐ol; Leblond et al, 2010; Volkman, 1986, 2003; Volkman et al, 1998). As evidenced by the first review by Volkman (1986) and continuing to the present with the latest studies (Leblond et al, 2022), dinoflagellates have had their sterols characterized by many research groups over many years to elucidate their rich variety of sterols, which are arguably unparalleled in their diversity when compared to other algal groups.…”

Sterols of Amphidinium species in the Operculatum Clade: Predominance of cholesterol instead of Δ8(14) sterols previously considered Amphidinium‐specific biomarkers

“…The genus Amphidinium is indeed large and diverse [see discussion in Leblond et al, 2022], and it should not be a surprise that there is no sterol(s) that amply covers the entire genus as a possible major sterol biomarker. Indeed, these results demonstrate that caution should be used when considering Δ 8(14) sterols, such 4α‐methyl‐5α‐cholest‐8(14)‐en‐3β‐ol and 4α,23,24‐trimethyl‐5α‐cholest‐8(14),22E‐dien‐3β‐ol, as Amphidinium ‐wide major sterol biomarkers because of their lesser abundance (compared with previously determined Δ 8(14) sterol distributions in A. carterae and A. corpulentum ).…”

“…As mentioned in the recent study of the sterols of T. testudo by Leblond et al (2022), there have been two known exceptions to Δ 8( 14) nuclear unsaturation as a major sterol biomarker trait in sterols of Amphidinium species. Specifically, photosynthetic Amphidinium massartii Biecheler and heterotrophic Amphidinium longum Lohmann (Chu et al, 2009;Leblond et al, 2010) have been observed to produce cholesterol as a major sterol instead of the Δ 8( 14) sterols mentioned above-in fact, A. massartii was observed to cluster with cholesterol-and dinosterol-producing dinoflagellates rather than with A. carterae and A. corpulentum (Leblond et al, 2010).…”

“…They were thus considered sterol biomarkers of the Kareniaceae that drove formation of a Karenia / Karlodinium cluster (Leblond et al, 2010). More recently, however, gymnodinosterol has been identified as the sole major sterol in Testudodinium testudo (Herdman) Horiguchi, Tamura, Katsumata, et A. Yamaguchi (Leblond et al, 2022), which is phylogenetically close to members of the Kareniaceae but morphologically similar to the genus Amphidinium (Horiguchi et al, 2012). This new discovery calls into question the ability of gymnodinosterol (and possibly brevesterol) to serve as a Kareniaceae‐specific sterol biomarker and is one example of how new discoveries await in the field of dinoflagellate sterol biology.…”