The Monthly Fern—More Quirks of Quillworts

Jon Keeley with several of his beloved quillworts (date unknown).

Once again The Monthly Fern series is featuring the Prairie Quillwort and its relatives—genus Isoetes. One post was not enough for these fascinating plants! Not only are they the sole survivors of a plant group that dominated 300 million years ago (see last month's post), they use CAM photosynthesis (1) ... that's astonishing! In fact it's so unexpected that when Jon Keeley announced it 40+ years ago, he was written off as ignorant (Keeley 2014).

Here's the conundrum. CAM photosynthesis is thought to have evolved in flowering plants (angiosperms) in hot arid environments. Many succulents, including most cacti, are CAM plants. But quillworts are primitive spore-producing lycophytes predating flowering plants by c. 200 million years. And almost all are aquatic.

Isoetes and other lycophytes split from ferns and seed plants long ago (source; black, red labels added).

Lycophyte diversity by Kingfiser (click link for full names and more info).

When I was an undergraduate long ago, only one type of photosynthesis was known (or so we were taught). As a grad student a decade later, I learned there were three: C3 is the common type; C4 and CAM are restricted to certain groups and situations (2). Since then I've largely ignored photosynthesis. But when I read that quillworts are CAM plants, I was intrigued! It was time to learn more. (Information here is from Khan Academy's Biology Unit 8, Photosynthesis unless noted otherwise.)

Photosynthesis: 1st stage powered by sunlight; 2nd makes food & oxygen for us to consume.

Photosynthesis is complicated and very chemical, but the basic process is simple. There are two stages. In the first, energy from sunlight is captured and converted to chemical energy. In the second, this chemical energy is used to convert carbon dioxide and water into glucose and similar carbon-based compounds, releasing oxygen in the process.

These are the benefits we reap. We consume carbon-based compounds for energy and to build proteins, DNA, muscles and more. And we breathe oxygen. If photosynthesis were to stop, we would die—either starve or suffocate.

As wonderful as photosynthesis is, there's room for improvement. The widespread C3 type, used by 85% of plants, is surprisingly inefficient. Carbon dioxide is captured and a sugar molecule created only about 65–80% of the time. The problem lies with an important but indiscriminate enzyme—rubisco—which will happily bind oxygen instead of carbon dioxide if given the chance (more here). 

Rubisco is the "molecular equivalent of a good friend with a bad habit" (KA, modified slightly).

This inefficiency is significantly less in C4 and CAM photosynthesis. But there's another problem and it's a big one—water loss. Plants take in carbon dioxide from air via stomata (pores) on leaf surfaces. But water vapor is lost at the same time, especially on hot dry days. Many plants close their stomata at night to prevent water loss, and when it's hot, some close stomata during the day as well. But then there's no source of carbon.

This is where CAM plants excel. They open their stomata at night and collect carbon dioxide, storing it for use the next day when the sun is shining. That way they can photosynthesize without opening their stomata and losing water. So clever!!

CAM photosynthesis requires extra energy compared to the common C3 type, but apparently it's worth the cost. CAM is used by at least 16,000 species, c. 7% of all plants. Most are desert plants, including at least 99% of the 1700 species of cacti (source). And then there are the quillworts, nearly all of which are aquatic at least part of their lives. Why would they bother with energy-expensive CAM?

Isoetes melanopoda, Prairie Quillwort, uses CAM even though it's aquatic (©2015 Robbin Moran).

Prairie Quillworts photosynthesizing by the light of day, with CO2 they gathered before dawn (Andrey Zharkikh).

Like terrestrial CAM plants, aquatic quillworts gather and store carbon dioxide at night but for a different reason. Terrestrial CAM plants have no access to CO2 during the day because their stomata are closed to prevent water loss. Quillworts have no risk of water loss, but for them daytime uptake of CO2 is difficult. It diffuses poorly in water to begin with, and most of the other plants in the pond are better at sucking it up for photosynthesis (4).

By the end of the day, the amount of CO2 in pond water is quite low. But as soon as night falls and photosynthesis stops, it quickly rises. "This must be when quillworts open their stomata to collect CO2" you may be thinking—as I did. But then a memory floated to the surface. Stomata don't work underwater! Aquatic quillworts have none, or non-functional ones at most.

