Around 2005 we, the Diatom Research Group at the BGBM Berlin, cooperated with colleagues from the Georg-August-University Göttingen who wanted to identify diatoms and cyanobacteria using a predecessor method, today called DNA metabarcoding. Since our group had not yet gone molecular, our task was to identify the few newly established cultures and to assist in naming the OTUs (Operational Taxonomic Unit) which were produced by 18S sequencing data. Different OTUs usually represent different species. A specific OTU appeared in all samples showing that it seemed to play an important role in calcifying biofilms of hard water creeks. At that time, sequence databases (e.g. GenBank) did not have much data on diatoms. However, this OTU could not even be assigned to a genus, which is a coarser level than species, so we had to stick to the order name Cymbellales. When screening the other OTUs in the samples, we noticed that one of the most important species of Central European rivers – Gomphonema olivaceum – was missing from the list of taxa. The assumption that this OTU-Cymbellales might be Gomphonema olivaceaum was discarded because its phylogenetic position showed that it could in no case be a Gomphonema.

During the Project German Barcoding of Life 2 (2016-2019), we were able to establish cultivation capacity and therefore strains of Gomphonema olivaceum. The cultivation turned out to be complicated since cells of G. olivaceum are often joined due to the slime (mucous) they produce (Figure 1). In addition, their identification was difficult since the typical club shape disappeared during cultivation and the cells started to appear more even in shape (specifically, naviculoid). When we were finally able to compare the molecular data from our cultivated cells with the OTU-Cymbellales, we were flabbergasted because the sequences matched! We had finally – more than 10 years later – solved this question, but it was too late to include it in the publication of this research (Brinkmann et al. 2015).

Figure 1. Gomphonella olivacea (living cells joined by the slime they produce)

This result initiated taxonomical and nomenclatural questions: if this G. olivaceum is not a Gomphonema, to which genus does it belong? Maybe it should be named Gomphoneis olivacea as it was called in the English-speaking countries (e.g. Kociolek 2011)?

To find out if a species in question is really the same as described by the first author, the type of the name of the species needs to be consulted. Databases (e.g. INA, AlgaTerra) helped us to identify the first name of this species and its author (“authority“). Prof. Hornemann published a colored drawing with the name Ulva olivacea in Flora Danica (1810), showing a macroscopic olive-green mass of mucous (Figure 2). When asking the curator of the Copenhagen Herbarium to loan us the historical material, her reaction was: this cannot be a diatom! Nevertheless, when searching for the material, she realized that it was still on loan to the Philadelphia Diatom Herbarium and had been since 1965. The curator of this Herbarium also started a search and some months later the material was sent to us. The suspense was almost unbearable: would this material collected 200 years ago really be the species in question? Using light and scanning electron microscopes, we took photos of the historical type population and were relieved when we found out that the current species concept did correspond to the original type! To understand its variability, we compared the morphometrics and the micro-morphology of the historical type population with our cultivated material.

Figure 2. Ulva olivacea as illustrated in Flora Danica (1810)

After we had done its typification, we wanted to understand the history of its name. During its nomenclatural trip from Ulva olivacea to its current name, more name combinations had been created: Echinella olivacea (Lyngbye 1819), Frustulia olivacea (Kützing 1833), Cymbella oliva­cea (Brébisson & Godey 1835), Gomphonema olivaceum (Ehrenberg July/August 1838 & Brébisson October 1838). Already in 1853, Rabenhorst had invented the new genus name Gomphonella and published the combination Gomphonella olivacea (Figure 3).  But Gomphonella was degraded to a subgenus of Gomphonema (Brun 1880), and finally sunk into oblivion. 100 years later a British researcher (Dawson 1974) studied the micro-morphology of Gomphonema olivaceum with the newly available electron microscope and noticed that the micromorphology of the areolae is different from the typical Gomphonema. She proposed to recombine it with the genus Gomphoneis Cleve (1894). Since some features did not fit, continental European identification books stuck to the old name (e.g. Hofmann et al. 2013).

Our molecular and micromorphological research had clearly demonstrated that our species did not fit into the genus Gomphonema (Abarca et al. 2014, 2020) but neither did it fit into the genus Gomphoneis in its current concept (Kociolek & Stoermer 1989). Current genus Gomphoneis is not monophyletic (Skibbe et al. 2018), meaning that it includes species that did not descend from a common ancestor. We decided to revive the name Gomphonella olivacea for our studied species (Jahn et al. 2019) (Figure 3).

Figure 3. Gomphonella olivacea under Scanning Electron Microscope

Nevertheless, Gomphonella olivacea did not remain the only species in this genus. We described further new species from our cultures: Gomphonella acsiae from Lake Balaton, Hungary, Gomphonella coxiae from Lake Helenesee, Eastern Brandenburg in Germany, and Gomphonella tegelensis, a neobiont from Lake Tegel, Berlin, which we had seen already in 2009 due to its large size but later, during GBOL Project, were able to analyze this taxon molecularly and morphologically (Skibbe et al. 2018). In addition, a number of recently described species from China with similar morphology were recombined with Gomphonella (Jahn et al. 2019).

The final question is yet to be answered: where does this genus and its species belong? All these taxa show the typical club shape with a footpole as Gomphonema, but they differ in the internal features (specifically, they have different raphe and areolae structures, and no stigma). Molecularly they do not belong to the family Gomphonemataceae but instead are closer to the family Cymbellaceae (Jahn et al. 2019). Since only few taxa from the Cymbellales and especially from the Cymbellaceae have been studied molecularly and micro-morphologically, more research is needed to find out where they phylogenetically really belong to.

 

Regine Jahn was Head of the BGBM Diatom Research Group, Freie Universität Berlin, until March 2019. She is now officially retired but keeps on working as an Honorary Researcher. Email Regine or drop a message below if you have any questions about the post.

 

References
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