Concentrating on carbon …

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 …

References

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.

 

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At last … a red alga that really is red …

The bed of some parts of the River Ehen on my last visit had a distinctly pinkish-red hue, which is a sure sign that we are moving towards autumn and winter.   This is Audouinella hermanii, which we have already met in earlier posts.   Having written recently about a green alga that is actually red, and also described Lemanea, a red alga that is typically olive-green in colour, it was reassuring to finally encounter a red alga that is unequivocally red in colour.

Ehen_Audouinella_131009

A rock from the River Ehen, near Ennerdale Bridge, with a thick coating of Audouinella hermanii. Photographed in October 2013.

The red colour in Audouinella and other red algae is due to a pigment called phycoerythrin.  This is a protein-based pigment which is also found in blue-green algae.  By contrast, the carotenoids which gave Haematococcus its bright colour are lipid-based.     The astaxanthin of Haematococcus is, as we saw on 6 October, a natural sunscreen but phycoerythrin and many carotenoids have a different role inside the algae.

Phycoerythrin appears red to us because it is reflecting red light whilst at the same time absorbing other wavelengths.   Beccause blue light penetrates water to a greater depth than light of longer wavelengths, phycoerythrin allows red algae to absorb this and use the energy for photosynthesis.  We can think of the phycoerythrin (and, indeed, many carotenoids) as “turbochargers” for the photosynthetic apparatus, allowing the algae that contain them to live in a wider range of habitats.

This argument works in coastal areas, where red algae can live at great depths where relatively little light penetrates but why should Audouinella be so common in the River Ehen?    The same reasoning applies: the stretch of river where I found Audouinella was heavily shaded and this species is most common in autumn and winter when the sun’s rays are much weaker.  Again, the phycoerythrin gives the normal photosynthetic apparatus a boost to help it make the most of the light that is available.

Ehen_Gom_trunc_131009

Cells of Gomphonema truncatum growing within the Audouinella colonies in the River Ehen near Ennerdale Bridge, 9 October 2013.   The two left hand images are in “valve view” whilst those on the right are in “girdle view”.   Scale bar: 10 micrometres (= 100th of a millimetre).

Whilst examining the Audouinella under my microscope, I noticed some clumps of diatom cells that looked like inverted Coke bottles.   These are a species known as Gomphonema truncatum which grows on long mucilaginous stalks which branch to form bush-like colonies.   Unlike Coke bottles, Gomphonema cells are not round, so the characteristic shape is only seen when peering down onto the flat surface of the cell.   When viewed from the side, the cell is wedge-shaped.  Diatomists refer to these two different views as “valve view” and “girdle view” respectively.   Note, too, the characteristic yellow-brown appearance of the chloroplasts.  This is due to the carotenoid fucoxanthin which, like Audouinella’s pigments, gives a boost to the photosynthetic machinery when natural light is in short supply.

Ecological yin and yang …

One of the sites we visited had a lot of fine, silty material at the margins, washed into the river following floods a few weeks before our visit.   There were a few light green patches on this silt which were dotted with oxygen bubbles as the algae made the most of the bright sunlight.  Under the microscope the green patches resolved into filaments of the blue-green alga Phormidium (probably P. autumnale or a relative).  You can see from the picture that this is a much simpler organism than the Stigonema that we met in the previous post, consisting just of straight, unbranched filaments.  However, it is effective at growing around the silt particles, creating a “mat” of algal filaments. The Phormidium filaments are capable of limited gliding motion which means that they can adjust their position to get the maximum benefit from the light.

Phormidium_in_Atma

A patch of Phormidium autumnale (or a close relative) growing on silt at the side of the Atma River, July 2013.   The air bubbles are about two millimetres across.

The next time there is a flood in the Atma, these banks of silt will probably be washed away, along with the Phormidium colonies.   However, we have seen very similar colonies form more substantial growths in the River Ehen (see post of 24 April 2013), perhaps reflecting a more stable habitat though these, too, could be washed away by the larger floods.

It is often hard to convince people of the importance of algae in lakes and rivers.   My work can seem abstract and esoteric but these oxygen bubbles help us put it all into perspective.   Put simply, the algae are the engines of rivers, particularly fast-flowing rivers such as the Atma where higher plants cannot get established.  They use the sunlight to create simple sugars out of carbon dioxide and water and this, in turn, is the food for the midge larvae and other bugs which are, ultimately, eaten by fish.  The oxygen is a by-product of this process but also plays a role in keeping the river healthy. All of the other organisms in the river need oxygen if they are to survive, so there needs to be a source that can constantly replenish the supply.   Algae contribute to the yin and the yang of freshwater ecology: capturing the sun’s energy and then balancing this by producing the oxygen that other organisms need to release this energy again for their own needs. Those of us who study algae tend to get bogged down with putting names on all the microscopic shapes we find and too easily forget to explain the role that they play.

Phormidium_from_Atma

A network of filaments of Phormidium autumnale (or a close relative) growing amongst silt particles in the Atma River.  The inset shows a single filament (scale bar: 10 micrometres = 1/100th of a millimetre).