Species showcase: Desmid

Desmid 

The winter skies are dark and heavy. White snow falls and blankets the bog. But under the pool’s frozen surface, green snowflakes hang in the water. They are as diverse and beautiful as the white, ice crystal forms in the air above. But they are seldom noticed. These green snowflakes - or desmid algae - represent some of the most beautiful but perhaps most unknown diversity in our peatlands.

We are still discovering new peatlands around the world. We have a fairly good picture of where most peatlands are within the UK, but there are still treasures to discover. Understudied microbial groups - such as testate amoebae, rotifers, tardigrades and desmid algae - are critical to peatland function and their ecological supporting roles are poorly quantified.   

What is a desmid? 

Simply put, desmids are a group of microscopic green algae that are found mostly in fresh (occasionally slightly brackish) waters globally. They are microscopic organisms with a chlorophyll-rich chloroplast and can fix carbon through photosynthesis, just like plants. They are hugely diverse in terms of their species and the cellular forms they take.  ‘Desmid’ is derived from the Greek word ‘desmos’ meaning a bond or chain, but few species actually have colonial or chain-like forms.  

Desmids are characterised by symmetrical cells and broadly have two forms: saccoderm and placoderm: 

• Saccoderm desmids are unconstructed in the middle portion of the cell. 

• Placoderm are composed of two halves and joined in the middle by a narrowing termed the ‘isthmus’.  

Form and function 

Within these two broad types there are a range of different forms which tend to reflect the microhabitat they inhabit. Smoother forms with less surface area inhabit the aerial water films on Sphagnum leaves. Planktonic forms – free-floating in the water column in bog pools - tend to have a larger surface area and a variety of different ‘arms’ or ‘projections’ which extend their surface area and enable them to maintain their position: floating in the water column near the surface where light is amply available.    

A microscope is critical for viewing these organisms and the cell shape, dimensions, chloroplast structure and ornamentation of the cellulose cell wall are all key identification features. Some species have a characteristic mucilage (thick, gluey) coating which helps them adhere to each other, or to their surroundings, and interact with other microbes to help them scavenge scarce nutrients in acidic bogs. This mucilage has also been found to protect them from attack from aquatic fungi .  

This group of green algae play a key role in carbon fixation as photosynthetic organisms, but this has not been well quantified. Studies suggest that phototrophic microorganisms in peatlands, which obtain energy from sunlight, account for between 10-30% of carbon fixation.  Desmids are likely to play a key role in this in peatlands where they are dominant.   

Where can you find them? 

In the UK desmids can be found in lakes, rivers, ponds, puddles and wetlands. But they are particularly diverse and species rich in nutrient poor, acidic bogs. Our best quality and healthiest bog peatlands have the microhabitat structure - hummocks, lawns, pools, aquatic vegetation and open water - which support a diversity of desmid communities.  

Desmid algae are readily collected by scraping the film of algae from leaf surfaces or even squeezing plants like Sphagnum mosses and collecting the resulting water. Free-floating aquatic forms can be swept up using a 30 micron mesh plankton net. Once collected, they will quite happily sit in a bottle on a windowsill for a few weeks. If community composition is the reason for sampling, then the species present at the time of collection can be preserved in Lugol’s iodine or formaldehyde to prevent them being eaten by other organisms in the sample.  

Desmid algae are tiny - some are as small as 50 micron long, meaning you could fit 20 individuals end to end between two millimetre marks on a ruler! Because they don’t take up much space, many thousands of individual cells can exist on the thin water film around a Sphagnum leaf. In a study of Scottish blanket mires¹, one of the richest sites contained over 200 species of desmid algae in a single droplet of water on a microscope slide. The species list from that same site totalled over 300 species of desmids alone. When you consider all the other microscopic plants, animals, fungi and bacteria that will be present on a high-quality site, you can really start to appreciate how understudied peatland microscopic life is and just how much invisible life is bustling around beneath your feet! 

Distribution and Threats 

The distribution of known species is poorly understood. We do know desmid algae are species rich in good quality bog habitats, especially blanket bogs. We also know these organisms are lost (or a large proportion of the diversity is lost) when peatlands are degraded. Draining and drying of the peat removes the aquatic microhabitats they require and decreases the availability of other niches offered upon a diversity of Sphagnum species.  Nutrient increases or changes in chemistry mean they are quickly outcompeted by other species (e.g., other green algae, diatoms, etc.) and lost from sites.  

The ‘Important Plant Areas for algae’ report in 2007² highlighted a variety of peatland areas across the UK which should be considered of high conservation concern for the preservation of groups such as desmid algae. Multiple studies have shown since the early 1800s that desmids are particularly species rich in healthy wetlands and peatlands.  This is yet another reason why preservation of healthy peatlands is so critical.  

