Taxonomic re-evaluation of Ericiolus and Mercedesia (Prymnesiophyceae) and description of three new species

ABSTRACT The genera Ericiolus and Mercedesia are distinctive extant coccolithophores that are characterized by monothecate, monomorphic coccospheres with one type of triradiate star-shaped nannoliths. The two genera were described from the Danish coastal waters, the surface waters of the Arctic, and the Southern Oceans. During a study of samples from the low photic zone of the Mediterranean and Sargasso Seas, and from the subtropical gyres of the South-eastern Pacific and the South Atlantic Oceans, 44 collapsed coccospheres with triradiate star-like nannoliths were observed via scanning electron microscopy. Observations on the morphologies and biometric assessments of these specimens revealed that three distinct sets of nannoliths can be distinguished and that these were morphologically differentiated from all currently known species of Ericiolus and Mercedesia. The new forms and the previously described species of Ericiolus and Mercedesia were, however, similar, as they all demonstrated a distinctive set of collectively shared morphological characters and almost identical size ranges. On the basis of this, and instead of describing a third genus for the same group of nannoliths, we preferred to taxonomically synonymize Mercedesia with Ericiolus and revise the definition of Ericiolus. Therefore, we describe three new species, Ericiolus bendifii sp. nov., Ericiolus sheldoniae sp. nov. and Ericiolus mattioliae sp. nov., and an incompletely defined taxon, as Ericiolus cf. bendifii, and establish the new combinations E. aspiphorus comb. nov., E. multistellatus comb. nov. and E. pusillus comb. nov.


INTRODUCTION
The taxonomy of extant coccolithophores (Prymnesiophyceae, Haptophyta) has arguably been more comprehensively monographed and reviewed than that of any other phytoplankton (Jordan et al. 1995;Cros & Fortuño 2002;Jordan et al. 2004;Malinverno et al. 2008;Chang 2019) and most living coccolithophores can be readily identified to species-level with a high degree of confidence (Young et al. 2003;Jordan et al. 2004;Young et al. 2023). In addition, the isolation into culture of a diverse set of coccolithophores has enabled the revision of taxonomy through precise molecular sequencing and phylogenetic evaluations for diversity and evolutionary history (e.g. Sáez et al. 2004;Probert et al. 2007;Bendif et al. 2011;Edvardsen et al. 2011;Filatov et al. 2021;Bendif et al. 2023). Observations on coccolithophore life cycle associations, where coccoliths of different phases (diploid and haploid) and morphologies are found to co-occur (e.g. Cros et al. 2000;Keuter et al. 2021), have provided further valuable contributions to 1) understanding relationships of diverse morphotypes, 2) uniting taxa that were previously considered separate, and therefore, 3) resolving complex taxonomic and nomenclatural issues. All of the above-mentioned factors combined with their abundant fossil record make coccolithophores an ideal group for studying evolutionary patterns and processes in phytoplankton and the robust interpretation of their biodiversity patterns. Nonetheless, many species are yet to be successfully cultured, their life cycle is still unknown, and therefore, studies of coccolith morphology and ultrastructure, including identification of taxa in natural populations using microscopical techniques, remain prime tools for taxonomy.
An intriguing aspect of coccolithophore biodiversity is the presence of many seemingly rare but globally distributed species, especially in the low photic zone (LPZ). Many of these coccolithophores have only been informally proposed, but description of these and of entirely new forms has greatly progressed over recent years (e.g. Kleijne & Cros 2009;Andruleit & Young 2010;Young et al. 2014;Thomsen et al. 2016;Andruleit & Jordan 2017;Archontikis & Young 2021;Keuter et al. 2021), in which many of the practical nomenclatural issues faced by taxonomists have been resolved. Here, we use electron micrographs from our individual collections to describe further elements of this biota.
The genera Ericiolus H.A. Thomsen and Mercedesia H.A. Thomsen & J.B. Østergaard are minute (3-5 μm) planktonic protists that produce coccospheres consisting of numerous monomorphic, star-shaped nannoliths (Thomsen et al. 1995;. The cells bear two long similar, naked flagella and a haptonema, so they are unambiguously haptophytes. Ericiolus and Mercedesia were separated on the grounds of showing nannoliths with, respectively, four and three spine-like elements. In Ericiolus the nannoliths are formed of four radiate spine-like elements, three of them arranged broadly parallel to the cell surface, and the fourth perpendicular to it. Conversely, in Mercedesia the nannoliths are formed of only three units, all arranged parallel to the cell surface, forming a triangular or triradiate nannolith. The nannoliths are calcified, but lack the annular structure of heterococcoliths, i.e. instead of being formed by a ring of crystal units, they are made from radially arranged units. This feature separates them from other living coccolithophores, and so they are regarded as incertae sedis nannoliths (Young et al. 2003;Young et al. 2023 Thomsen & J.B. Østergaard) have been described. All five species were described from Danish waters and/or the surface waters of the Arctic and Antarctic, using transmission electron microscopy (Thomsen et al. 1995;. Using high-resolution scanning electron microscopy (SEM) we have imaged minuscule (c. 2-5 μm) and wellpreserved star-like nannolith specimens with morphologies comparable to Ericiolus and Mercedesia that were collected from samples originating in the LPZ of the Mediterranean and the Sargasso Seas, and the subtropical gyres of the Southeastern Pacific and the South Atlantic Oceans (Fig. 1). We report on observations and morphometric assessments of these specimens that suggest the circumscription of three new morphospecies and discuss their taxonomic implications for the genera Ericiolus and Mercedesia.

