Diatoms: the jewels of palaeoclimatology

Jack Wilkin (UK)

My PhD at the University of Exeter focuses on using micropalaeontology and various geochemical methods from Holocene marine sediment cores, to try to find out how the climate of South Georgia has changed over the past 15,000 years. One of the microfossil groups I’m using to achieve this is diatoms (Fig. 1).

Diatoms are microscopic algae that play a vital role in marine ecosystems. They have circular or elliptical to rod-like shapes and are perforated by minute apertures called areola. They are divided into two orders: the Pennales and the Centrales. Diatoms can reproduce both sexually and asexually, using a unique “shrinking division” form of a sexual reproduction called binary fission. They are photosynthetic and form the basis of food chains in many aquatic ecosystems. Diatoms also have a wide range of geological applications, especially in palaeoclimate studies.

What do diatoms look like?

Diatoms are usually colonial, photosynthetic, single-celled algae, made of biogenic silica, with an estimated 20,000 to two million species. One could imagine them as microscopic plants that live inside tiny glass greenhouses, stacked on top of each other. They form as iliceous shell, known as a frustule, which can range in length from 1μm to 2000μm, depending on species, with most species falling within the 10μm to 100μm range. The frustules is sometimes compared to a pillbox, as it consists of two overlapping values or thecae, with the larger (the epitheca) fitting over the smaller one (the hypotheca) in a box-like fashion, to enclose the living (protoplasmic) mass (Fig. 2).

The thecae are linked together by a thickened ring of silica called a girdle band (also called a copuae), and the different valves often separate after death. The frustules are commonly circular (centric) or elliptical to rod-like (pennate) in shape, and this morphology forms the basis of diatom classification (Fig. 3).

The frustules are perforated by minute apertures called areola (plural areolae), which can cover from 10% to 30% of the valve’s surface area. The shape and arrangement of the areolae are among the most important diagnostic characteristics of diatoms. The areolae are often arranged into lines called striae.

Diatoms are divided into two orders: the Pennales and the Centrales, which are then subdivided into five suborders and numerous families (detailed in Fig. 2). Members of the order Pennales, the pinnate diatoms, have frustules that are elliptical or rectangular in shape, and process bilateral symmetry along the central line. In one group of pennates – the Raphidineae – this central line is comprised of a longitudinal unsilicified groove running down the middle called the raphe. The raphe allows for the flow of mucus that enables the diatom to move around.

The order Centrales (syn. Coscinodiscophyceae) have frustules that are circular, triangular or quadrate. Most Centrales are planktonic and non-motile, with all taxa lacking raphe.

Lifecycles

Diatoms can reproduce both sexually and asexually. They have a unique “shrinking division” form of asexual reproduction called ‘binary fission’. During this process, two new daughter cells form within the parent cell frustule. Each daughter cell receives one parent cell theca that becomes the overlapping epitheca valve.

Because the parent cells theca cannot expand, the daughter cells gradually get smaller and smaller with each successive generation. After enough generations, the daughter cells become so small that they cannot divide any further, so resort to sexual reproduction that “resets” the next generation to its original size (Fig. 4).

Diatomecology

Diatoms are aquatic, being found in a wide range of environments from freshwater to hypersaline, and even soils. Their habit can include bottom-dwelling (benthic), attached (epiphytic = attached to plants; epilithic = attached to stones), and free-floating (planktonic) forms.

In the sea, diatoms can be found in the intertidal zone, lagoons, shelf seas, and the open oceans (but only within approximately the top two hundred meters). The freshwater, more terrestrial and marine benthic are dominated by pennates, while the centrics tend to be more planktonic in the oceans, especially in the subpolar regions.

The cells may be single or colonial, the latter bound together by mucous filaments bands into long chains (Fig. 5).

Much of the “pond scum” you see in stagnant water and the green discolouration on icebergs (Fig. 6) are diatom colonies.

