Due to the paucity of studies and the lack of recently developed regional floras for marine benthic diatoms, we face challenges to identify the species in marine benthic samples. In addition, taxonomically complicated genera, such as Amphora (sensu lato), often form the majority of species in the samples. We studied some small-celled and finely striated taxa belonging to Amphora s.l. in samples from the Adriatic Sea and the Black Sea. The Adriatic samples originated from Pula, Croatia, and were taken from both environmental benthic habitats and turtle carapaces. Samples from the Black Sea included the biofilm on plexiglass tiles placed at two sites off the southern Bulgarian coast. Combining both light (LM) and scanning electron microscopy (SEM), we differentiated nearly 30 small-celled and/or finely striated Amphora s.l. taxa. A few taxa were shared between the studied seas and substrates, such as Halamphora coffeaeformis, H. kolbei, H. bistriata, several other Halamphora species with uncertain identity, and an unknown Seminavis taxon. The diversity among the small-celled taxa in SEM was higher than the one we observed in LM. In the group of Halamphora pseudohyalina two taxa were discovered. Five taxa were distinguished in SEM from a provisionally single taxon identified as “Amphora crenulata” in LM. Often, it was impossible to clearly relate the species observed in SEM to those observed in LM, especially the different small-celled “Halamphora coffeaeformis”- like taxa present in the Black Sea samples. Zidarova et al. (2022) attempted to separate these taxa, but this study indicated that some of them represent complexes in LM. Finally, there were also taxa that we discovered only in SEM, and they did not match any of the taxa or complexes observed in LM. Survey of the older literature showed that some of the species might have been described in the past from the Black Sea, but since these taxa were never (re)studied in SEM, their identities are uncertain. Apart from a few such exceptions, most of the small-celled Amphora s.l. taxa that we observed are apparently unknown and yet to be described.

References:
Zidarova R., Ivanov P., Hristova O., Dzhurova B., Hineva E. 2022. The unexpected diversity in Amphora sensu lato (Bacillariophyta) at Sozopol Bay, the western Black Sea. Phytotaxa 544(2): 103-127.