Fragilaria longwania sp. nov. (Bacillariophyceae), a New Araphid Diatom from Sihailongwan Maar Lake, Northeastern China

1. Introduction

In the past few years, considerable progress has been made on the revision of the genus Fragilaria Lyngbye (1819: 185)[1], with the reanalysis of historic samples and type materials. These analyses have provided a better understanding for some of the most reported species belonging to Fragilaria as well as the description of numerous new species (e.g., [2,3,4,5]).

By contrast, few new species have been described from material collected in China. According to the list compiled by Kociolek et al. [6], only one new species of Fragilaria has been described from China between 2000 and 2019 (i.e. Fragilaria crenophila var. sinensis Rioual [7]). Since the publication of this list, Fragilaria huebeneri A. Schwarz, KJ Krahn et CE Wetzel was described from material collected from Lake Namco, on the Tibetan Plateau [8]. When compared with the large number of new species belonging to Ulnaria, a genus closely related to Fragilaria, that were recently described from China (e.g., [9]), it is very likely that the diversity of the genus Fragilaria in China has been understudied.

As part of an ongoing research project on the modern and down-core diatom assemblages of Sihailongwan Maar Lake (Northeast China), a species of Fragilaria, that was found mostly in sediment trap samples, could not be identified using the currently available literature. On the basis of observations made using light microscopy (LM) and scanning electron microscopy (SEM) and after comparisons with known species of the genus, this Fragilaria is described as new-to-science.

2. Materials and Methods

2.1. Study Site

The study site, Sihailongwan Maar Lake, is located in the Long Gang volcanic field Jilin Province, in Northeast China (42°17’N, 126°36’E, 790 m a.s.l., Figure 1). This lake has is a typical maar lake, i.e. a volcanic lake formed by a phreatomagmatic explosion. Its morphology is therefore shaped like a bowl, with steep rocky slopes and a flat bottom, a relatively small surface area (0.5 km²) but a deep water depth (maximum 50 m). Detailed information on the geology, recent climate and vegetation of the region around the lake as well as on the lake limnology can be found in previous studies on this lake [10,11,12].

As shown by the analyses on short cores and trap material, the sediment sequence of this lake is composed of seasonal laminations, or varves, which correspond to the alternation of allogenic atmospheric dust inputs that correspond to the winter/spring season and authigenic material, mainly composed of algal remains including diatoms, deposited during the spring, summer and autumn seasons [13,14]. The high quality of its sediment sequence, that store a record of environmental and human impacts at a continental and global scale over the past millennia [11], made this site particularly suitable for palaeolimnological investigations. Considering these characteristics, Lake Sihailongwan was selected as a candidate site to become a Global boundary Stratotype Section and Point (GSSP) for the demarcation of the Anthropocene [15].

2.2. Limnology of Lake Sihailongwan

Between August 2020 and November 2014, chemical variables such as pH, alkalinity (Gran titration), electrical conductivity and the concentrations for the main cations, anions and nutrients were measured on water samples collected at the same time as sediment trap samples were retrieved. On a few occasions, the depth of the photic zone was assessed using a Secchi disk. Details on the water chemistry methods that were used to measure these variables are provided in Rioual et al. [16] and a summary of the results is given in Table 1.

2.3. Diatom Sampling and Sample Preparation

Material for diatom analysis was obtained from sediment traps deployed at the center of Lake Sihailongwan between May 2003 and July 2017. The mooring was designed so that the traps collected all material, organic and inorganic, that settle into the lake water column but that the content of the trap would not be affected by possible resuspension from the bottom sediment. Samples were collected monthly between 2002 and 2005 and bimonthly from 2006. In total 187 sediment trap samples were collected and analyzed for this study. In addition to the trap samples, were also analyzed three periphyton samples scrapped from the surface of the buoy that maintain the trap mooring in the water column, two surface sediment samples (the top ~1cm, representing recent epipelic assemblage as well as several years of sedimentation), that were collected from the middle of the lake (~50 m water depth) on the 21^st October 2005 and 14^th August 2007, and a series of benthic samples (epilithon, epiphyton, epipelon and epixylon) collected in the littoral zone of the lake at different times during this study. Details of these samples are given in Table 3. The purpose of analyzing these additional samples was to determine the preferred habitat of the new species.