Sample collection and SEM analyses
The observed specimens originate from a collection of samples that are summarized as follows: 1) Two water samples collected from stations 69-6 (37°25.8ʹN, 0°25.2ʹW; 90 m depth) and 69-11 (37°25.8ʹN, 0°25.2ʹW; 42.5 m depth from the deep chlorophyll maximum (DCM)) in September-October 1999 on cruise MATER II of the R/V Hesperides in the North-western Mediterranean and Alboran Seas (Font 1999); 2) A pair of samples from the low photic waters of stations CTD184 ( Table 1. On each cruise, sampling was performed using Niskin Bottles attached to rosette samplers. Seawater samples were filtered using three types of filters: 1) Whatman membrane track-etched filters (0.8 μm porosity, 25 mm diameter); 2) Isopore hydrophilic, nonsterile membranes (0.8 μm porosity, 47 mm diameter); and 3) Pall Life Sciences Supor-800 filter membranes (0.8 μm porosity, 25 mm diameter). During BATS-BIOS cruise, samples were pre-filtered through a membrane of 50 μm porosity to remove larger zooplankton and contaminants. A low-pressure vacuum pump was used in all expeditions  Thomsen et al. (1995) and for all other previously published species from this study and/or .  to avoid breakage of coccosphere specimens. The filters were then washed with either buffered distilled water (adjusted with NaOH; pH c. 8.0) or a 20 mM sodium carbonate solution (Na 2 CO 3 , 2 g l -1 ; pH c. 10) to remove salt, placed individually in plastic Millipore Petri-dishes, and oven-dried at 40°C for at least an hour. A portion of each filter was subsequently cut out and mounted on an aluminium stub using double-sided adhesive tape. The stubs were then sputter-coated with gold-palladium for 1-2 min. Observations and imaging were conducted at the Natural History Museum London (NHM), UK, on a Phillips XL-30 FEG Field Emission SEM or a Zeiss Ultra Plus Field Emission SEM, and on a VEGA3 TESCAN SEM at the Department of Earth and Environmental Systems, Indiana State University, USA.

Morphology, terminology and biometry
Terminology and morphological observations largely followed the guidelines of  and the approaches of  on starshaped nannoliths. Morphometric measurements were conducted via ImageJ software (Schneider et al. 2012) and frequency plots were produced using the R package ggplot2 (Wickham 2009).

