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).

Monthly Ferns—Prairie Quillwort & Scale Tree

Prairie Quillwort is 50 centimeters tall (© 2015 Robbin Moran); Scale Tree is 50 meters tall (source). Both are lycophytes—formerly "Fern Allies".

This episode of The Monthly Fern was going to feature Isoetes melanopoda, the Prairie Quillwort, mentioned last month in the wildly popular Prairie Spikemoss post (1). But in my search for information I fell down a rabbit hole and landed with a splash in an ancient Wonderland—a wet lush forest 350 million years ago near the start of the Carboniferous Period (2). After extricating myself from the muck I looked around. Tree ferns, horsetails and dragonflies looked familiar, though a bit large. But the trees were very strange.

It was during the Carboniferous that wetland forests with tall trees first appeared in the fossil record. These were hot humid riotous tangles of vegetation growing in shallow water and muddy peat that reeked of decay. Dense stands of curious trees rose high above the understory. The most common (or best preserved) was Lepidodendron, the Scale Tree. "Scale" refers to the distinctive bark—a network of diamond-shaped leaf scars (Halliday 2022).

Wetland forest of the Carboniferous; large Lepidodendron on left (Meyers Konversationslexikon 1885–1890).

Though they've been gone for 300 million years, we know a lot about these trees. Their fossilized remains are among the most extensive for any plant from any geological period, and for good reason. Not only was Lepidodendron large and ecologically dominant, it lived in waterlogged conditions conducive to preservation. Paleontologists have been able to describe features ranging from spores to leaves to trees, and even stands of trees (Hetherington et al. 2016).

Lepidodendron differed in many ways from the trees we know. Stems of young trees were covered in long ascending needle-like leaves. These fell off as the tree grew taller and wider, leaving a spiraling network of diamond-shaped scars on the trunk. With age the trunk developed a thick tough outer layer—bark of sorts—but underneath was soft spongy tissue instead of wood. At maturity the stem branched dichotomously (repeatedly forked), forming a high crown as much as 50 meters above the ground. The final branchlets were tipped with strobili (cones) filled with spores, to be dispersed by wind (source). This so-called "tree" was an arborescent lycophyte, a fern relative.

Juvenile and mature Lepidodendron on left; trees to right are related lycophytes. Reconstruction from fossils, by Falconaumanni.

Given Lepidodendron's massive build, its lack of internal wood and the waterlogged habitat, it seems it would fall before reaching such heights. What kept it upright? Some credit the thick tough bark. But others argue convincingly for the robust root system. From very long rhizomes grew a profusion of highly-branched rootlets covered in root hairs—on the order of 26,000 rootlets per meter of rhizome! They intertwined with those of adjacent trees, forming a strong anchoring network—"trees holding onto one another for stability" (Hetherington et al. 2016; Halliday 2022).

Lepidodendron truly was an arboreal superstar, dominating the wetland forests and producing immense amounts of biomass for tens of millions of years (3). But it was doomed. By 300 million years ago, the Carboniferous rainforests and arborescent lycophytes were gone, destroyed by widespread drought. Lepidodendron's only surviving relatives are little herbaceous plants—Isoetes, the quillworts.

Bolander's Quillworts in a lake in the Wasatch Mountains, Utah. Andrey Zharkikh photo.

Now we return to the present—to a roadside waterhole along highway #44 in Mellette County, South Dakota, where W. H. Over (4) made his 15,878-th plant collection on July 10, 1924. Isoetes has not been collected in the state since.

Over correctly identified his collection to genus—Isoetes. Four months later, TC Palmer called it I. melanopoda, and Daniel F. Brunton agreed in 1995. The specimen resides at the Academy of Natural Sciences, where it has been digitized for all to enjoy (cropped here).

Like all quillworts, the Prairie Quillwort has no stem. Instead a cluster of leaves develops from the rhizome. From a distance these look like clumps of grass, but up close the leaves are distinctive—long, slender, quill-like, and bright green. The leaf bases are broad and pale, forming a swollen rootstock. These usually become black with age, hence the alternative common name—Black-foot Quillwort (plants with leaf bases that remain pale have been called I. melanopoda f. pallida).

Isoetes melanopoda; MWI photo.

Being a lycophyte, Prairie Quillwort produces no seeds. Nor does it have rows of spore-bearing sori on the undersides of its leaves as do most true ferns. Instead, spores are produced in a sac on the inner side of leaf bases.

Isoetes melanopoda; MWI photo.

Surely some readers are wondering ... how is little Isoetes like immense Lepidodendron? Why do botanists think they're related? Answer: It's the roots, especially the way they branch.

"this architecture is conserved among [Lepidodendron's] only extant relatives, herbaceous plants in the Isoetes genus. Therefore, despite the difference in stature and the time that has elapsed, we conclude that both ... have the same rootlet system architecture." (Hetherington et al. 2016)

Lepidodendron and Isoetes rootlets branch dichotomously, narrowing in a stepwise manner. A is a diagram of a rootlet with 4 levels of branching. B and C show rootlets of Isoetes and Lepidodendron (scale bars are 5 mm). Hetherington et al. 2015, Fig. 1 in part.

A final question: Is Prairie Quillwort gone from South Dakota? Has it suffered its own extinction? Maybe so. Skilled botanists have searched for it with no luck. But perhaps they were limiting themselves, looking only in "roadside waterholes" and such. Of great interest to me is its occurrence nearby in Minnesota where it's a state Endangered species. It's distribution there is quite limited—rainwater and seepage pools in quartzite rock outcrops, in the southwest corner of the state.

MWI photo.

This rock—Sioux quartzite—also crops out in the southeast corner of South Dakota. I visited several locations a few years ago, and am tempted to go back now that I have a such a vivid search image for Prairie Quillwort in my head!

Sign at Palisades State Park, SD. Pink marks exposures of Sioux Quartzite in southwest MN and southeast SD.

See any habitat?

Notes

(1) Last week I noticed that views of my Prairie Spikemoss post had skyrocketed. Now Desert Mountain is getting the same level of attention. I'm suspicious. Are chatbots visiting, searching for information? Are AI models being built? Have you had this experience?

(2) In North America the Carboniferous often is treated as two periods— Mississippian followed by Pennsylvanian.

(3) Though long extinct, Lepidodendron remains vitally important. Fossilized remains of those wetland forests with giant trees and abundant peat (aka Coal Forests) drove rapid industrialization in the 18th and 19th centuries, and continue to sustain our dependency on fossil fuels (oil and gas are more common in Cretaceous rocks).

(4) W. H. Over must have been a bright and highly motivated autodidact. He quit school in Illinois after finishing the 8th grade, homesteaded in South Dakota, and by the time of his Isoetes collection, was Museum Curator at the University of South Dakota. He would go on to become one of the state's great botanists. Among his many achievements is the "Flora of South Dakota"—the first comprehensive treatment of plants known for the state. For more about Dr. Over (he was awarded a Doctor of Science degree at age 70), start here and here. I'm still looking for a comprehensive biography.

Sources (in addition to links in post)

Halliday, T. 2022. Otherlands; a Journey through Earth's Extinct Worlds. Random House.

The Carboniferous Period is in Chapter 11, "Fuel". Halliday's descriptions of past worlds are surprisingly detailed, and supported with many citations.

Hetherington, AJ, Berry, CM, Dola, L. 2016. Networks of highly branched stigmarian rootlets developed on the first giant trees. PNAS 113:6695–6700.  https://www.pnas.org/doi/full/10.1073/pnas.1514427113

Moran, Robbin. 2004. A Natural History of Ferns. Timber Press.