Charales, commonly known as stoneworts, are an order of multicellular green algae that grow rooted in the sediments of lakes, ponds, and slow-moving waterways. Despite looking something like scraggly underwater weeds, they hold a remarkable distinction in evolutionary biology: multiple lines of genetic evidence identify them as the closest living relatives of all land plants.1PubMed. The closest living relatives of land plants That makes stoneworts far more than a niche curiosity for algae enthusiasts. They are a living window into one of the most consequential transitions in the history of life on Earth.
What Charales Actually Look Like
If you have spent time around clear, shallow freshwater, you may have seen stoneworts without realizing what they were. They grow upright from the bottom, anchored by root-like structures called rhizoids, and branch in whorls that give them a superficial resemblance to the horsetail plants you might find on land. Some species are stiff and crusty to the touch because they deposit calcium carbonate on their surfaces, which is where the common name “stonewort” comes from. Others feel softer and more pliant. They range in height from a few centimeters to over a meter in favorable conditions, and dense stands of them can carpet an entire lake bed.
One feature that surprises people is their smell. Crush a stonewort and you often get a pungent, garlicky or sulfurous odor. Some species in the genus Chara are sometimes called “muskgrass” for this reason. The smell comes from sulfur compounds in the tissue and has nothing to do with decay. It is just what they produce.
The Bridge Between Algae and Land Plants
The evolutionary relationship between Charales and land plants has been tested and confirmed through nuclear, chloroplast, and mitochondrial DNA. A landmark multi-gene analysis combining sequences from all three genomes placed Charales as the sister group to land plants with strong statistical support.2Molecular Biology and Evolution. The Chloroplast Genome Sequence of Chara vulgaris Sheds New Light into the Closest Green Algal Relatives of Land Plants A separate study sequencing the full mitochondrial genome of Chara vulgaris reached the same conclusion and described the support as “unequivocal.”3The Plant Cell. The Mitochondrial Genome of Chara vulgaris: Insights into the Mitochondrial DNA Architecture of the Last Common Ancestor of Green Algae and Land Plants Another order of charophytic algae, the Coleochaetales, sits just outside this pairing as a slightly more distant relative.4PubMed. The closest living relatives of land plants
This does not mean that modern stoneworts are the ancestors of land plants. The two lineages diverged from a shared ancestor hundreds of millions of years ago and have been evolving independently since. But because Charales branched off closest to the base of the land plant lineage, studying them can reveal which traits were already present before plants colonized dry ground and which had to evolve afterward.
What the Chara Genome Reveals About Moving Onto Land
In 2018, an international team published the genome of Chara braunii, the first full genome for any member of the Charales. The findings were striking. Chara already possesses genes associated with several hallmarks of land plants, including the machinery for cell division using a phragmoplast, unique xylan synthases for building cell walls, and the biochemical pathways for producing multiple plant hormones.5PubMed. The Chara Genome: Secondary Complexity and Implications for Plant Terrestrialization The genome also revealed molecular signatures suggesting that auxin transport through PIN proteins, certain transcription factor families, photorespiration, and seed-storage-like proteins were already in place before the divergence that eventually gave rise to land plants.6Cell. The Chara Genome: Unlocking the Genomes of Early Land Plant Ancestors
What this means, practically, is that the toolkit for becoming a land plant was partly assembled while the ancestors of modern plants were still aquatic. The move to land did not require inventing everything from scratch. Many of the genetic foundations were already there, waiting to be repurposed.
Giant Cells and Electrical Signaling
Charales have played an outsized role in laboratory science because of one anatomical oddity: their internodal cells are enormous. A single cell in the stem of a Chara plant can be several centimeters long and wide enough to see clearly without magnification. This made them a favorite subject for biophysicists long before modern molecular tools existed. Researchers could insert electrodes directly into a single cell, manipulate its internal contents, and measure how its membranes behave.7Functional Plant Biology. How characean cells have contributed to the progress of plant membrane biophysics
One of the more fascinating discoveries from this work is that Chara cells produce action potentials, the same type of rapid electrical signals that animal nerve cells use, though the underlying chemistry is different. In animals, action potentials rely primarily on sodium and potassium channels. In Chara corallina, the depolarizing phase involves calcium ions flooding into the cell, which triggers calcium-activated chloride channels to open, allowing chloride to rush out. A delayed potassium current then helps restore the resting state.8PubMed Central. Action potentials in a giant algal cell: a comparative approach to mechanisms and evolution of excitability Patch-clamp experiments have identified two types of chloride channels and one type of potassium channel that become transiently active during these electrical events.9PubMed. Ion channel activity during the action potential in Chara: new insights with new techniques
These action potentials can be triggered by touch, injury, or direct electrical stimulation. Their function appears to be defensive: the electrical signal halts cytoplasmic streaming, the vigorous internal flow of cell contents that is itself a spectacular phenomenon visible under a low-power microscope. Stopping the streaming may limit the spread of damage from a wound or discourage grazing by small animals.