Morphological observations
Our examination of SEM micrographs yielded, in total, 44 collapsed coccospheres each consisting of numerous monomorphic triradiate nannoliths. The nannoliths showed three bifurcate rays in plan view, all equally positioned and angled, with tapering ray-tips. In side view, the nannoliths had a central process on one side, while their ray-tips were deflected upward. The nannoliths around the edge of collapsed coccospheres typically showed their process oriented outward, making clear that the side with the process is distal. On the proximal side the nannoliths appeared flat, but with a small central pore. Through our analyses, it also became apparent that there is considerable variability in our specimens, as these differed consistently in the ultrastructure of their nannolith centre and the degree of bifurcation. We distinguished three sets of specimens (Figs 2, 3, 4): (a) nannoliths with the bifurcation occurring near the centre of the nannoliths, and with a long and elongate central spine with a terminal knob (Fig. 2); (b) nannoliths with a similar bifurcation to the first, but with, instead of a spine, a calyx of three vertically-oriented laths, extending between the bifurcations (Fig. 3); and (c) nannoliths showing bifurcation near the tip of their rays, and a calyx formed by delicate laths (Fig. 4); the laths, however, were seen to develop along the rays or across their axis, and they extended from the nannolith centre to approximately one-quarter from the tip of the ray, as in (b). In addition, we observed a limited number of specimens with morphologies similar to (a), bearing, however, a short central spine on the distal side. Based on these morphological differences, we propose three new species, namely Ericiolus bendifii sp. nov., E. sheldoniae sp. nov. and E. mattioliae sp. nov., and describe an incompletely defined taxon under the designation of Ericiolus cf. bendifii.

Biometric analyses
To test for morphometric differences between our new forms and the previously published species of Ericiolus and Mercedesia, we carried out biometric measurements on 728 star-shaped nannoliths originating from specimens from our collections and/or other available sources (Thomsen et al. 1995;; Table S1). To minimize subjectivity in the resulting dataset, we measured only nannoliths in plan view, and given their three-fold dimension, we considered the distance between the tips of the rays (Figs 5, 6) as representative of the nannolith size/diameter. The frequency histogram plots (Fig. 7) highlighted that all species exhibited a (broadly) unimodal distribution pattern of their nannolith size, with our new forms, E. bendifii sp. nov., E. sheldoniae sp. nov., E. mattioliae sp. nov., and Ericiolus cf. bendifii, demonstrating, respectively, a cluster at 0.30-0.60 μm (mean value 0.45 μm), 0.35-0.80 μm (mean value 0.55 μm), 0.60-1.30 μm (mean value 0.80 μm) and 0.25-0.55 μm (mean value 0.35 μm). The species E. spiculiger and E. frigidus showed an obvious clustering at, respectively, 0.35-0.55 μm and 0.25-0.55 μm (both with a mean value of 0.40 μm). Similarly, the species E. pusillus comb. nov., E. multistellatus comb. nov. and E. aspiphorus comb. nov. (see below for taxonomic discussion and treatments) displayed, respectively, clusters at 0.18-0.34 μm (mean value 0.22 μm), 0.25-0.80 μm (mean value 0.40 μm) and 0.35-0.85 μm (mean value 0.50 μm). We thus observed that all nannolith morphotypes fell within the size range 0.2-1.3 µm with the individual morphotypes showing more restricted, but overlapping, size ranges.

Synonymization of Mercedesia with Ericiolus
As highlighted from our morphologic and biometric analyses, our new species and the genera Ericiolus and Mercedesia demonstrate numerous collectively shared features (see Table  2). They are all characterized by coccospheres 2-7 µm in size with numerous monomorphic nannoliths that are triradiate and show three symmetrically arranged, similar rays radiating from a common centre. The nannoliths vary 1) in the presence and development of the central process; 2) in whether the rays are coplanar or arranged in a low angle cone; 3) in the presence or absence of bifurcations; and 4) in details of size and shape. These common features make this a distinctive group that is well-separated from all other extant coccolithophores. As noted above, Ericiolus and Mercedesia differ in the ray angle and the process development, i.e. in Ericiolus the process is welldeveloped and the rays are arranged conically, whilst in Mercedesia the rays are coplanar, and the process is absent or rudimentary. Our specimens have coplanar rays and a welldeveloped process, and do not, therefore, readily fit in either genus. Consequently, we could either emend the generic definitions to accommodate our species, or alternatively, create a third genus exclusively for the new forms that are distinguished by the presence of bifurcations. However, we consider the similarities amongst the taxa more important than their differences and that proliferation of paucispecific genera being separated on minor differences in nannolith morphology is unhelpful. Based on the above, we prefer to place all forms in a single genus, Ericiolus, with an emended diagnosis.  Thomsen et al. (1995). REMARKS: All known species form small monomorphic coccospheres (2-7 µm) consisting of numerous very small (<1.3 µm) nannoliths. All polar species are known to be motile with two flagella and a well-developed haptonema; equivalent observations are not available for the subtropical LPZ species.  fig. 14).

