Diatom of the Month - October 2017: Lindavia (Pantocsekiella) comensis complex
by Elizabeth Alexson*
Twice a year, in early spring and late summer, our lab boards the Environmental Protection Agency’s Lake Guardian research vessel to collect samples as a part of their Great Lakes National Program Office biological monitoring program. Our role is to monitor phytoplankton in all five of the Great Lakes. In addition to assessing modern phytoplankton, we collect cores for paleolimnological investigations to examine historical shifts in diatom indicators.
Figure 1. Photographs taken aboard R/V Lake Guardian: (A) rosette sampler collecting water samples at sunrise, (B) sunset on Lake Huron, (C) box corer collecting sediment cores for paleolimnological analysis.
In our paleolimnology and monitoring work, we’ve encountered an abundance of the small centric diatom Lindavia comensis (Grunow) T. Nakov et al., which forms a diverse complex with high morphological plasticity. A species complex is a group of related species that share many characteristics and can be difficult to tell apart. A recent rise in relative abundance of the L. comensis complex and its closely related species like Lindavia atomus “fine form” (Cyclotella atomus var. 1; Reavie and Kireta 2015) indicates an apparent response of the Great Lakes to atmospheric warming (Reavie et al. 2017; Bramburger et al. 2017).
We distinguished two morphotypes of L. comensis, Lindavia comensis var. 1 and Lindavia comensis var. “rough center with process” (RCWP), by examining modern and fossil specimens from the Great Lakes. We examined specimens with light microscopy (LM) and scanning electron microscopy (SEM) to identify differences in morphological features and used multivariate statistical and autoecological analyses to assess whether these are unique species.
Table 1. LM morphometric data for L. comensis, L. comensis var. RCWP, and L. comensis var. 1.
Lindavia comensis var. RCWP
The valve of L. comensis var. RCWP is disc-shaped with uniform, radial striae around the valve margin. The central region is colliculate (rough or sculptured). Within the central region is a singular off-center fultoportula. In some rare cases valves have 2 or 3 central fultoportulae. Valves have one rimoportula opposite of the central fultoportula.
Figure 2. SEM images of L. comensis var. RCWP. Scale bar = 2 μm.
In the Great Lakes, based on relative cell densities of the most common (>5% of assemblage) algae found, L. comensis var. RCWP is found equally in spring and summer. It is most common (based on relative abundance to other phytoplankton) in Lake Huron, but can be found in all five Laurentian Great Lakes. Lindavia comensis var. RCWP is found in sites with relatively high alkalinity and relatively low chlorophyll a, nitrate, dissolved phosphorus, and turbidity.
Figure 3. LM images of L. comensis var. RCWP
Previously, this diatom has been identified as Lindavia delicatula, and recorded as L. delicatula (Burge and Edlund 2016) and Cyclotella (Lindavia) cf. delicatula (Reavie and Kireta 2015). Though valve morphology is similar between L. delicatula and L. comensis var. RCWP, L. delicatula has a smaller central area (~1/3 of valve diameter according to Houk et al. 2010) and has uneven striae (Houk et al. 2010). Lindavia comensis var. RCWP’s central area occupies 1/2-3/4 of valve diameter and has uniform striae. Also, the colliculate pattern of the central area of L. comensis var. RCWP appears to be different from that presented for L. delicatula specimens in Hustedt’s type material (Simonsen 1987).
Lindavia comensis var. 1
The valve of L. comensis var. 1 is disc-shaped with uniform, radial striae around the valve margin. The central region is tangentially undulate with a pore field on one half of the central area. The pores do not penetrate the valve face. There is one central fultoportula and one rimoportula opposite of the central fultoportula.
Figure 4. SEM images of L. comensis var. 1. Scale bar = 2 μm.
L. comensis var. 1 is most abundant in summer months (Reavie et al. 2014). It is present in all five Great Lakes, but is most prevalent in Lake Huron. Lindavia comensis var. 1 is associated with relatively high temperature, pH, and alkalinity and relatively low chlorophyll a, phosphorus (dissolved and total), and turbidity.
Figure 5. LM images of L. comensis var. 1.
Lindavia comensis’ pore field is uniform and often radial across the entire valve center with little to no tangential undulation. Lindavia comensis var. 1 has a strong undulation, its pore field is limited to one side of the valve, and is smaller than L. comensis (4-6 μm versus 4-12 μm). Lindavia michiganiana, another common taxa in the Great Lakes, has similar but stronger undulation and is larger (5-21 μm versus 4-6 μm).
So, are they different?
A principal components analysis (PCA) of the morphological features of these diatoms belonging to the L. comensis complex, showed a clear separation between L. comensis, L. comensis var. RCWP, and L. comensis var. 1. Because of these morphological differences as well as differences in environmental preferences, the separation of these morphotypes is essential to infer change in the environment and maintain taxonomic precision.
Figure 6. PCA of the morphological variation of the specimens observed using LM. Vectors represent the variables measured/observed.
Why do we care?
The Laurentian Great Lakes are an important resource, yet they are incredibly vulnerable to human influences. As primary producers, diatoms play an important role at the base of the aquatic food web; changes to that diatom community can alter food web dynamics. By using precise taxonomy we are able to detect the most subtle changes in diatom communities, and consequently use results to predict changes in the larger aquatic environment.
Bramburger, A. J., Reavie, E. D., Sgro, G. V., Estepp, L. R., Shaw Chraïbi, V. L., & Pillsbury, R. W. 2017. Decreases in diatom cell size during the 20th century in the Laurentian Great Lakes: a response to warming waters?. Journal of Plankton Research, 39(2), 199-210.
Burge, D.R.L, and Edlund, M.B. 2015. Lindavia delicatula. In Diatoms of the United States. Retrieved September 26, 2017, from http://westerndiatoms.colorado.edu/taxa/species/lindavia_delicatula
Houk, V., Klee, R. & Tanaka, H. 2010. Atlas of freshwater centric diatoms with a brief key and descriptions. Part III. Stephanodiscaceae A: Lindavia, Tertiarius, Discostella. Fottea 10 (Supplement): 1-498.
Reavie, E.D., Sgro, G.V., Estepp, L.R., Bramburger, A.J., Shaw Chräbi , V.L., Pillsbury, R.W., Cai, M., Stow, C.A. & Dove, A. 2017. Climate warming and changes in Lindavia sensu lato in the Laurentian Great Lakes. Limnology and Oceanography 62, 768-783.
Reavie, E.D. & Kireta, A.R. 2015. Centric, Araphid and Eunotioid Diatoms of the Coastal Laurentian Great Lakes. Biblioteca Diatomologica, vol. 62, J Cramer, Berlin.
Reavie, E.D., Heathcote, A.J. & Shaw Chraïbi, V.L. 2014. Laurentian Great Lakes phytoplankton and their water quality characteristics, including a diatom-based model for paleoreconstruction of phosphorus. PLOS ONE 9(8), e104705.
Simonsen, R. 1987. Atlas and catalogue of the diatom types of Friedrich Hustedt, vol. 3, J Cramer, Berlin & Stuttgart.