Cell Walls That Foreshadow Land Plants
The cell walls of Charales share important features with those of land plants but are not identical copies. Land plant cell walls rely on cellulose, hemicelluloses, and pectin. Charales also use cellulose and produce pectin-like substances, and pectate has remained one of the most conserved wall components across both groups.10PubMed Central. Enzyme-Less Growth in Chara and Terrestrial Plants However, the “pectins” found in charophytic algae differ from land plant pectins in their detailed composition, even though they can be extracted using the same laboratory methods.11PubMed Central. Same but different – pseudo-pectin in the charophytic alga Chlorokybus atmophyticus
This pattern of “same basic architecture, different details” runs throughout the comparison between stoneworts and land plants. The skeleton of the cell wall design was inherited from a common ancestor, but each lineage has modified it in its own ways over the intervening hundreds of millions of years.
Reproduction and Dormant Oospore Banks
Charales reproduce sexually with conspicuous structures that are visible to the naked eye. The male organs, called antheridia, turn bright orange at maturity. The female organs, called oogonia, each contain a single egg cell wrapped by elongated sheath cells that spiral upward as the egg grows. When the egg is mature, gaps open between these sheath cells, allowing sperm to swim in and fertilize it.
Once fertilized, the egg develops into a tough-walled resting stage called an oospore. Oospores are diploid zygotes capable of surviving in the sediment for extended periods, forming what ecologists call propagule banks analogous to the seed banks of terrestrial plants.12Aquatic Botany. Deposition, germination and spatio-temporal patterns of charophyte propagule banks: a review These banks accumulate oospores from multiple generations and act as insurance against reproductive failure in any given year. Freshly harvested oospores can germinate at rates approaching 100%, but oospores drawn from the sediment bank germinate at much lower rates, which appears to be a bet-hedging strategy: by not germinating all at once, the population spreads its risk across multiple seasons.13Aquatic Botany. Can oospore germination patterns explain charophyte distribution in permanent and temporary wetlands?
This strategy is particularly important in temporary wetlands that dry out unpredictably. Charophyte species that colonize these habitats rely on their dormant oospore banks to repopulate when water returns.
A Fossil Record Stretching Back Over 400 Million Years
The sheath cells surrounding a charophyte oospore can become calcified, forming a structure called a gyrogonite. Because of their calcium carbonate composition, gyrogonites fossilize readily and provide a nearly continuous record of charophyte evolution stretching back to the Late Silurian period, roughly 425 million years ago.14Aquatic Botany. Gyrogonites and oospores, complementary viewpoints to improve the study of the charophytes (Charales) Over that span, paleontologists have recognized three orders, 12 families, and 81 genera of fossil charophytes. The fossil gyrogonites serve as biostratigraphic markers, helping geologists date non-marine rock formations and reconstruct the conditions of ancient lakes and waterways.
Because the trace-element chemistry of gyrogonites reflects the water conditions in which they formed, researchers also use them as paleoenvironmental proxies. The ratios of magnesium to calcium and strontium to calcium in the gyrogonite wall, for example, are sensitive to the temperature of the water at the time of growth, making it possible to reconstruct past temperature changes in lake systems.15Aquatic Botany. Trace-element and stable-isotope chemistry of gyrogonites of the euryhaline charophyte Lamprothamnium
Ecosystem Services in Lakes and Wetlands
Where they grow abundantly, stoneworts function as ecosystem engineers. Dense charophyte meadows help maintain clear water by dampening the resuspension of sediments, absorbing dissolved nutrients, and exerting chemical effects on competing phytoplankton.16PubMed. Interactions between stoneworts (Charales) and waterbirds They build up substantial biomass in both freshwater and brackish systems and contribute to nutrient removal, carbon storage, habitat provision for invertebrates and fish, and food for waterbirds.17Aquatic Botany. The role of charophytes (Charales) in past and present environments: An overview
This creates a positive feedback loop. Charophyte meadows promote water clarity, which allows more light to reach the bottom, which supports more charophyte growth. When this cycle is broken, the consequences cascade. Lakes that lose their charophyte beds tend to shift into a turbid, algae-dominated state that is much harder to reverse.
Why Stoneworts Are Disappearing From Enriched Lakes
The primary threat to Charales worldwide is eutrophication, the excess loading of nutrients like nitrogen and phosphorus into water bodies from agricultural runoff, sewage, and stormwater. Charophytes have disappeared from several enriched lakes in southern Sweden since the 1940s, and the most probable explanation is poor light conditions rather than any direct toxic effect of phosphorus or competitive pressure from fish.18Freshwater Biology. Decline of charophytes during eutrophication: comparison with angiosperms When nutrient loading triggers dense phytoplankton blooms, the water column becomes opaque. Stoneworts, which grow on the bottom and depend on light reaching the sediment, are among the first submerged plants to be squeezed out.
Compared to rooted vascular plants that can grow tall enough to reach the surface or store energy in bulky root systems, charophytes are at a disadvantage in eutrophic conditions. They are essentially low-growing and rely on a clear water column for photosynthesis. Once shaded out, the positive feedback loop described above reverses: the loss of charophytes reduces the biological clarifying effect, which further deteriorates light conditions.