Ericiolus bendifii
Figs 2-4. Schematic representation of the diagnostic morphological features of nannoliths of the newly described species of Ericiolus as observed in proximal, distal and side views. Drawings are not to the same scale. Fig. 2. Ericiolus bendifii sp. nov. Fig. 3. Ericiolus mattioliae sp. nov. Fig. 4. Ericiolus sheldoniae sp. nov.
DESCRIPTION: Coccosphere shape unknown, possibly subspherical or saddle-shaped but found collapsed. It consists of c. 50-70 triradiate, star-shaped nannoliths probably forming a single-layered cover. Nannoliths with three coplanar bifurcate rays. The rays are symmetrically arranged (120° angle between them) and bifurcate near the centre of the nannolith with the bifurcations directed radially. The bifurcations are parallel-sided, and tips of each bifurcation are directed distally. On the distal side of the nannolith, there is an elongate central spine with a robust terminal knob. On the proximal side of the nannolith, axial grooves run along the rays from the bifurcations to the centre where they meet to form a small central pore (Fig. 12 Young et al. (2003) previously reported the holotype when examining plankton assemblages of Mediterranean waters and identified via SEM the possession of bifurcate rays in its nannoliths. The authors argued that this specimen (their p. 85, pl. 39, fig. 14, labelled as 'Ericiolus? sp.') was reminiscent of Ericiolus, and although its morphology was incompatible with the generic description, the form probably represented a closely related taxon. This is now confirmed via our morphologic and biometric findings and therefore, it is established as a discrete morphospecies. The species differs in possessing nannoliths with three bifurcated rays, vertically directed ray-tips and an elongate central spine with a terminal knob. The bifurcation occurs near the nannolith centre.

Ericiolus mattioliae Archontikis, J.G. Millán, A. Winter & Jer.R. Young sp. nov.
Figs 16-20 DESCRIPTION: Coccosphere shape unknown, possibly subspherical or saddle-shaped but found collapsed, and with 30-60 nannoliths. Nannoliths with three coplanar rays, equally positioned, with delicate bifurcations near their tips; the bifurcations are short and have short, upturned tips. On the distal side, laths extend vertically up from each ray, developing from the centre of the nannolith to about two-thirds of the length of the ray. The laths are slightly higher at the centre giving a curved profile, and are slightly thickened along their distal edge. On the proximal side of the nannolith, grooves run along the rays from the bifurcation to the centre of the nannolith, where they unite to form a small central pore (Fig. 18,  REMARKS: The species differs in showing three coplanar rays with a small bifurcation occurring near the tips of the rays. In addition, the nannoliths bear delicate laths that form a calyx distally and they develop from the nannolith central spine towards the tips of the rays by becoming progressively shorter. The laths are also seen to develop along the nannolith rays, across their axis. Figs 5-6. Biometric approach for quantifying nannolith size. Given the three-fold dimension of nannoliths, we considered as nannolith size/diameter, 1) the distance between the tips of the diametrically opposite rays (Fig. 5) for Ericiolus bendifii sp. nov. and E. sheldoniae sp. nov.; and 2) the 'tip-to-tip' distance of the nannolith rays ( Fig. 6) for all other species. Scale bars = 0.5 μm. Fig. 5. Distance between the tips of diametrically opposite rays, used for Ericiolus bendifii sp. nov. and E. sheldoniae sp. nov. Fig. 6. 'Tip-to-tip' distance of the nannolith rays.

Ericiolus sheldoniae Archontikis, J.G. Millán, A. Winter & Jer. R. Young sp. nov.
Figs 21-29 DESCRIPTION: Coccosphere shape unknown, possibly saddle-shaped or sub-spherical but seen collapsed. It shows 30-60 star-shaped triradiate nannoliths with three coplanar bifurcate rays, equally positioned and angled. The rays are symmetrically arranged (120° angle between them) and bifurcate near the centre of the nannolith. The bifurcations are parallel-sided and directed radially, and show upturned tips that are directed distally. On the distal side of the nannolith, laths extend vertically down from the centre of the nannolith, in between the rays (Fig. 23) and to about one-quarter from their tips. The laths form a calyx that is slightly thickened along its distal edge. On the proximal side of the nannolith, axial grooves run along the rays from the bifurcation to the nannolith centre where they unite to form a small central pore (Fig. 22); laths are usually seen collapsed in proximal view.  Fig. 8. Holotype. Coccosphere with star-shaped nannoliths showing three bifurcate rays and a central spine (arrow) with a robust terminal knob on distal side. Fig. 9. Paratype. Central spine with a terminal knob at the centre of the nannolith and ray tips deflected upwards (arrow). Fig. 10. Collapsed monomorphic coccosphere with star-shaped nannoliths. Fig. 11. Nannoliths with three coplanar rays showing bifurcation near their centre (arrow). Fig. 12. Detail of star-shaped nannoliths in proximal view showing a central pore (arrow) made by axial grooves that run along the rays.

