I’ve written about a curious group of algae called stoneworts (or charophytes) on a couple of occasions (see “The desert shall rejoice and bloom” and “Croft Kettle through the magnifying glass”. The significance of the name “stonewort” becomes obvious when you pick up a Chara plant, expecting it to be soft and pliable, and are struck by the rough texture of the axes, caused by the deposition of lime.

Chara hispida, photographed by Chris Carter.   Note the main axis and branches, from which whorls of branchlets arise at intervals.

The stoneworts are asssociated with hard water, so this deposition should not be a great surprise (the process by which kettles are coated with lime scale in hard water areas is very similar) however, most of the other plants in these habitats don’t share this property, so what is so special about Chara?   The answer is that, in hard waters, the carbon dioxide that plants need for photosynthesis is in short supply, but much more carbon is available as the bicarbonate ion.   Some aquatic plants can absorb the bicarbonate and then use an enzyme, carbonic anhydrase, to convert this bicarbonate to carbon dioxide. Chara, however, has a different strategy, actively pumping out hydrogen from inside the cells which, in turn, react with the bicarbonate and release carbon dioxide, which can then be absorbed by the plant.   However, as the water is also rich in calcium, a further series of reactions produces insoluble calcium carbonate, generating some additional carbon dioxide in the process As this series of reactions occurs very close to the cells from which the hydrogen ions are leaking, the precipitates end up on the plant surface, creating the rough texture.   The chemistry is way beyond this blog (meaning “… this blogger”) but you can follow it up in the references below.

Marcroscopic view of Chara intermedia showing an internode with a whorl of branchlets, along with spine cells and cortex cells (photograph: Chris Carter).

Another ion that is not very soluble is phosphate and this often gets caught up with the precipitating lime to form calcium phosphate.   This can be beneficial, as this phosphorus might otherwise fuel growth of phytoplankton which, in turn, would shade the Chara.   This means that Chara meadows should be resilient to artificial enrichment of nutrients to a limited extent at least.   However, there is some evidence that this capacity might be much less than was previously thought.   Hawes Water, a small tarn in Lancashire (not to be confused with Haweswater in Cumbria), for example, used to have rich and diverse communities of Chara spp, even in the deepest parts, but now the Chara and other submerged aquatic plants are confined to the shallow margins of the lake.   There is also good evidence of artificial enrichment in this catchment. The surprise is that concentrations of phosphorus in the water are still relatively low, yet the Chara meadows are much reduced compared with their condition fifty years ago.   The team that did this work also looked at another small marl lake, Cunswick Tarn, near Kendal in Cumbria, and found very similar changes.

It suggests a sensitivity to eutrophication that, perhaps, has previously been under-estimated, but it also points to the importance of balancing mechanisms in nature. On the one hand, Chara has some inbuilt capacity to counter-act increased nutrient concentrations. But others have shown that the ability of Chara to precipitate calcium carbonate is, itself, based on the photosynthesis rate.   The Chara meadows will reach a point when their natural capacity to absorb this extra phosphorus will be exhausted and then, as the phytoplankton take advantage of this, the water will get more turbid, reducing the amount of light reaching the Chara.   Less light means less photosynthesis and that will reduce the need for bicarbonate and, in turn, mean less carbonate deposition and less phosphorus removed. The evidence from Hawes Water is that this change happens very quickly: an ecological “domino effect”, if you like. As ever, everything is connected; sometimes in surprising ways.

Chara virgata (with oospores) from the Isle of Skye, photographed by Chris Carter.

Reference

McConnaughey, T. (1991). Calcification in Chara corallina: CO₂ hydroxylation generates protons for bicarbonate assimilation. Limnology and Oceanography 619-628.

Pentecost, A. (1984). The growth of Chara globularis and its relationship to calcium carbonate deposition in Malham Tarn. Field Studies 6: 53-58.

Walker, N.A., Smith, F.A. & Cathers, I.R. (1980). Bicarbonate assimilation by freshwater charophytes and higher plants: I. Membrane transport of bicarbonate ions is not proven. Journal of Membrane Biology 57: 51-58.

Wiik, E., Bennion, H., Sayer, C.D., Davidson, T.A., McGowan, S., Patmore, I.R. & Clarke, S.J. (2015). Ecological sensitivity of marl lakes to nutrient enrichment: Evidence from Hawes Water, UK   Freshwater Biology 60: 2226-2247.

Wiik, E., Bennion, H., Sayer, C.D., Davidson, T.A., Clarke, S.J., McGowen, S., Prentice, S., Simpson, G.L. & Stone, L. (2015). The coming and going of a marl lake: multi-indicator palaeolimnology reveals abrupte cological change and alternative views of reference conditions.  Frontiers in Ecology and Evolution 3:82. doi: 10.3389/fevo.2015.00082.