Diatom samples were prepared according to the standard procedure described in Battarbee et al. [17] using hydrogen peroxide. Slides for light microscopy were prepared by mounting the cleaned suspension in Naphrax®. LM observations were made at ×1000 magnification using a Leica DM 2500 equipped with immersion objectives (N Plan, 1.25 numerical aperture) and differential interference contrast (DIC). Photomicrographs were captured using a Leica DFC450 camera (Leica Microsystems Ltd). For scanning electron microscope analyses, aliquots of cleaned suspensions were filtered and rinsed through a 3-µm Isopore™ polycarbonate membrane (Merck, Darmstadt, Germany). The membrane filters were the mounted on aluminum stubs using double-coated carbon conductive adhesive tape and coated with gold using a Quorum Q150R S Plus sputter coater (Quorum Technologies, Laughton, UK). To illustrate the morphological variability of the species, we provide images captured from various samples collected at different locations in the lake (e.g. sediment trap, surface sediment from the deepest point, epipelon from the littoral zone of the lake) and in different years and season. The type of sample and the time of collection are given in the captions of the images. Through the years, different scanning electron microscopes were used. Most of the images reproduced in this paper were taken with a Phenom XL desktop SEM (Nanoscience Instruments, Phoenix, USA) at IGG-CAS (Beijing, China) operated at 10 kV.

3. Results

3.1. New Species Description

Division: Bacillariophyta

Class: Fragilariophyceae – F.E. Round 1990 [18].

Subclass: Fragilariophycidae – Round & Crawford

Order: Fragilariales – Silva, 1962 sensu emend. [19].

Family: Fragilariaceae – Greville 1833 [20].

Genus: Fragilaria Lyngbye 1819 [1].

Fragilaria longwania Rioual sp. nov.

Description: LM (Figure 2): Frustules in girdle view rectangular, solitary or occasionally two cells connected to each other (Figure 2V). Valves spindle-shaped to elongated, narrowly lanceolate with clearly convex, never parallel margins, tapering from a broadened central area towards protracted, distinctly capitate apices, often deflected. Teratological forms with irregularly undulating or bent valve outlines often observed (Figure 2B, Figure 2T). Valve dimensions: length 15—44 µm (n= 1078, LM and SEM), width 2.0—3.5 µm (from SEM images only, n=36). Sternum distinct but narrow, gradually widening towards the central area. Central area swollen, forming small, irregularly shape hyaline zone, occasionally absent. Striae alternating, parallel throughout entire valve, 21—25 in 10 µm. Areolae sometimes discernible in LM under differential interference contrast.

SEM (Figure 3Figure 4Figure 5Figure 6Figure 7 and Figure 8): Series of marginal spines, generally emerging from vimines, but sometimes from virgae, occurring more or less evenly from apex to apex (Figure 3C, Figure 5C, Figure 6C, Figure 7B). Spines acute, shaped like shark tooth (Figure 3B-C and Figure 8A). Up to three tiny spines present at apices (Figure 4B, Figure 5B and Figure 6B). Valve face flat with weakly raised virgae (Figure 3A, Figure 5c and Figure 7c). Central area with ghost striae (Figure 3D, Figure 5C, Figure 6C and Figure 7B). Striae uniseriate, composed of several (up to 6) rounded areolae (Figure 5C, Figure 6C, Figure 7B) becoming transapically elongated on the mantle (Figure 8A-B), 62—74 in 10µm (n= 35). Virgae much wider than the striae. Apical pore field of the ocellulimbus type, not well developed, composed of only one or two rows of small rounded poroids (Figure 3A and Figure 3E, Figure 4B-C, Figure 5B, Figure 5D and 8B-C). One rimoportula is present per valve, positioned at a distal end (Figure 3A, Figure 4A, Figure 5A and Figure 7A). Externally, rimoportula opening small, rounded but not rimmed, generally located at the centre of the last stria (Figure 3B, Figure 4B, Figure 5B, Figure 6B and Figure 7D). Internal rimoportula opening more or less elongated, oblique (Figure 4C). Girdle composed of only two or three open bands, ligulate, tapering and opening at the valve pole (Figure 8A-C). Each band with one row of poroids (Figure 4C and Figure 8A-C).