Ecology
Very little information can currently be extracted in relation to the ecology of Ericiolus bendifii sp. nov., E. sheldoniae sp. nov., Figs 16-20. SEM micrographs of Ericiolus mattioliae sp. nov. Scale bars = 1 μm. Fig. 16. Holotype. Coccosphere with star-shaped nannoliths showing three rays with small bifurcation at their tips (arrows). Fig. 17. Paratype. Coccosphere with nannoliths showing, in distal view, delicate laths that develop from approximately one-quarter from the tip of the ray (arrow; side view) towards the nannolith centre, vertically up. Fig. 18. Detailed view of star-shaped nannoliths of the specimen shown in Fig. 17. Proximal nannolith side with a central pore made by axial grooves (upper arrow); distal nannolith side showing a star-shaped calyx reaching approximately one-quarter of the tip of the ray (lower arrow). Fig. 19. Collapsed individual nannoliths with bifurcation at the tip of the rays. Arrow indicates the lath-made calyx in side view. Fig. 20. Collapsed coccosphere specimen. Nannoliths bearing delicate laths that become progressively higher towards the nannolith centre, forming a calyx. The elements of the calyx appear blockier at its distal end (arrow).
E. mattioliae sp. nov. and Ericiolus cf. bendifii, other than that our specimens have been obtained from the LPZ layers (within or close to the DCM) of subtropical to tropical waters. Our records, however, expand the findings of Thomsen et al. (1995) and , who noted occurrences of star-like nannolith-bearing species in the surface waters (10-40 m) of high latitude environments, namely the Arctic, Danish, Antarctic and Weddell Seas.

DISCUSSION
The genus Ericiolus (including now species previously classified in Mercedesia) is a minuscule extant coccolithophore that accommodates saddle-shaped, ovoid and/or sub-spherical monothecate monomorphic coccospheres with one type of nannoliths showing three-fold symmetry. The nannoliths are remarkably small (0.2-1.3 μm) in size and they show a star-like shape. They are convex on distal side and flat or concave in proximal view, and they demonstrate a peculiar morphology and ultrastructure, predominantly composed of an assembly of rays (Thomsen et al. 1995;our observations). The rays overlie an organic baseplate, are almost equal in length and width, and they are seen almost equally positioned and angled across the nannolith. They develop from the centre of the nannolith distally by becoming progressively thinner with noticeably tapering ends. In addition, the nannolith rays may or may not show bifurcation; this typically occurs either near the nannolith centre or close to the tips of the rays. The ray base is narrow and usually bears a central pore in proximal view (e.g. Thomsen & Østergaard 2015, p. 158, fig. 3;our Figs 12, 18, 22). The nannolith centre may vary in showing a central structure with either: 1) a narrow and almost flat surface (e.g. E. spiculiger,

Taxonomic affinities
Ericiolus is currently classified within the informal category 'nannolith incertae sedis' (Young et al. 2003

ALISPHAERACEAE -POLYCRATER PHASE
The Alisphaeraceae are a group of heterococcoliths, which, in their alternate life cycle phase, produce aragonitic nannoliths that were formerly placed in the genus Polycrater Manton & Oates (Cros et al. 2000;Cros & Fortuño 2002;Šupraha et al. 2018). Well-developed nannoliths of the 'Polycrater' phase are monomorphic and form monothecate coccospheres but, unlike Ericiolus, they are typically asymmetrical and four-sided, i.e. they are seen as quadrate in plan view and with an hour-glass shape in side view. The nannolith long axes are directed equatorially, and the nannoliths are seen to embrace the cell in regular meridian rows. The size of the nannoliths (c. 0.4-1.0 μm) is similar to that observed in Ericiolus. A range of different analogues (e.g. forms with holes, tubercles, ladle-like or petal-like forms) to the ray structures of Ericiolus have previously been documented (Cros & Fortuño 2002;Young et al. 2003), and these also partially resemble the more skeletal Polycrater-type species; however, the basic four-fold symmetry seems to be a significant difference between these and the Ericiolus forms. In addition, it is still unclear whether Ericiolus nannoliths are calcitic or aragonitic, although the presence of calcium in the periplast has been confirmed (Thomsen et al. 1988). Therefore, it would be useful to determine if Ericiolus nannoliths were formed of aragonite or calcite and, if aragonitic, then affinity with Polycrater nannoliths would be indicated.

