Shortly after I posted my piece on calcification in Chara last week (see “Everything is connected …”), I came across another paper that discussed similar processes in a very different organism. I’ve written about the diatom Didymosphenia geminata before (see “A journey to the headwaters of the River Coquet …”) and commented on the long stalks that it produces. I mentioned in this post that the stalks were composed of carbohydrates and that this may be part of the reason why Didymosphenia can growth in such large quantities in rivers that are naturally nutrient-poor. As carbohydrates are composed of just carbon, hydrogen and oxygen, they can be built by the miraculous rearrangement of carbon dioxide and water that we call photosynthesis.
Although other diatoms produce stalks too, the stalks produced by Didymosphenia are intriguing because they are so much larger than those of other species. An organism that is only a tenth of a millimetre long can produce a stalk ten times as long – enormous, by the standards of the microscopic world. Just as a builder might need to adjust his methods when building a skyscraper, compared to a normal-sized house, so it may be that Didymosphenia has acquired some structural reinforcements to make sure that the polysaccharide stalk can support the cells amidst the rigours of a fast-flowing stream.
A paper by a large team of researchers from Germany, France, USA, Russia and Poland sheds some intriguing light on this subject. At the heart of the story is the same inorganic chemistry that we encountered for calcification in Chara, and the same enzyme, carbonic anhydrase, to enhance the process. In the case of Didymosphenia, however, there are some intriguing differences. The researchers suggest that the stalk of Didymosphenia is strengthened by calcite nanofibres within the polysaccharide matrix. They pointed out that the “foot” of the Didymosphenia cell is rich in mitochondria, which provide the energy for the production of the stalk. Carbonic anhydrase is an enzyme that can both produce bicarbonate and protons from carbon dioxide and water, and the reverse. This means that it can regulate the concentration of carbon dioxide, ensuring a constant supply for photosynthesis whilst, at the same time, preventing a build-up in those parts of the cell that are busily respiring. The same carbonic anhydrase-mediated process that we saw in Chara can take place inside the Didymosphenia cell to capture calcium to build the nanofibres for the stalk. However, intriguingly, a parallel reaction can take place outside the cell.
A long stalk is no advantage to an organism unless it is well-anchored and the suggestion now is that the carbonic anhydrases can generate localised patches of acid conditions that erode the rock surface and allow the stalk to form rhizoid-like “holdfasts” within the substrate. About half of all the carbonic anhydrase activity seems to take place outside the cell, and so contribute to these processes. It is an interesting hypothesis that makes sense when the substrate contains a high proportion of limestone; whether it explains the success of Didymosphenia on other rock types (such as basalt, found in Didymosphenia-rich streams of the Cheviots) remains to be seen. The researchers (who approach the topic from the perspective of materials scientists rather than ecologists) describe the outcome as “mechanically stable and simultaneously very flexible under challenging hydrodynamic conditions of rivers with especially strong flow”.
Other evidence points to stalk production being at least partially controlled by the need to acquire nutrients, so a picture is starting to emerge of a single-celled organism with a range of physiological adaptations that enable it to survive in fast-flowing nutrient-stressed environments where relatively few other organisms can survive. Having grumbled a few times in the past about diatom scientists wanting to know the shape of everything and the meaning of nothing, it is great to see that, in a few cases at least, we are beginning to get a more rounded understanding of the ecology of these fascinating organisms.
Note: the picture at the top of the post shows Didymosphenia stalks smothered in epiphytes, based on material collected from the headwaters of the River Coquet, Northumberland, May 2011.
Bothwell, M.L. & Kilroy, C. (2011). Phosphorus limitation of the freshwater benthic diatom Didymosphenia geminata determined by the frequency of dividing cells. Freshwater Biology 56: 565-578.
Ehlich, H., Motylenko, M., Sundareshwar, P.V., Ereskovsky, A. et al. (2016). Multiphase biomineralization: enigmatic invasive siliceous diatoms produce crystalline calcite. Advanced Functional Materials DOI:10.1002/ADFM.201504891.