Holotype: 20 April 2005, collected by P. Rioual, slide number IGGDC-DB-SIHA-0504 (herb. IGGDC) partly shown here in Figure 2, holotype specimen: illustrated in Figure 2Z, located using England Finder J27/1.

Paratype: BM 101 673 in the Natural History Museum, London, UK. (Note: this is the same slide as for Staurosira longwanensis that was described in Rioual et al. [21].

Registration:http://phycobank.org/xxxxxxxxxx (pending)

Type locality: China, Lake Sihailongwan, Jilin Province (E: 126°35’51’’–126°36’24’’, N: 42°17’0’’–42°17’24’’; Elevation: 797 m above sea level)

Etymology: For naming this species we followed Dubois [22] who recommends keeping species names as simple and short as possible in order to be euphonious in all languages and easily memorable. The epithet of the new species refers to “Long wan” the Chinese name for the maar lakes of the Long Gang volcanic field in Jilin Province and which means “Dragon bowl” in English.

Distribution: So far only observed in the type locality.

3.2. Analysis of the Trap Samples

In Lake Sihailongwan, during the 15-year period of investigation, two centric diatoms, Discostella stelligeroides (Hustedt) Houk & Klee and Lindavia balatonis (Pantocsek) Nakov, Guillory, Julius, Theriot & Alverson largely dominated the sediment trap assemblages. Fragilaria longwania was present in 139 of the 187 trap samples analyzed, with a maximum abundance of 19.8% recorded at the beginning of the study in June 2003 and a mean abundance of 2.1%. Over the study period, a general decline in relative abundances is observed (Figure 9A). On the other hand, during the same period the mean valve length follows a positive trend (Figure 9B).

The seasonal distribution of F. longwania was investigated by plotting its relative abundances against Julian day on a year-to-year basis (Figure 10). In most years, such as 2003, 2004, 2007, 2008, 2009, 2010, 2011, 2103, 2014, abundances of F. longwania exhibit a peak during the spring period (April-May-June). In some years (2003, 2004, 2013), a second peak occurred in the autumn (September-October). Only in 2006, the seasonal distribution of F. longwania was characterized by a single autumnal peak. Generally, F. longwania had low abundances during the summer season, which is characterized not only by high temperature, but also high precipitation, low wind speed and low sunshine (due to high cloud cover).

3.3. Analysis of Additional Samples

Discostella stelligeroides and Lindavia balatonis, the two centric diatoms that dominate the assemblages collected in the sediment trap samples, are also present and often dominant in all the other types of samples collected in the lake. These include periphyton samples collected from the mooring buoy and in the various benthic samples from the littoral zone (Table 2).

For F. longwania, the highest percentages were found in sediment trap samples (up to 19.8%). Unsurprisingly considering its abundance in trap samples, F. longwania is also among the most abundant species in the surface sediment samples collected from the middle of the lake reaching 1.8% in October 2005 and 1.2% in August 2007 (Table 2). It is also found in the periphyton samples collected from the mooring buoy, although its abundance in this type of sample is also declining and therefore follows the same trends observed in the trap samples. F. longwania can be completely absent in the benthic samples collected from the littoral zone and when present this species never reaches 3% of the assemblages.

