ecology │ adaptation │ ontogeny │ taxonomy │ habitats

The research of euglenoids and all other microorganisms is connected to the development of a microscope and its gradual improvement since the 16th century, which is traditionally associated with Antoni van Leeuwenhoek (1632-1723) or Robert Hooke (1635-1706). The first mention of observation of Euglena is attributed to Antoni van Leeuwenhoek – “These animalcules had divers colours, some being whitish and transparent; others with green and very glittering little scales; others again were green in the middle, and before and behind white; others yet were ashen grey” (Dobell, 1932). The first description of the photosynthetic euglenoids was made by Ehrenberg at the beginning of the 19th century – Euglena (Ehrenberg, 1830), Cryptoglena (Ehrenberg, 1832), Colacium (Ehrenberg, 1834) and Trachelomonas (Ehrenberg, 1834). During the 19th century, other traditional genera were described – Phacus (Dujardin, 1841), Lepocinclis (Perty, 1849), Monomorphina (Mereschkowsky, 1877) and Ascoglena (Stein, 1878), the last traditional genus Strombomonas was described at the beginning of the 20th century by Deflandre (1930). Studies of euglenoids, as well as other algal or protistian groups were based on morphological features, which were the key factor of the taxonomic studies of euglenoids during the whole 19th and most of the 20th century. During that relatively long research period, a huge number of species and intraspecific taxa in these listed genera were described. According to the AlgaeBase (Guiry & Guiry, 2018), a commonused source about algal diversity and taxonomy, there was 1553 species described in these nine genera from which about 60 % have been flagged as accepted taxonomically, on the intraspecific level 1956 taxa are in the database listed. These numbers suggest that the question of taxonomic position or validity of a huge number of species and infraspecific taxa is still a challenge for the further research. This problem of a high number of described species and taxa was simply commented by Pringsheim (1953), who explains that certain authors could not resist the temptation to give a name to every minor deviation from the typical form previously described or prevailing at same or similar places. The huge morphological variability could be probably explained by using two main factors: (1) the impact of environmental conditions to the euglenoid morphology and (2) the role of the ontogeny stage in which species is observed.

How to apply modern taxonomy to classical ecology?

How does variability of euglenoids affect their systematics?

Several studies support the theory of the environmental impact on the euglenoid morphology. Conforti (1998) studied morphological changes of several euglenoids (Lepocinclis acus, L. spirogyroides, Monomorphina pyrum, Phacus curvicauda and P. tortus) in response to the organic enrichment of laboratory cultures of these algae. Results show cell deformation, the extraordinary accumulation of paramylon grains and the development of larger paramylon grains in higher frequency than which is typical for euglenoids from enriched cultures. An organic enrichment of medium has a significant impact on shortening and widening of cells of Lepocinclis acus, observed in cultures by Conforti et al. (2017). In addition, similar results were obtained in the study realized by Nannavecchia et al. (2014) on the culture of Phacus brachykentron. Moreover, similar results have been shown in studies on a natural condition, e.g. Bauer et al. (2012) designed bioassay based on Lepocinclis acus and Acutodesmus acutus as model organisms in order to study the impact of the chemical and textile industry on the environment of river in Argentina. Their results, again, show significant increase in the cellular volume for species together with abnormal shape in the studied sites.

Second important factor which could explain a huge number of described species and infraspecific taxa is the variability of euglenoids during their ontogeny. The morphological variability of the several species of the genus Trachelomonas was studied by Pringsheim (1953) and Singh (1956a,b). Their results show that the Trachelomonas species studied in the clonal cultures exhibit a morphological variability on their monads as well as their loricae during the life cycle. Authors conclude that this variability is a rather complicating factor in describing new species from natural populations, because descriptions of new taxa were based on few individuals from the population without deeper study of their life cycle at all. Owing to the modern research, the role of ontogeny is well documented in the work focused on the common euglenoid species Monomorphina pyrum by Kosmala et al. (2007b), who provided a revision of M. pyrum and morphologically similar taxa using a morphological-molecular approach and one of the results was synonymization of several Monomorphina taxa with M. pyrum. Authors support their results by detailed discussion about reasons why such a high number of morphologically similar organisms were described by different authors as separate species. The description of these species was based on differences from the “typical” M. pyrum, e.g. the presence of two chloroplasts, the absence of large paramylon grains, and the degree of cell flatness. Authors connect this description with the observation of M. pyrum in different stages of its life cycles and support this claim with a detailed description of changes in M. pyrum during ontogeny. The second study connected with the variability of euglenoids during their ontogeny is the study of natural populations of the common Trachelomonas caudata species (Wołowski, et al., 2016). Authors used basic morphometrical data in the combination with the literature sources to compare T. caudata with morphologically similar taxa (T. caudata f. pseudocaudata, T. fusiformis, T. allorgei, T. mollesta and T. bernardinensis) – as a result of this comparison, authors claim that all studied species should be synonymized with the T. caudata. According to this study, we could assume that the description of these species was connected with the observed stage of T. caudata life cycle as was shown in the previous study about Monomorpina pyrum.

Figure 1. Lepocinclis acus (above) and Lepocinclis spirogyroides (below) variability affected with the organic enrichment. Typical form on the left and examples of two altered species (modified according Conforti, 1998)

Figure 2. Variability of the outline of the loricae, collar and development of the Trachelomonas lefevrei studied in the culture (modified according Pringsheim, 1953).

In the past, several authors tried to solve some problems of the euglenoid taxonomy and several revisions were published, e.g. Chu (1947), Gojdics (1953), Pochmann (1942) or Pringsheim (1956). In addition, some revisions and taxonomical conclusions are part of monographs focused on euglenophytes, e.g. Huber-Pestalozzi (1955), Wołowski (1998) and Yamagishi (2013, 2016). Furthermore, all of these revisions are based on morphological features comparison using data from optical or electron scanning microscope. Additionally, they are sometimes supplemented with information about the structure of some organelles using cytochemical reactions or ultrastructure using transmission electron microscope. Despite the large number of taxonomic works, the situation in the taxonomy of euglenophytes is still not satisfactorily solved in many cases.


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