Diatoms are photosynthetic and form the basis of food chains in many aquatic ecosystems. They are the largest producers of organic matter in the ocean, supporting the ocean ecosystems and accounting for approximately one-fifth of global photosynthesis and 40% of marine primary production. Each year, these tiny plants generate about the same amount of oxygen as all the rainforests combined. Being a major primary producer, diatoms form the foundations of most marine ecosystems.

Geological history

The origin of the diatoms remains enigmatic. While molecular data indicate an origin during the Triassic or Early Jurassic (see Medlin, 2011), earliest unequivocal recorded diatom frustules are centrics from the Late Jurassic amber deposits from Thailand (Girard et al., 2020). However, they remain rare until the Late Cretaceous.

Pennates are thought to have first appeared during the Upper Cretaceous (see Harwood et al., 2007), and raphid pennates in the Palaeocene. It seems that the Mesozoic diatoms were mostly marine, with large-scale colonisation of freshwater environments occurring during the Cenozoic.

Forty six percent of diatoms survived the end-Cretaceous extinction, with planktonic genera showing substantially higher survivorship rates (50% to 90%), relative to benthic genera (10% to 40% survivorship). This high survivorship might be due to the ability of modern planktonic (but not benthic) diatom species to form resting stages.

However, modern marine diatom resting stages have not been known to remain viable for more than two years. This suggests that, if resting stages were the mechanism used by diatoms to survive the K-Pg event, the event must have had a very short effect that limited diatom proliferation.

Diatoms are very common in Cenozoic sediments. Periods of turnover in diatoms species have coincided with steps in global cooling leading to the increasing latitudinal thermal gradients through the Cenozoic. Before the Oligocene, diatom assemblages were dominated by robust species, which became progressively replaced by more finely silicified forms.

This trend continued throughout the Cenozoic and has accompanied global cooling, more vigorous circulation, and raised nutrient levels. Therefore, it seems that diatoms have adapted to the increasing competition for scarce silica in surface waters, by reducing the need for silica in their frustules and girdle bands.

Applications (geosciences)

The abundance and wide distribution of diatoms make them ideal tools for a wide range of applications in geology.  Issues to overcome: Diatoms are very light, so can be easily transported hundreds of miles. Therefore, assemblages may contain some allochthonous diatoms. Another potential cause of uncertainty is the selective destruction of diatoms. Complete or partial dissolution of the frustules can occur under pressure and the less robust/more weakly silicified forms tend to dissolve in very alkaline conditions.

In such a context, the diatom assemblage would be biased towards the more robust and more heavily silicified forms. Reworked (or derived fossil) diatom frustules can sometimes be detected in samples, when some of the hypotheca seem to have been subjected to differing degrees of mechanical abrasion or dissolution. However, derived diatoms within an assemblage may be difficult to differentiate from in-situ material. Diatomaceous earth: Diatoms are so common that they can form their own sedimentary rock, called diatomaceous earth. This siliceous rock has many important economic uses including:  

• acting as filtration aids (even removing radioactivity from liquid nuclear waste);
• as mild abrasives in metal polishes and toothpaste;
• for absorbency in cat litter;
• as a stabilizing component in dynamite.

 Zone fossils: Diatoms have been used as zone fossils in Cretaceous and Cenozoic successions. The use of diatoms in biostratigraphy is outlined by Barron and Baldauf (1995) and Scherer et al. (2007).

Palaeoenvironmental analysis: Individual species can be extremely sensitive to physical and chemical conditions, so provide a valuable tool for studies of modern water quality and for the reconstruction of palaeoenvironments. The palaeoenvironmental and palaeoecological value of diatoms is very well established, particularly during the Cenozoic of the Arctic and Southern Oceans.

The analysis of diatoms has provided detailed insights into a wide range of palaeoenvironmental issues, such as:  

• the reconstructions of changes in water levels;
• variation in water chemistry;
• temperature variations;
• sea-ice/marine-terminating glacier proximity; and
• disturbances caused by human activity.