4. Discussion

4.1. Generic Placement of the New Species

As discussed by Williams [23], the genus Fragilaria is currently aphyletic, i.e. without identified synapormophy, and so far, recent attempts to separate new genera from Fragilaria have not been entirely convincing [7,24,25]. Nevertheless, F. longwania shares many characters with what Williams [23] refers as `classical’ species of Fragilaria, including the presence of one rimoportula on one apex, apical pore fields of the ocellulimbus type, uniseriate striae and open girdle bands. F. longwania is therefore best placed in that genus. Among the groups of Fragilaria species that Williams [23] recognized according to the occurrence and placement of their marginal spines, F. longwania is closest to the group 3, i.e. `classical’ species with spines that emerge from the vimines, even if some spines appear to emerge from virgae. In that group, are included Fragilaria tenera var. nanana and Fragilaria billingsi, two of the species that closely resemble F. longwania (see below).

4.2. Comparison with Other Species of the Genus Fragilaria

The new species described here is comparable to several other species of Fragilaria (Table 3), especially to species of the Fragilaria rumpens complex which has been recently reviewed by Van de Vijver et al. [3] who analyzed the type material of numerous species described in the 19^th century.

One of the most similar species is Fragilaria campyla (Hilse) Van de Vijver, Kusber et DM Williams, which largely overlaps with F. longwania in terms of valve length (35—45 µm vs 15—44 µm) and width (2.5—3.0 µm vs 2.0—3.5 µm). F. campyla however lacks a swollen area and its valves are linear to linear-lanceolate, not spindle-shaped to elongated and narrowly lanceolate. In F. campyla the ocellulimbi are also larger than in F. longwania [3].

Fragilaria parva (Grunow) Tuji et DM Williams, is another species of the F.rumpens complex with similar ranges of valve dimensions compare to F. longwania. In addition, F. parva also possesses a swollen area and shark tooth shaped spines. However, F. parva differs by having linear to linear-lanceolate valve outline, a large hyaline rectangular zone in the central area and well-developed ocellulimbi [3].

Fragilaria metcalfeana Van de Vijver, DM Williams et Kusber, is a fossil species originally described from Neogene material collected in Dubravica (Croatia). This taxon has also a distinctly inflated central area, but its valves are much longer (60—120 µm) and characterized by a well-delimited hyaline central area and large ocellulimbi at the apices [3].

Fragilaria pseudofamiliaris Van de Vijver, TM Schuster, Kusber et DM Williams, has similar valve dimensions (length 30—50 µm and width 2.0—3.0 µm) and protracted subcapitate to capitate apices. However, this species differs from F. longwania by having elongated, narrowly lanceolate valve, with broad linking spines that help this species in producing long ribbon-like colonies, that we never observed in F. longwania. In addition, this species has a lower stria density (18—19 striae in 10 microns) and well-developed ocellulimbi composed of at least 4 rows of small poroids [3].

Last among the species of the Fragilaria rumpens complex that could be usefully compared to F. longwania is Fragilaria scotica (Grunow) Van de Vijver, CE Wetzel et Ector. This species also possesses a clearly inflated central area. Its outline is however not spindle-shaped, but linear-lanceolate with gradually narrowing margins and protracted, rostrate apices. In addition, F. scotica has much lower stria density than F. longwania with 15 instead of 21-25 striae in 10 microns [3].

Outside the F. rumpens complex, several more species can be compared to F. longwania. Fragilaria nanoides Lange-Bertalot, which was described from Julma Ölkky, an oligo-dystrophic lake from Finland, has a very similar silhouette, and overlapping ranges of stria density (22.5—23 vs 21—25 striae in 10 µm). However, F. nanoides has longer (40—90 µm) and narrower valves (1.8—2.4 µm) [26].

Fragilaria billingsi Wengrat, C.E.Wetzel et E.Morales, that was described from a reservoir in southern Brazil, is another species with an inflated central area, but its valves are longer (54—76 µm), have a lower stria density (17—20 striae in 10 microns), clearly raised costae and well-developed ocellulimbi [27].

Fragilaria huebeneri A.Schwarz, KJ Krahn et CE Wetzel, which was described from Nam Co, a large subsaline lake in Tibet, has spindle-shaped valves that are often curved, like what can be observed in F. longwania. However, in F. huebeneri the central area is less inflated, the valves are more narrowly tapering towards subcapitate apices. In addition, F. huebeneri has much longer valves (99—121 µm), possesses raised costae and spatula-shaped spines that allow this species to form ribbon-like colonies. The ocellulimbi are also well-developed with 3-5 rows of poroids [8].

