On the other side of Ennerdale Water I could see plenty more submerged stones, all covered with green filaments but these belonged to different genera to those that I wrote about in my previous post. Both are genera that we have met previously – Mougeotia, which has flat, plate-like chloroplasts which rotate around a central axis in order to control its rate of photosynthesis – and Spirogyra. When light levels are low, Mougeotia’s flat chloroplast is perpendicular to the light in order to capture as much energy as possible, but in bright light it rotates so that the plate is parallel to the direction of the light, in order to slow the photosynthesis mechanism down and prevent internal damage (see “Good vibrations under the Suffolk sun” for another approach to this problem).
However, too much sunlight is the least of an alga’s problems in the Lake District. This post looks at a different challenge facing freshwater algae and our starting point is the spherical nodules, “pyrenoids”, that you should be able to see on the chloroplasts of both Mougeotia and Spirogyra in the images below. Photosynthesis involves a reaction between water and carbon dioxide to make simple sugars (turning fizzy mineral water into “pop”, in other words). A submerged alga does not have a problem obtaining the water it needs, but what about carbon dioxide? Gases are not very soluble in water, so this presents a much bigger problem to the algae. Explaining why also presents a big problem to a blogger who conscientiously avoided physics and chemistry from age 16 onwards. Here goes …
Mougeotia from the littoral zone of Ennerdale Water, April 2017. Scale bar: 20 micrometres (= 50th of a millimetre).
The concentration of a gas in a liquid depends upon the concentration of that gas in the surrounding atmosphere. As far as we know (and this is still an area of contention amongst geologists), concentrations of carbon dioxide in the deep past were much higher than they are today, in part because there were no land plants to suck it out of the atmosphere for their own photosynthesis. So the earliest photosynthetic bacteria and, subsequently, algae, lived in water that also had higher concentrations of carbon dioxide. As land plants spread, so the carbon dioxide concentration in the atmosphere dropped as they used it to fuel their own growth. As a result, carbon dioxide concentrations in the water also dropped, thus depriving the algae of an essential raw material for photosynthesis.
However, carbon dioxide is not the only source of carbon available to aquatic organisms. There is also carbon in many rocks, limestone in particular, and this can mineralise to carbonate and bicarbonate ions dissolved in the water. Aquatic plants can get hold of this alternative carbon supply via an enzyme called carbonic anhydrase. By concentrating the carbonic anhydrase activity in a small area of the chloroplast, the algal cell can boost the activity of the Rubisco enzyme (which evolved to function at a higher concentration of carbon dioxide). This whole process is one of a number of forms of “carbon concentrating mechanism” that plants use to turbocharge their photosynthetic engines (see “CAM, CAM, CAM …” on my wife’s blog for more about a terrestrial version of this).
A two-chloroplast form of Spirogyra from the littoral zone of Ennerdale Water, April 2017. Scale bar: 20 micrometres (= 50th of a millimetre).
Pyrenoids are widespread amongst algae, though a few groups (notably red algae and most chrysophytes) lack them. Cyanobacteria (blue-green algae) use an organelle called a “carboxysome” for a similar purpose. The only group of land plants with pyrenoids are the hornworts, relatives of mosses and liverworts. About half of all hornworts have pyrenoids and a recent study has suggested that the ability to form pyrenoids has evolved up to five times in this group during their evolution. The appearance of pyrenoids in distinct evolutionary lineages of algae also suggests that there may have been several evolutionary events that precipitated their formation. And, it is important to stress, some algae which lack pyrenoids have alternative methods of concentrating carbon to enhance Rubisco activity.
So let us end where we started: in the littoral zone of Ennerdale Water on an April morning, gazing at a fine “fur” of filamentous algae clinging to the submerged rocks. Back in October last year, I talked about how Ennerdale fitted into a pattern of increasing productivity of Cumbrian lakes first noticed by Pearsall in the early part of the 20th century (see “The power of rock …”). Now we can start to understand that pattern in terms of basic biochemical processes: getting enough carbon from a combination of atmospheric carbon dioxide and the surrounding rocks for Rubisco and the other photosynthetic enzymes to convert to sugars. In Ennerdale Water, one of the least productive of the Cumbrian lakes, we can see these algae during the winter and spring because the amount of biomass that those biochemical reactions produces is still just ahead of the amount that grazing invertebrates such as midge larvae can remove. In a month or so, the grazers will have caught up and the rock surfaces will be, to the naked eye at least, bare.
Rubisco is the enzyme whose gene, rbcL, we use for molecular barcoding, subject of many recent posts (see “When a picture is worth a thousand base pairs …”). My early desire to avoid physics and chemistry at school translated into as little biochemistry as possible whilst an undergraduate and, over the past few-years, I’ve developed a frantic urge to catch-up on all that I missed. Just wish that those lectures explaining the Calvin cycle had been a little less … tedious …
Giordano, M., Beardall, J. & Raven, J.A. (2005). CO2 concentrating mechanisms in algae: mechanisms, environmental modulation, and evolution. Annual Review of Plant Biology 56: 99-131.
Villareal, J.C. & Renner, S.S. (2012). Hornwort pyrenoids, carbon-concentrating structures, evolved and were lost at least five times during the last 100 million years. Proceedings of the National Academy of Science of the USA 109: 18873-18878.