Diatom of the Month March 2019 - The hidden diversity of small-sized araphid diatoms

by Chunlian Li*

What we've got here? Chunlian Li presents her latest research, and how different small araphid diatoms are when we look at them more closely

Small-celled araphid diatoms (bilaterally symmetrical diatoms without a slit on the valve face, and with diameters < 20 mm) are important contributors to primary production in marine and freshwater habitats (Round et al. 1990; Witkowski et al. 2000). These diatoms mainly colonize benthic substrates, e.g. epiphytic (plants), epizoic (animals) or epipsammic (sand) habitats (Fig. 1) and are often the most diverse benthic algae. Yet, benthic habitats are relatively under-sampled possibly because of their higher heterogeneity than open water habitats, which results in poorly described araphid diatom floras. In addition, the identification of these diatoms is difficult, even for the most experienced diatomists, because of their small size and the scarcity of reliable diagnostic characters.

 

Fig. 1. Substrates colonized by araphid diatom. a) Algae; b) Rock; c) Turtle carapace; d) Sand (Photos taken by Chunlian Li & Aydin Kaleli).

 

In the early days, the identification of small-celled araphid diatoms was performed with the aid of light microscope (LM); however, many important characters could not be discerned. Williams et al. (1987) began to redefine the taxonomy of small-celled araphid diatoms using the ultra-structural characters under Electron Scanning Microscope (SEM), and separated several new genera from Fragilaria, uncovering diversity in this group non previously recognized before. By means of SEM, Morales et al. described dozens new species and revised other descriptions of existing taxa (Morales et al. 2001, 2002, 2005, 2010, 2012a, 2012b, 2015). In recent years, Li et al. (2016, 2018) investigated global diversity patterns of small-celled araphid diatoms, showing that taxa are geographically distributed across different oceans (Fig. 2). Based on morphological and molecular data, the authors not only refined the diagnostic features for some confusing genera (e.g. Nanofrustulum, Pseudostaurosira, Staurosirella), but also established several new genera (e.g. Castoridens, Cratericulifera, Hendeyella, Psammotaenia) including dozens of new species. Therefore, the diversity of these small-celled taxa is higher than previously thought.

 

Fig. 2. Distribution of the isolated small-celled araphid diatoms (Source: Li et al. 2018, Fig. 1)

 

Small-celled araphid diatoms are widely used in biomonitoring and paleoenvironmental reconstructions (Witkowski et al. 2005，2009). The accurate identification of these diatoms is therefore very important to produce reliable data for both modern and past ecological studies. However, ecologists tend to categorize every small araphid valve under the broad “staurosiroid” group (genera Staurosira, Fragilaria, Pseudostaurosira) that utilizes a certain size of small-celled araphid diatoms to indicate particular environmental conditions. Diatoms with small size may have been indicating different ecological conditions so focus on ultra-morphological (examined with SEM) or phylogenetic differences instead of LM morphological features should be made to unequivocally link species identity and environmental data. For instance, the specimens shown in Fig. 3a and Fig. 3b share many similarities: oval valves, narrow-linear sternum and alternate striae. When examined under SEM, these specimens look alike (Fig. 3c and Fig. 3d), with the exception of small particles observed on the valve face of the latter.  The species Gedaniella flavovirens (Fig. 3c) thrives in freshwater, brackish or marine environments, while the taxon Gedaniella boltonii (Fig. 3d) is a marine species.

 

Fig. 3. LM images of a) Gedaniella flavovirens and b) Gedaniella boltonii, and SEM images of c) Gedaniella flavovirens and d) Gedaniella boltonii. (Photos taken by Chunlian Li)

 

As noted by different studies and by our own studies, relatively simple diagnostic features also hide the diversity of small-celled araphid diatoms. Pseudostaurosira (Fig. 4a) is a well-known freshwater or brackish genus; a researcher might assign a marine diatom (e.g. Serratifera, Fig. 4b) to that genus because of an overall similarity in most of diagnostic characters (e.g. lanceolate-elliptical valve, small apical pore fields, transapically-elliptical areolae, one areola on both valve face and mantle, the presence of marginal spines). However, the sternum width is widely-lanceolate in Pseudostaurosira and narrow-linear in Serratifera. Therefore, these diatom species can be used as ecological indicators in future ecological survey. Phylogenetic analysis also supports the separation of Pseudostaurosira and Serratifera; both are several clades away from each other, that is, they do not share a common ancestor (Li et al. 2018).