PAPPOSPHAERACEAE
The family Papposphaeraceae includes a wide range of small coccolithophores, which, like Ericiolus, occur sporadically in both polar waters and in the LPZ of the tropics and subtropics. Indeed, our key samples with common Ericiolus specimens also contained various members of the Papposphaeraceae. However, unlike Ericiolus, this group is characterized by predominantly dimorphic coccospheres with unambiguous heterococcoliths that are both made by R-and V-units, not monomorphic nannoliths. The heterococcoliths predominantly show four-fold symmetry and display narrow murolith rims (see Andruleit & Young 2010 for extended discussion), whereas Ericiolus is characterized by a three-fold symmetry pattern and a rim is absent; instead, a narrow ray base is always seen. Therefore, affinity with the Papposphaeraceae seems unlikely.

PILEOLOSPHAERA
The extant nannolith-bearing species Pileolosphaera longistirpes K.J.S. Meier, Kinkel & Jer.R. Young, which is the type species of the genus Pileolosphaera, is composed of about six to eight circular shield-like nannoliths. The nannoliths are formed exclusively of V-units (Meier et al. 2014) showing on the coccosphere, radially oriented calcite axes under light microscopy (Meier et al. 2014). The nannoliths possess three radial segments with a central triradiate process. Light microscopy observations indicate that all three elements are formed of crystal units with their c-axes vertical relative to the nannolith, and therefore, radial relative to the coccosphere (Meier et al. 2014). Pileolosphaera shares with Ericiolus the key features of triradiate symmetry and radial growth of the elements from the centre, rather than around a rim. There are, however, notable differences between the two taxa: 1) Pileolosphaera coccospheres have far fewer nannoliths than those of Ericiolus (6-8 vs 30-70); and 2) the Pileolosphaera nannoliths are considerably larger than those of Ericiolus (3.0-4.0 μm vs 0.2-1.3 μm). Despite these quantitative differences, Pileolosphaera nannoliths are structurally the closest extant forms to Ericiolus, and affinity between the two genera remains likely.

DISCOASTER
Discoaster is an important genus of nannofossils that were abundant through most of the Cenozoic, especially in warm oligotrophic waters. The last two species, D. brouweri S.H. Tan and D. triradiatus S.H. Tan went extinct 1.8 million years ago, in the Early Quaternary. Like Ericiolus, Discoaster nannoliths have radial symmetry, being formed of rays that grow from a central axis, rather than a proto-coccolith ring as in heterococcoliths (Young et al. 1999 Meier et al. (2014) noted that the crystallographic origin of the elements in Pileolosphaera was the same as that in Discoaster, with the c-axes vertical/ parallel to the axis of rotational symmetry of the nannolith. Unfortunately, Ericiolus is too small to determine crystallographic orientation optically and we do not have suitable samples for X-ray diffraction studies. Nonetheless, this type of analyses would be an interesting test, as would molecular genetics, to confirm whether Ericiolus is instead related to another group of extant coccolithophores.

Concluding Remarks
In summary, our findings showed that Ericiolus is a more diverse genus than was previously known and includes at least eight morphologically different species. The radial construction with three-fold symmetry separates the genus from other extant coccolithophores except the equally poorly known species Pileolosphaera longistirpes. In parallel, this construction pattern suggests a possible origin from the otherwise extinct Discoasterales, and therefore the possibility that these species may prove valuable keys to past biodiversity.
cataloguing our material at, respectively, the Natural History Museum London, UK, and the U.S. National Herbarium of the Smithsonian Institute, and Luc Beaufort for providing environmental data for the BIOSOPE samples. Helge A. Thomsen, an anonymous reviewer and the Editor-in-Chief, António J. Calado, are warmly acknowledged for their insightful suggestions on the article.

DISCLOSURE STATEMENT
No potential conflict of interest was reported by the authors.