Desmid community response to environmental change – Emma Hinchcliffe

Microbial communities are key to nutrient cycling in peatlands: primary producers undertake the initial fixing of carbon and other nutrients and others, such as bacteria and fungi, play a role in the release of these nutrients. Most climate impact analyses (generally, and for peatland ecosystems) have investigated a small sub-sample of net biodiversity (see Table 1.3), often excluding organisms which perform important under-pinning ecosystem functions. This is particularly true for microorganisms, which can be critically important in ecosystem function, but being difficult to identify and to quantify ecologically, they have therefore been severely neglected in climate impact analyses.

Microorganisms respond rapidly to changing environmental conditions, such as the pH gradient in patterned mires³. This rapid response is evidenced by changes in distribution and microbial community structure⁴ and has been attributed to the physical size of the organism and reduced physiological resistance to external factors³.

As a study group, desmids offer a potentially sensitive ecological guild with which to investigate microorganismal changes driven by climate and other anthropogenic impacts. Within the limiting environmental conditions of the blanket bog habitat, desmids show remarkable sensitivity and marked biogeographic structure across climatic gradients – pointing to their possible sensitivity to climatic impacts on peatland hydrology and chemistry. However, the majority of desmid research over the last 200 years has been taxonomic, with studies concentrating on community structure focused locally at small sites, and there is a noticeable lack of work which draws together knowledge on specific species niches and the ecological functioning of desmid groups⁵. There have been recent calls by peatland experts⁶ to fund research which will allow us to characterise and understand the underpinning function of microbial communities. This situation with respect to desmids – and microorganisms more generally - represents a key knowledge gap in our understanding of peatland ecological dynamics and trophic interactions.

There are several bodies of work which suggest that desmid algae - and other microbial communities - do respond positively to habitat restoration work. In the only structured sampling of desmid algae to date in Scotland, it was found that the desmid community responded positively to restoration with a c. 70% recovery of the community 12 years post-restoration compared to nearby intact peatlands¹. Similar results have been reported in other studies, perhaps indicating that it is the recovery of mosses and the microhabitats they provide that could be a key driver of changes occurring in the structure of the microbial communities in restored peatlands⁷. Microbial functional diversity was found to be reduced in damaged or recovering peatlands⁸ and this could partially be linked to altered physical surface structure of the peatland vegetation, passive dispersal meaning it takes time for these microscopic organisms to recolonise, or other unknown factors.  

We cannot rely on conservation designations alone to protect these understudied groups. Future conservation strategies for peatlands must ensure they give utmost importance to protecting whole ecosystem function and limit outside threats: this approach is essential to ensure we do not neglect underappreciated peatland riches, such as the jewel-like desmid algae, and other unknown treasures which our peatlands have yet to reveal to us.  

References

1. Goodyer E. Quantifying desmid diversity of Scottish Blanket Mires. PhD thesis, University of Aberdeen; 2014. 

2. Brodie J, John DM, Tittley I, Holmes MJ, Williamson DB. Important Plant Areas for algae: a provisional review of sites and areas of importance for algae in the United Kingdom. Plantlife International. 2007. 

3. Hájek M, Poulíčková A, Vašutová M, Syrovátka V, Jiroušek M, Štěpánková J, et al. Small ones and big ones: cross-taxon congruence reflects organism body size in ombrotrophic bogs. Hydrobiologia. 2014;726(1):95–107. doi: https://doi.org/10.1007/s10750-013-1754-8. 

4. Klemenčič AK, Smolar-Žvanut N, Istenič D, Griessler-Bulc T. Algal community patterns in Slovenian bogs along environmental gradients. Biologia. 2010;65(3):422–37. doi: https://doi.org/10.2478/s11756-010-0033-7.  

5. Domozych D, Domozych C. Desmids and biofilms of freshwater wetlands: development and microarchitecture. Microbial Ecology. 2008;55(1):81–93. doi: https://doi.org/10.1007/s00248-007-9253-y.  

6. Ritson JP, Alderson DM, Robinson CH, Burkitt AE, Heinemeyer A, Stimson AG, et al. Towards a microbial process-based understanding of the resilience of peatland ecosystem service provisioning - A research agenda. Science of the Total Environment. 2021;759:143467. doi: https://doi.org/10.1016/j.scitotenv.2020.143467. 

7. Andersen R, Grasset L, Thormann MN, Rochefort L, Francez AJ. Changes in microbial community structure and function following Sphagnum peatland restoration. Soil Biology and Biochemistry. 2010;42(2):291-301. doi: https://doi.org/10.1016/j.soilbio.2009.11.006. 

8. Andersen R, Wells C, Macrae M, Price J. Nutrient mineralisation and microbial functional diversity in a restored bog approach natural conditions 10 years post restoration. Soil Biology and Biochemistry. 2013;64:37-47. doi: https://doi.org/10.1016/j.soilbio.2013.04.004.