Stable isotopes from diatoms have been used as a geochemical proxy. The δ¹⁸O values of the silica within diatom frustules have been used as an absolute palaeotemperature proxy in Quaternary deposits, although it is vital effects cannot be fully discounted (for example, Swann et al., 2007; Swann and Leng, 2009).

The δ¹³C diatom records have been used to model whether the Southern Ocean was a source or sink for carbon dioxide during the last glacial (for example, Jacot DesCombes et al., 2008). However, because diatoms are so small, it requires a huge amount of laboratory work to be able to extract stable isotope data from them.

Final thoughts

I like diatoms – they are one of the most interesting and beautiful type of microfossils to study. And their fossils have been so important for reconstructing Cenozoic climate change; and in the Arctic and Southern oceans, diatoms are often the only microfossils abundant enough to use as a palaeoclimate proxy. Today, diatoms produce more oxygen than all the rainforests combined, forming the foundations of entire ecosystems. They are the unsung heroes of life on Earth and their protection is paramount for the survival of entire ecosystems.

Further reading and references

Armbrust, E.V. (2009) The life of diatoms in the world’s oceans. Nature 459: 185-192: https://www.nature.com/articles/nature08057.

Armstrong, H.A. and Brasier, M.D. (2005) Microfossils (2^nd ed.) Blackwell Publishing, Oxford. Barron, J. and Baldauf, J. (1995) Cenozoic marine diatom biostratigraphy and applications to palaeoclimatology and paleoceanography. Short Courses in Paleontology 8: 107-118: https://doi.org/10.1017/S2475263000001446.

Girard, V. et al. (2020) Thai amber: insights into early diatom history. BSGF – Earth Sciences Bulletin 191(23): https://doi.org/10.1051/bsgf/2020028.

Harwood, D.M. et al. (2007) Cretaceous records of diatom evolution, radiation, and expansion. Paleontological Society Papers 13: 33-59: https://www.cambridge.org/core/journals/the-paleontological-society-papers/article/abs/cretaceous-records-of-diatom-evolution-radiation-and-expansion/893A75A1319567566873DE9B75AF2BC3.

Hasle, G.R. and Syvertsen, E.E. (1996) Marine diatoms. In Tomas, C.R. (ed) Identifying marine diatoms and dinoflagellates. Academic Press, New York: 5-386.

Jacot DesCombes, H. et al. (2008) Diatom δ¹³C,δ¹⁵N, and C/N since the Last Glacial Maximum in the Southern Ocean: Potential impact of Species Composition. Paleoceanography and Paleoclimatology 23(4): https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2008PA001589.

Medlin, L.K. (2011) A review of the evolution of the diatoms from the origin of the lineage to their populations. In Seckbach, J. and Kociolek, J.P. (eds) The diatom world. Dordrecht, Springer: 95-118: https://doi.org/10.1007/978-94-007-1327-7_4.

Scherer, R. et al. (2007) Methods and applications of Cenozoic marine diatom biostratigraphy. The Paleontological Society Papers 13: 61-83: https://www.cambridge.org/core/journals/the-paleontological-society-papers/article/abs/methods-and-applications-of-cenozoic-marine-diatom-biostratigraphy/99A548A884088FD1E610A84D2BCECCF2.

Swann, G.E.A. et al. (2007) Diatom oxygen isotopes: Evidence of a species effect in the sediment record. Geochemistry, Geophysics, Geosystems 8(6): https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2006GC001535.

Swann, G.E.A. and Leng, M.J. (2009) A review of diatom δ¹⁸O in palaeoceanography. Quaternary Science Reviews 28(5-6): 384-398: https://www.sciencedirect.com/science/article/abs/pii/S0277379108003284?via%3Dihub. Theriot, E.C. (2012) Diatoms. eLS: https://onlinelibrary.wiley.com/doi/10.1002/9780470015902.a0000330.pub2