Diatom of the month - March 2018: Afrocymbella barkeri

by Heather Moorhouse*

The tropical diatom genus Afrocymbella has only 12 known species, all of which are found in the African Rift Valley lakes¹. They have been observed both free-living in the water column and attached to rocks and plants, and are solitary or colonial¹. One such species is Afrocymbella barkeri Cocquyt & Ryken sp. nov. (19.9-63.8 µm) (Fig. 1), a newly described diatom found in Lake Chala¹ (Fig. 2), a tropical crater lake that lies directly on the border of Kenya and Tanzania, just south of the equator (Fig. 3). 

This species was named after Prof. Philip Barker whose seminal work on diatoms in the East African Rift valley lakes has helped understand past climate and environmental change in the region. Afrocymbella barkeri is common at the end of the dry and windy season in Lake Chala, which corresponds to the northern hemisphere summer. The summer winds mix the lake water column and cause nutrient-rich deep water to rise to the surface, providing the nutrients that fuel diatom blooms. As paleolimnologists, we can use observations of how the current climate shapes diatom communities to help reconstruct historical environmental changes using fossilized diatoms found in lake sediment records.

Fig. 1. SEM images of Afrocymbella barkeri Cocquyt & Ryken sp. nov. found in the sediments of Lake Chala. Image taken by H.Moorhouse.

Lake Chala is a deep crater lake (maximum depth of 97 metres, 4.2 km² in size) which lies on the lower eastern flanks of Mount Kilimanjaro, Africa’s tallest mountain and a dormant volcano. Lake Chala is a hydrologically simple system as it has no major river inflows or outflows, which makes it an ideal study site to investigate changes in precipitation versus evaporation of lake water. Currently, more water is lost from evaporation than is replaced by rainfall at Chala, but the lake water level is maintained by subsurface or groundwater flows from rainfall that has fallen on the Mount Kilimanjaro area and seeped underground. 

Fig. 2. Lake Chala, a tropical crater lake. Image courtesy of Loes van Bree.

Fig. 3. Location of Lake Chala in eastern Africa.

Interestingly, the diversity of diatoms in Lake Chala is extremely low, with assemblages dominated by Nitzschia and Afrocymbella species. This may be a result of the lake’s isolation or lack of in-lake habitat diversity. Nevertheless, whilst we still know relatively little about the ecology of the diatoms in this lake, we can use other information on the chemistry of diatom cells to reconstruct environmental change.

Fig. 4. SEM images of Nitzschia spp. found in the sediments of Lake Chala, which, along with Afrocymbella spp., dominate this lake's diatom community. Image taken by H. Moorhouse.

This is the one of the motives behind the DeepCHALLA project, an International Continental Scientific Drilling Programme which aims to study over 214 meters of sediment cores retrieved from the depths of Lake Chala. The sediment in these cores are estimated to have been deposited as far back as >250,000 years, allowing scientists the opportunity to investigate long-term changes in climate, the lake and surrounding terrestrial ecosystems, volcanic activity, and the role of these in shaping human evolution. We are particularly interested in the African Megadroughts period that occurred between 90,000 and 130,000 years ago and is thought to have caused the dispersal and evolution of our modern human ancestors. The fossilized diatoms of Lake Chala will be key to discern the nature of the African Megadroughts and will help identify how the lake and surrounding terrestrial ecological communities responded to prolonged aridity.

My role in the DeepCHALLA project is to clean sediment samples so all that remains are pure fossilized diatoms. We will then look at the stable isotopes of oxygen and carbon found in the diatom silica. Stable isotopes are elements with the same number of protons, but different number of neutrons. Oxygen and carbon isotopes provide snapshots of what the ambient environment of the diatoms was like at certain points in time, because we know different environmental conditions effect their cell isotopic composition. Diatoms are useful hosts of stable isotopes because their silica frustule (cell wall) acts as protection against degradation. Different layers of the diatoms silica frustule host different isotopes; oxygen is found in its inner isotopically homogenous silica layer, protected by the outer layer, while carbon is found in the organic inclusions or proteins that form within the silica frustule.

By looking at the oxygen isotopes captured by the diatoms, we can estimate the amount of precipitation to that of evaporation of the lake water. We measure the oxygen isotope (δ¹⁸O_diatom) which is calculated by analyzing the ¹⁸O:¹⁶O ratio³. Higher values mean that more of the lighter ¹⁶O has evaporated out of the lake; the heavier ¹⁸O left behind is then incorporated by diatoms into their frustules³. This is a great way to reconstruct the past hydro-climate, as we can derive periods of aridity from heavier oxygen values.

We are also going to investigate the occluded carbon (δ¹³C_diatom) within the diatom silica, which is determined by the ratio of ¹²C and ¹³C and broadly tells us about changes to the supply and demand of carbon over time⁴. For example, previous work found that high values of δ¹³C_diatom during the Last Glacial Maximum (which resulted in dry conditions), may be explained by not only high diatom productivity, but also inputs of drought-adapted terrestrial vegetation⁴. This will help us to understand how climate can shape carbon cycling in lakes, which are integral components of the global carbon cycle.

Ultimately, the stable isotopes of oxygen and carbon found in diatom silica will help provide unique insights into tropical climate and carbon cycling over two glacial-interglacial cycles. Understanding long-term variation in climate and its impact on ecosystems is important to more accurately predict future climate change in this drought-sensitive region. Diatoms greatly help us understand environmental history; environmental geochemistry techniques, such as stable isotope analysis, can further complement diatom taxonomy and ecology.

*Diatom Isotope Research Technician, Lancaster University

References

1. Cocquyt, C. and Ryken, E., (2016). Afrocymbella barkeri sp. nov.(Bacillariophyta), a common phytoplankton component of Lake Challa, a deep crater lake in East Africa. European Journal of Phycology 51: 217-225.

2. https://www.icdp-online.org/projects/world/africa/lake-challa-kenya-tanzania/details/

3. Leng, M.J. and Barker, P.A., (2006). A review of the oxygen isotope composition of lacustrine diatom silica for palaeoclimate reconstruction. Earth-Science Reviews 75: 5-27.

4. Barker, P.A., Hurrell, E.R., Leng, M.J., Plessen, B., Wolff, C., Conley, D.J., Keppens, E., Milne, I., Cumming, B.F., Laird, K.R. and Kendrick, C.P., (2013). Carbon cycling within an East African lake revealed by the carbon isotope composition of diatom silica: a 25-ka record from Lake Challa, Mt. Kilimanjaro. Quaternary Science Reviews 66: 55-63.