Restoration Efforts and the Challenge of Bringing Stoneworts Back
Restoring charophyte meadows is a common goal in lake management, but getting them to recolonize is not straightforward. If water quality improves and dormant oospores still persist in the sediment, spontaneous recovery is possible. When it does not happen on its own, transplantation becomes an option. Species that grow slowly and produce large amounts of biomass can be collected from a healthy donor site and planted directly. Rarer species with faster life cycles and smaller body sizes often need to be propagated in laboratory cultures first before being introduced on-site.19PubMed Central. Re-Establishment Techniques and Transplantations of Charophytes to Support Threatened Species
A study of the Albufera de València Lagoon in Spain found that reestablishment of charophytes could succeed if underwater light conditions improved and nutrient inputs were reduced, either through natural recolonization from sediment-banked oospores or through active planting programs. But the researchers stressed that none of these measures would work without first improving the quality of the water flowing into the lagoon and increasing water turnover to flush out accumulated nutrients.20Ecological Engineering. Assessing the potential of Albufera de València Lagoon sediments for the restoration of charophyte meadows The underlying message across restoration studies is consistent: you cannot plant your way to healthy charophyte beds without fixing the water quality first.
Cleaning Up Contaminated Water
Beyond their role in nutrient cycling, some charophyte species show genuine promise for removing heavy metals from polluted aquatic sediments. Chara australis tolerated cadmium concentrations up to 20 mg per kilogram of soil in laboratory cultures, accumulating cadmium in both its cell walls and cytoplasm. Its bioconcentration factor exceeded the threshold commonly used to identify hyperaccumulators for shoots at the highest treatment level and for rhizoids at somewhat lower levels.21PubMed. Cd tolerance and accumulation in the aquatic macrophyte, Chara australis: potential use for charophytes in phytoremediation Follow-up work showed that Chara australis could also take up both cadmium and zinc from sediments contaminated with a mixture of metals, and that the chelating agent EDTA enhanced uptake further.22PubMed. Combined effects of cadmium and zinc on growth, tolerance, and metal accumulation in Chara australis and enhanced phytoextraction using EDTA
Other species also perform well. Chara aculeolata removed more than 95% of cadmium and lead from experimental wastewater, and both C. aculeolata and Nitella opaca showed bioconcentration factors above 1000 for those metals.23PubMed. Phytoremediation potential of charophytes: bioaccumulation and toxicity studies of cadmium, lead and zinc These are laboratory results, and scaling phytoremediation from controlled experiments to real-world contaminated sites is always harder than the numbers suggest. But the ability of charophytes to pull metals directly from sediments, rather than from the water column alone, fills a niche that most conventional aquatic phytoremediation approaches miss.
A Taxonomy Tangled by Looks
Identifying stonewort species by appearance alone turns out to be unreliable, largely because their physical form varies enormously depending on environmental conditions. Features long used to distinguish species, such as the number and length of spine cells, stipulodes, and bract cells, often differ between individuals of the same genetic lineage growing in different habitats. A DNA barcoding study of the genus Chara found that molecular data grouped samples into six main clusters, while the traditional morphological approach recognized 14 different taxa from the same material.24PubMed. DNA barcoding the genus Chara: molecular evidence recovers fewer taxa than the classical morphological approach A large unresolved group of eight named species shared identical barcode sequences across nearly a thousand nucleotide positions, suggesting that what taxonomists had been calling separate species may be variants of a single genetically coherent entity.
This does not mean those morphological differences are meaningless ecologically. A “species” that grows long spines in one lake and short spines in another may be responding to local grazing pressure or water chemistry. But it does mean the actual species-level diversity of Charales is probably lower than the taxonomic literature suggests, and conservation assessments that rely on morphological identification may overcount the number of distinct lineages that need protection.
The Microbial Communities Living on Stoneworts
Like any submerged surface in an aquatic ecosystem, stonewort thalli are colonized by complex communities of bacteria. A study comparing the epiphytic bacteria on Chara aspera and a flowering aquatic plant found that certain bacterial groups were consistently abundant on both hosts, with members of the Cytophaga-Flavobacteria-Bacteroidetes group representing between 5% and 35% of total cell counts across all samples, and Alphaproteobacteria forming the second largest group at 3% to 17%.25PubMed Central. Epiphytic bacterial community composition on two common submerged macrophytes in brackish water and freshwater The composition shifted between freshwater and brackish water, and differed between the two plant hosts, suggesting that the stonewort surface is not just a passive substrate but actively influences which microbes settle on it.
These biofilms matter ecologically. They contribute to nutrient cycling, may mediate the allelopathic effects that help stoneworts suppress phytoplankton competitors, and serve as a food source for grazing invertebrates. The microbial ecology of charophytes is still a relatively young field, but understanding it is likely to be important for making sense of how stonewort meadows sustain clear-water conditions in the lakes they inhabit.