 

Fig. 4. a) Pseudostaurosira elliptica; b) Serratifera corallina. (Source: Li et al. 2018, Fig. 341 & Fig. 399)

 

In our previous investigation, we found dozens of species in natural samples that resemble the existing taxa, but lacking differences in diagnostic characters. We feel that as diatomists we are still a long way away from revealing the true biodiversity of these small araphid taxa. Ongoing and future research, especially through molecular analyses, will shed more light on the diversity of small-celled araphid taxa. Although there have been some efforts to revise the diagnostic characters for the difficult-to-identify species, we also need to increase sampling efforts. We encourage further investigations on these diatoms, especially with DNA data, to make their use easier for freshwater and marine ecologists.

 

*Institute of Ecological Sciences, South China Normal University, Guangzhou 510631, China

You can email Chunlian if you have questions about her post or want to know more about her research.

 

References:

Li CL, Ashworth MP, Witkowski A, Lobban CS, Zgłobicka I, Kurzydłowski KJ, Qin S (2016) Ultrastructural and molecular characterization of diversity among small araphid diatoms all lacking rimoportulae. I. Five new genera, eight new species. Journal of Phycology 52: 1018–1036.

Li CL, Witkowski A, Ashworth MP, Dąbek P, Sato S, Zglobicka I, Witak M, Khim JS, Kwon CJ (2018) The morphology and molecular phylogenetics of some marine diatom taxa within the Fragilariaceae, including twenty undescribed species and their relationship to Nanofrustulum, Opephora and Pseudostaurosira. Phytotaxa 355: 001–104.

Morales EA (2001) Morphological studies in selected fragilarioid diatoms (Bacillariophyceae) from Connecticut waters (U.S.A.). Proceedings of the Academy of Natural Sciences of Philadelphia 151:105–120.

Morales EA (2002) Studies in selected fragilarioid diatoms of potential indicator value from Florida (USA) with notes on the genus Opephora Petit (Bacillariophyceae). Limnologica - Ecology and Management of Inland Waters 32: 102–113.

Morales EV (2005) Observations of the morphology of some known and new fragilarioid diatoms (Bacillariophyceae) from rivers in the USA. Phycological Research 53:113–133.

Morales EA, Edlund MB, Spaulding SA (2010) Description and ultrastructure of araphid diatom species (Bacillariophyceae) morphologically similar to Pseudostaurosira elliptica (Schumann) Edlund, et al. Phycological Research 58: 97–107.

Morales EA, Novais MH, Chavez G, Hoffmann L (2012a) Diatoms (Bacillariophyceae) from the Bolivian Altiplano: three new araphid species from the Desaguadero River draining Lake Titicaca. Fottea 12: 41–58.

Morales EA, Manoylov Kma, Bahls LL (2012b) Three new araphid diatoms (Bacillariophyta) from rivers in North America. Proceedings of the Academy of Natural Sciences of Philadelphia 160: 29–46.

Morales EA, Wetzel CE, Van deVijver B, Ector L (2015) Morphological studies on type material of widely cited araphid diatoms (Bacillariophyta). Phycologia 54: 455–470.

Round FE, Crawford RM,, Mann DG (1990) The diatoms: biology and morphology of the genera. Cambridge University Press, Cambridge, 747 pp.

Williams DM (1987) Revision of the genus Fragilaria. Diatom Research 2: 267–288.

Witkowski A, Lange-Bertalot H, Metzeltin D (2000) Diatom flora of Marine Coasts I. Iconographia Diatomologica 7, 925 pp.

Witkowski A, Broszinski A, Bennike O, Janczak-Kostecka B, Jensen J B, Lemke W, Endler R, Kuijpers A (2005) Darss Sill as a biological border in the fossil record of the Baltic Sea: evidence from diatoms. Quaternary International 130: 97–109.

Witkowski A, Cedro B, Kierzek A, Baranowski D (2009) Diatoms as a proxy in reconstructing the Holocene environmental changes in the south-western Baltic Sea: the lower Rega River Valley sedimentary record. Hydrobiologia 631: 155–172.