Diatom of the Month December 2018 - Pantocsekiella ocellata complex

Melina Feitl writes about links between diatom valve size and climate variability

Why Pantocsekiella ocellata?

Diatoms are widely used proxies for reconstructing past climatic and environmental change because of their ecological sensitivity to physical and chemical lake characteristics [1]. These fossil reconstructions are often based on the known ecological ranges of modern taxa.  The Arctic, although particularly interesting climatically, has relatively sparse studies on diatom taxonomy, ecology, and biogeography because of logistical issues involving its remote location. 

One of the most important members of the Arctic (and sub-arctic) diatom flora is the planktic species Pantocsekiella ocellata (Pantocsek) K.T. Kiss & E. Ács, formerly reported as Cyclotella ocellata Pantocsek.  P. ocellata was first described by Pantocsek in 1902; valves were circular with a marginal ring of striae and a flat valve face with 3 large “pearls”.  Since its original description, a number of studies have yielded a much wider range of morphological variability (Table 1).  In a 2018 study by Cvetkoska [2] and others different variations of P. ocellatawere associated with different lake physico-chemical characters. Understanding the relationship to valve variability and lake physico-chemical characteristics will lead to more accurate paleoecological reconstructions.

Where?

The Lake El’gygytgyn (Lake E), located in northeastern Siberian Russia (Fig. 1) is an approximately 3.6 million-year-old impact crater lake. Locally, there is no evidence of glaciation, making Lake E the longest continuous terrestrial archive of Arctic paleoclimate. The Lake E Drilling Project was created, which resulted in the collection of a well-dated sediment core that documents glacial-interglacial climatic periods. Members of the P. ocellata complex dominate the planktic assemblage for the last 1.2 million years giving us an opportunity to observe morphological variability through time, and how that variability is related to paleoclimatic change.

Fig.1 Location and bathymetry map of the Lake El’gygytgyn (Russia)

What did I do? 

My thesis research involved characterizing morphological variability in the P. ocellata complex throughout the last 1.2 million years using light microscopy (LM) and scanning electron microscopy (SEM). First, quantitative features were measured on both external and internal valves from SEM photographs (Fig. 2). The external valves were considered to show the widest array of variability, and so only those valves were used for multivariate analysis. Since size can be highly influential on valve morphology, the valves were divided into four different size classes (0-5 μm, 5-10 μm, 10-15 μm, and >15 μm), and a Principal Component Analysis (PCA) was run on each size class to evaluate morphological patterns independent of size. 

Fig. 2 Measurements taken on SEM valves. Top: external. Bottom: internal. (D1: valve diameter, D2: central area diameter, RD: distance of the rimoportulafrom the margin, Striae: number of striaein 10 µm,Costae: number of costaein 10 µm, CF: number of central fultoportulae, MF: number of marginal fultoportulae)

Plate showing valves typical of class 3 (10-15 μm) in the Lake El’gygytgyn record

What are the results?

The P. ocellata complex in Lake E exhibited a wide array of morphological variability (Table 1).  The majority of individuals fell between 5-15μm.  PCA on different size classes reveals that in valves of larger size, morphology is variable through time (Fig. 3).  From present to 220 ka (thousand years before present), morphology is highly variable with individuals plotting scattered across the PCA.  From 220 to 550 ka, morphology is more restricted on the two axes, with a similar pattern occurring from 550 to 1.2 ka.  The most variable valve features in this record are the number/orientation of central fultoportulae, number/orientation of the depressi, and number/orientation of the puncta.  The highly distinct and constrained P. ocellata morph from 220 to 550 ka constitutes a new subspecies in the P. ocellata complex distinct to Lake E. Images of typical P. ocellata valves observed during the different time periods can be seen below. We observed that most changes in the dominant P. ocellata morphology occur after sedimentologically and geochemically inferred extreme cold events in the lake (13), where diatoms are completely absent or sparse.  These cold events may be driving these species variability shifts.  Alternatively, during some time spans, P. ocellataco-occurs with another large-valved planktic diatom, Pliocaenicus.  Valve size in P. ocellatadecreases during these intervals, perhaps suggesting biotic competitive exclusion [3]

Fig. 3 Principal Components Analysis (PCA) on the different size classes of P. ocellatain the Lake El’gygytgyn. (Blue: Recent-220 ka, Red: 220-550 ka, Green: 550-1.2 ka).  Separation of valves are observed through time demonstrating shifting morphology. Class 1: 0-5 μm; Class 2: 5-10 μm; Class 3: 10-15 μm; Class 4: >15 μm.

*Melina Feitl is a PhD student at the Department of Earth and Atmospheric Science in the University of Nebraska-Lincoln. Her research centers around questions of past ecological and climatic change in tropical South America.

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References

1. Douglas MSV, Smol JP (2010) Freshwater diatoms as indicators of environmental change in the High Arctic, in: The Diatoms: Applications for the Environmental Earth Sciences Second Edition, edited by: Smol JP, Stoermer EF, Cambridge University Press, Cambridge, 249-2662. 
2. Pantocsek J (1902) Kieselalgen oder Bacillarien des Balaton. Resultate der Wissenschaftlichen Errorschung des Balatonsees, herausgegeben von der Balatonsee-Commission der Ung. Geographischen Gessellschaft. Commissionsverlag von Ed. Hözel. Wien 2: 112 pp, 17 pls.
3. Feitl M & Snyder J Morphological analysis of the Pantocsekiella ocellata species complex (Bacillariophyceae) from the Lake El’gygytgyn, northeastern Russia. In preparation