What endemic diatoms of the most ancient lake of Europe can tell us about deep time evolution?

by Dušica Zaova |

When we think about deep-time evolution, we refer to million-year-old traces of fossil organisms that can be documented in a sedimentary record and whose systematic changes can be tracked through long sequences of geological history. Yet the fossil record is generally incomplete – fragmented by preservation gaps, eroded layers, and missing pieces in Earth’s archive, making such reconstructions difficult. To fill those gaps, researchers often look for sedimentary records from ancient aquatic ecosystems since they exhibit more continuous and long-term record of past life. Among the best-preserved microfossils in these systems are diatoms – organisms with siliceous shells whose peculiar ornamentation allows us to track morphological changes across extended time periods.

We discovered an incredible diversity of fossil diatoms in a palaeoecological and taxonomic study of the sediments from Lake Ohrid, the most ancient lake of Europe (Fig. 1a, b). There, in the sequence aged 1.3 million years (Fig. 1c), we identified more than five hundred species. Among them, the planktonic were the most intriguing, as they have persisted in the lake for long periods of time while exhibiting high morphological variability (Zaova et al., 2025). This increased our scientific curiosity for the lake and its living world even more, since such long, continuous, and species-rich sedimentary records with long-lived lineages are rare in the world, perhaps even among the very few of their kind.

Figure 1. Topographic and bathymetric map of Lake Ohrid with the DEEP drilling site. A) Location of Lake Ohrid on the Balkan Peninsula. B) Bathymetry of Lake Ohrid with the location of the DEEP drilling site. C) Seismic profile of Lake Ohrid showing undisturbed and continuous sediments at the DEEP drilled site. 

Cyclotella cavitata, with its almost one million years of existence, stands out as one of the longest-lived species in the lake. We carried out morphometric analyses of this taxon, together with assessments of its community structure, species abundance, and palaeoecological data, at high temporal resolution (ca. 2,000 to 6,000 years) to trace the species’ morphological evolution through this long-term period of its existence.

Figure 2. Morphological variability of the species Cyclotella cavitata over time. Only key time points are shown: 1040, when the species first appeared in the lake, and 630, when major shifts in the diversification process occurred, marking two distinct phases of species diversification. In the next figure (Figure 3), only part of the first diversification phase is represented. The second phase is still unpublished data.

Figure 3. Pattern of species evolution illustrating the first phase of species diversification. Each entity is listed with a distinct name; all are part of the Cyclotella cavitata species complex. Selected pollen and geochemical data are included to represent the dynamics of local and global environmental changes during the investigated time-period [from 1,045 thousand years ago (= 1Milin years ago) to 815 thousand years ago; Zaova et al. 2022].  

The results revealed remarkable intraspecific morphological variability, with more than twenty distinct entities (Fig. 2). The investigation of their morphological changes over time revealed an unusual pattern of species evolution. Appeared in the lake around one million years ago, this taxon exhibits a unidirectional evolutionary pattern that can be divided into two phases. In the first, the species expanded in the lake through step-by-step gradual diversification, accompanied by the emergence of long-lived co-existing morphologies, rapidly becoming the lake’s dominant species complex (Fig. 2; partially published in Zaova et al., 2022). This pattern of diversification was later disrupted by rapid, punctuated shifts and extinction events that coincided with extreme environmental changes (~630 thousand years ago; unpublished data). These extinction events are most probably associated with the end of the Mid-Pleistocene Transition, a period globally recognized for its profound reorganization of Earth’s climate system with marked changes in the duration and intensity of climate cycles (Lisiecki & Raymo, 2007).

After the extinction events, the diversification of the taxa resumed in the subsequent period but following a completely different pattern. In this phase, short-lived morphologies appeared in the lake, typically as solitary forms (rarely co-existing, often only during transitional periods) and within extremely short time intervals of existence. The reasons for these rapid and frequent morphological shifts for the period after 630 ka are still unclear. Are they related to more extreme glacial-interglacial cycles? Did intra- and interspecific competition play a larger role? Or were other environmental or ecological factors driving the evolutionary dynamics of the taxon? These remain open research questions for the further study.

However, what we have learned so far, shows that the species evolution is complex and cannot always be described as a simple, uniform pattern. It rather consists of multiple distinct patterns that can change and alternate over time – i.e. long-term stability with gradual diversification, and rapid, punctuated turnover – closely tied to external conditions, which can vary considerably over long timescales.

Further scientific discoveries

Only the dataset showing the dynamics and temporal progression of the first phase of this evolutionary history has been published in detail so far. We are currently working on releasing the complete dataset, covering the species’ entire lifespan. This will provide additional evidence for the links between environmental change and evolutionary patterns and modes of this taxon. Particular attention will be paid to valve size, as especially interesting evolutionary patterns are observed through this variable.

Until the complete dataset is published, we invite you to explore the fascinating world of unique fossil diatoms presented in ‘Fossil Diatom Diversity in Lake Ohrid over the Last 1.36 Ma’ (Zaova et al. 2025). We believe this publication will increase your curiosity for Lake Ohrid, as we truly believe that is one of the most unique places on Earth!

REFERENCES

Lisiecki, L.E., Raymo, M.E. (2005). A Pliocene-Pleistocene stack of 57 globally distributed benthic d18O records. Paleoceanography 20. https://doi.org/10.1029/2004PA001071.

Zaova, D., Cvetkoska, A., Wagner, B., Francke, A., Vogel, H., Levkov, Z., & Jovanovska, E. (2022). Diatom community responses to environmental change in Lake Ohrid (Balkan Peninsula) during the mid-Pleistocene Transition (Quaternary International, 622, 1–9. https://doi.org/10.1016/j.quaint.2022.03.001

Zaova, D., Jovanovska, E., Cvetkoska, A., Ognjanova-Rumenova, N. & Levkov, Z. (2025). Fossil diatom diversity in Lake Ohrid over the last 1.36 Ma. In: Iconographia Diatomologica Annotated Diatom Micrographs. Vol. 27. (Van de Vijver, B. Eds), pp. 1–749. Oberreifenberg: Koeltz Botanical Books.