From what I've read, there's still much to be learned about carbon dioxide uptake in Isoetes. However we do know that it varies with species and habitat. The few quillworts that are fully terrestrial—never submerged in water—have functional stomata and use C3 photosynthesis. They never use CAM, nor can they be converted to CAM even by keeping them underwater for a long time.

Isoetes histrix, Land Quillwort, is terrestrial (but often reported as aquatic). Late season photo by Sam Thomas; added insert by Peter de Lange.

Those quillworts that live part of their lives submerged, for example in vernal pools, are impressively versatile. They utilize CAM until water is low enough to expose their leaf tips to air. Then the stomata start to become functional and C3 photosynthesis begins to take over, progressing down each leaf cell by cell keeping just above the water! (Keeley 2014)

Isoetes howellii in dried vernal pool. It was in Howell's Quillwort that Jon Keeley stumbled upon CAM photosynthesis. © 2004 Carol W. Witham.

The many quillworts that are entirely aquatic are more puzzling. They have no stomata and their leaves are covered with a waxy cuticle. And yet they thrive, especially where other plants can't.

Aquatic Isoetes lacustris, the Lake Quillwort (Alina Ambrosova).

Isoetes lacustris in its favorite environment—lake bottom with sparse vegetation (5). (Alina Ambrosova) 

Aquatic quillworts seem to be more common in oligotrophic waters, where nutrients are scarce and there's little competing vegetation. So how do they survive if other plants can't? Probably with their unusual roots (6).

These roots have a large central air cavity that accumulates carbon dioxide gathered from sediments. Next to the cavity is bundle of vascular tissue that delivers it to the plant above. Furthermore, being CAM plants they collect CO2 at night as well as during the day, thereby doubling their harvest. Sometimes they truly flourish, covering the lake bottom in a dense green underwater carpet! (Moran 2004)

And with that, I will close. As you may suspect, this was one of my more challenging posts. Just when I had everything figured out, another puzzle would present itself. But I'm not complaining. In fact that's what I enjoy most about getting to know plants—pondering and unraveling their many little mysteries. And I know that the next time I meet up with a quillwort, it will be far richer experience.

So primitive, so simple in form, and yet so alluring (Isoetes englemannii, Nathan Aaron).

Notes

(1) C2 carbon concentration is sometimes considered a type of photosyntheses.

(2) CAM refers to crassulacean acid metabolism. To be clear, there is no "crassulacean acid"; the name refers to acid metabolism in the family Crassulaceae, where CAM was discovered (source).

(3) The widespread occurrence of CAM likely is due to repeated convergent evolution. After sequencing the pineapple genome, Ming et al. (2015) concluded that CAM arose from relatively simple reconfiguration of C3 pathways. See also Wickell et al. 2021.

(4) Many aquatic plants collect CO2 via bicarbonate; it appears that quillworts are unable to do this (Keeley 2014).

(5) Is that an alga on the leaves of Isoetes lacustris? If so, it might affect light capture but not CO2 uptake, which is done by the roots.

(6) Isoetes lacustris roots look very much like the fossilized roots of Lepidodendron trees, its ancient relatives.

Sources (in addition to links in post)

Keeley, JE. 1981. Diurnal acid metabolism in vernal pool Isoetes. Madroño 28:167-171. BHL

Keeley, JE. 1998. CAM Photosynthesis in submerged aquatic plants. The Botanical Review 64:122–158. PDF.

Keeley, JE. 2014. Aquatic CAM photosynthesis: A brief history of its discovery. Aquatic Botany 118: 38–44. http://dx.doi.org/10.1016/j.aquabot.2014.05.010

Lane, N. 2010. Life Ascending: The Ten Great Inventions of Evolution. WW Norton & Co.

Moran, Robbin. 2004. "Some Quirks of Quillworts" in A Natural History of Ferns. Timber Press.

Wickell, D, et al. 2021. Underwater CAM photosynthesis elucidated by Isoetes genome. Nat Commun. 12:6348 (open access).