Fragilaria stoermeriana Alexson, Reavie et Van de Vijver, described from plankton samples collected in the eastern basin of Lake Superior (Ontario, Canada), also shares some characteristics with F. longwania such as a distinctly swollen central area, protracted capitate ends, weakly raised virgae, acute spines irregularly scattered along the entire margin of the valve and small ocellulimbi at the apices. The two species have also similar stria density (20—24 vs 21—24 striae in 10 microns). However, F. stoermeriana is much longer (60—110 µm) and can form stellate colonies [28].

Finally, the apparently cosmopolitan Fragilaria tenera var. nanana (Lange-Bertalot) Lange-Bertalot et S.Ulrich partly overlaps with F. longwania in terms of valve dimensions as length and width of its valves range between 29—85 µm and 2.0—2.3 µm, respectively. However, the stria density of the variety nanana is lower (18.5—20 instead of 21—24 in 10 microns) as reported by Lange-Bertalot and Ulrich [29] and the outline of the valves, even for the short ones, is not spindle-shaped but lanceolate with a barely inflated central area.

Table 3. Selected morphological characteristics of Fragilaria longwania sp. nov. and of other species of Fragilaria sharing a broadly similar outline.

Taxon	Length (µm)	Width (µm)	Striae in 10 µm	Valve outline	Central area	Pore fields	Distribution	Reference
F.longwania	15—44	2.0—3.5	21—25	spindle shaped	clearly swollen	small, only one or two rows of small poroids	Lake Sihailongwan, NE China	This study
F.campyla	35—45	2.5—3.0	19—20	linear to linear-lanceolate	not swollen	well-developed, several rows of small poroids	Europe	[3]
F. parva	20—40	3.0—3.5	19—21	linear to linear-lanceolate	swollen	well-developed, 4 rows of large poroids	Europe	[3]
F. metcalfeana	60—120	2.5—3.5	ca. 18	linear but strongly elongated	distinctly inflated	large, 2-3 rows of large poroids	Fossil, Neogene, Dubravica, Croatia	[3]
F. pseudofamiliaris	30—50	2.0—3.0	18—19	elongated, narrowly lanceolate	not swollen	well-developed, at least 4 rows of small poroids	Artesian well, Dresden, Germany	[3]
F. scotica	65	4	15	linear-lanceolate	clearly inflated	not observed	Loch Leven, Scotland	[3]
F. nanoides	40—90	1.8—2.4	22.5—24	linear-lanceolate	weakly inflated	not observed	Lake Julma Ölkky, Finland	[26]
F.billingsii	54—76	2.0—2.5	17—20	narrow lanceolate	bilaterally gibbous	well-developed	Reservoir, Sao Paulo State, Brazil	[27]
F.huebeneri	99—121	2.0—4.0	17—21	spindle to needle-shaped, narrowly lanceolate	indistinct	well-developed, 3-5 rows of poroids	Lake NamCo, Tibet,China	[8]
F.stoermeriana	60—110	1.5—3.0	20—24	long, lanceolate	distinctly swollen	small ocellulimbi	Lake Superior, Ontario, Canada	[28]
F. tenera var. nanana	29—85	2.0—2.3	18.5—20	linear to linear-lanceolate	barely inflated	well-developed, 3 rows of poroids	cosmopolitan	[29]

Table 3. Selected morphological characteristics of Fragilaria longwania sp. nov. and of other species of Fragilaria sharing a broadly similar outline.

Taxon	Length (µm)	Width (µm)	Striae in 10 µm	Valve outline	Central area	Pore fields	Distribution	Reference
F.longwania	15—44	2.0—3.5	21—25	spindle shaped	clearly swollen	small, only one or two rows of small poroids	Lake Sihailongwan, NE China	This study
F.campyla	35—45	2.5—3.0	19—20	linear to linear-lanceolate	not swollen	well-developed, several rows of small poroids	Europe	[3]
F. parva	20—40	3.0—3.5	19—21	linear to linear-lanceolate	swollen	well-developed, 4 rows of large poroids	Europe	[3]
F. metcalfeana	60—120	2.5—3.5	ca. 18	linear but strongly elongated	distinctly inflated	large, 2-3 rows of large poroids	Fossil, Neogene, Dubravica, Croatia	[3]
F. pseudofamiliaris	30—50	2.0—3.0	18—19	elongated, narrowly lanceolate	not swollen	well-developed, at least 4 rows of small poroids	Artesian well, Dresden, Germany	[3]
F. scotica	65	4	15	linear-lanceolate	clearly inflated	not observed	Loch Leven, Scotland	[3]
F. nanoides	40—90	1.8—2.4	22.5—24	linear-lanceolate	weakly inflated	not observed	Lake Julma Ölkky, Finland	[26]
F.billingsii	54—76	2.0—2.5	17—20	narrow lanceolate	bilaterally gibbous	well-developed	Reservoir, Sao Paulo State, Brazil	[27]
F.huebeneri	99—121	2.0—4.0	17—21	spindle to needle-shaped, narrowly lanceolate	indistinct	well-developed, 3-5 rows of poroids	Lake NamCo, Tibet,China	[8]
F.stoermeriana	60—110	1.5—3.0	20—24	long, lanceolate	distinctly swollen	small ocellulimbi	Lake Superior, Ontario, Canada	[28]
F. tenera var. nanana	29—85	2.0—2.3	18.5—20	linear to linear-lanceolate	barely inflated	well-developed, 3 rows of poroids	cosmopolitan	[29]

4.3. Ecology, Seasonality, Cell-Size and Distribution

From the results of our water chemistry analyses (Table 1), Lake Sihailongwan has circumneutral, weakly alkaline water. Considering the median values for total phosphorus concentration and Secchi disk depth of ~15 µg/L and 3.8 m, respectively, the trophic status of Lake Sihailongwan can be considered as mesotrophic. The highest abundances of this species were observed in trap samples. It is also found in the periphyton attached to the mooring buoy and in the subfossil assemblages found in the surface sediment collected at the bottom of the water column. Together, these data on the occurrences and abundances in samples collected from the middle of the lake and the fact that it was never found in large abundance in the benthic samples collected from the littoral zone suggest that this species is part of the diatom plankton.

From the series of trap samples, F. longwania appears to be associated to the spring season (and in some years, to the autumn). In the region of Lake Sihailongwan meteorological conditions during spring are characterized by low temperatures, high wind speed and sunny conditions (Figure 10). During the 15 year-long period we investigated for this study, valve length of F. longwania tended to increase with decrease in valve abundance (Figure 9). A similar inverse relationship between cell size and density was observed for another species of Fragilaria by Soumya et al. [30] in their investigation of a natural planktonic population from Lake Biwa (Japan). According to these authors, this relationship indicates occurrence of cell division. If this is true for F. longwania in Lake Sihailongwan, it would suggest that conditions at the beginning of the study period (from 2003 to 2013) were more favorable for the growth of the population by vegetative cell division than at the end of the study period (2014-2017), when less numerous and longer cells were found, suggesting lower rates of cell division. Temperature and nutrients are thought to be key factors that affect such changes in cell size and density but our water chemistry data only cover a short part of the study period and a more detailed investigation would be required to unveil such relationships.

F. longwania is the second diatom species described from Lake Sihailongwan after Staurosira longwanensis Rioual, Morales & Ector [21]. So far, F. longwania has not been observed in other lakes from NE China or anywhere else but considering the relatively understudied diversity of the genus Fragilaria in China, it is probably premature to consider it as a species endemic to Lake Sihailongwan.

Finally, preliminary diatom analyses of the long sedimentary sequence of Lake Sihailongwan [11] indicate that Fragilaria longwanensis has been present in the diatom flora of this lake for at least the past 20,000 years. Its population however, appears to be declining as suggested by the trends we observed (Figure 9 and Figure 10) in the trap samples we collected over a 15 year-long period.