Red algae are a large, ancient group of seaweeds, almost all of them living in the sea. They range from the nori wrapped around sushi to the pink crusts on reef rocks. Their color is the key to their success, because it lets them harvest light in places too dim and deep for many other seaweeds.1Nature. What Are Red Algae and Why Are They Important? That same group quietly supports coral reefs, fills grocery products, and is now being tested as a tool against livestock methane.
What Makes an Alga a Red Alga
Red algae form one of three main branches of a group that also includes green algae and land plants.2PubMed Central / New Phytologist. Red macroalgae in the genomic era All three inherited the ability to photosynthesize from one ancient event, in which a cell swallowed a light-using bacterium and kept it.3PubMed Central / New Phytologist. Red macroalgae in the genomic era Unlike many algae, red algae have no swimming cells with whip-like tails at any stage of life.4PubMed Central / New Phytologist. Red macroalgae in the genomic era Most are seaweeds made of many cells, though some are single cells and a few live in fresh water.
Green seaweeds like sea lettuce share their chlorophyll with land plants and usually grow in shallow, bright water. Brown seaweeds like kelp are a separate, unrelated group, even though they can look similar. “Seaweed” is an everyday label, not a scientific group: red, green and brown seaweeds each evolved a seaweed-like body on their own. Red algae also store food and build their cell walls differently from green seaweeds, and their walls contain the gel-forming sugars that become agar and carrageenan.
Their color comes from a red pigment that absorbs green and blue-green light and reflects red.5Nature. What Are Red Algae and Why Are They Important? Green and blue-green light travels deepest in water, while red light is absorbed near the surface. So the red pigment captures light that chlorophyll alone would miss, helping them grow deeper than many seaweeds.6Nature. What Are Red Algae and Why Are They Important?7Chinese Science Bulletin. Structure, function and applications of phycoerythrin: a unique light harvesting protein in algae Some can keep photosynthesizing in extremely dim light, such as under winter sea ice in the Arctic.8Journal of Phycology. PRIMARY PRODUCTION OF CRUSTOSE CORALLINE RED ALGAE IN A HIGH ARCTIC FJORD
Not every red alga looks red. Extra green chlorophyll can make some look brown, purple, or almost black, so color is only a rough guide, and a lab or field guide checks structure instead.
You can find them yourself. Tide pools and rocky shores often hold feathery red or purple seaweeds, and pink or lilac crusts on rocks and shells are often coralline red algae. A few freshwater species grow in cool, clean streams as dark slimy strands.
One of Earth’s Oldest Complex Life Forms
A fossil red alga from arctic Canada is about 1.2 billion years old and closely resembles living red seaweeds.9Paleobiology. Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes That makes it among the oldest fossils that can be confidently placed in a modern group of complex cells.10Paleobiology. Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes Fossils from India may push red algae back to about 1.6 billion years, though scientists still debate them.11PubMed Central. Three-dimensional preservation of cellular and subcellular structures suggests 1.6 billion-year-old crown-group red algae Either way, red algae were among the first living things to build bodies from many cooperating cells.12PubMed Central / New Phytologist. Red macroalgae in the genomic era
Red algae also helped spread photosynthesis to other life. Some other algae, such as many plankton, got their ability to photosynthesize by swallowing a single-celled red alga long ago.13PubMed. A “green” phosphoribulokinase in complex algae with red plastids: evidence for a single secondary endosymbiosis leading to haptophytes, cryptophytes, heterokonts, and dinoflagellates
Red Algae You Already Eat and Use
Nori, the dark sheets wrapped around sushi, is made from red algae called Porphyra or Pyropia.14Seaweed Insights. Pyropia Production Overview It has been farmed in Japan for several hundred years and is a major industry in Japan, Korea and China.15Journal of Applied Phycology. What Are Red Algae and Why Are They Important? Nori farming is worth nearly $1 billion a year and is more automated than most seaweed farming.16Seaweed Insights. Pyropia Production Overview17Journal of Applied Phycology. What Are Red Algae and Why Are They Important?
The process starts at sea. Nori is grown on nets strung in shallow coastal water, where the seaweed attaches and grows through the cool season. Harvested blades are chopped, spread thinly into sheets and dried, much like making paper. Many sheets are toasted, which changes the color and flavor, so the dark-green sheet at the store looks quite different from the reddish seaweed that was growing in the sea. Dulse and Irish moss are other edible red seaweeds, sold whole.
Red seaweeds are nutritious. They tend to have more protein than green or brown seaweeds, up to about 19 grams per 100 grams of dried seaweed.18Journal of Food and Nutrition Research. Nutritional Composition and Bioactive Compounds of Red Seaweed: A Mini-Review They are also rich in fiber, especially the soluble kind, and low in calories.19Journal of Food and Nutrition Research. Nutritional Composition and Bioactive Compounds of Red Seaweed: A Mini-Review Typical servings are small, so protein per meal stays modest.
The catch is that red seaweeds are high in sodium and iodine.20Journal of Food and Nutrition Research. Nutritional Composition and Bioactive Compounds of Red Seaweed: A Mini-Review It makes sense to eat them as a regular side rather than in large daily amounts, especially with thyroid problems. A few sheets of nori around sushi is a small serving, so most healthy people eating it occasionally have little to worry about. Iodine content varies a lot between seaweed types and products, so heaping portions of a concentrated seaweed or supplement are the real concern.
Red seaweeds also thicken everyday products. Agar is a gel made from red seaweeds such as Gelidium and Gracilaria and is used in food, medicine and labs.21Frontiers in Plant Science. The Cell Wall Polysaccharides Biosynthesis in Seaweeds: A Molecular Perspective Carrageenan, a seaweed thickener from red seaweeds, gels or thickens foods and melts and resets with heat.22Cargill. Carrageenan Carrageenan appears in chocolate milk, plant milks, ice cream and some deli meats. Agar is a vegetarian substitute for gelatin in desserts and jellies, and agar plates are the jelly scientists grow bacteria on in labs.
The Hidden Builders of Coral Reefs
Crusty reef algae lay down a hard mineral inside their walls, the same type of stone as shells. The crust is part living tissue and part stone. As the algae grow, new layers spread across the surface and bind rubble, sand and old skeletons into a solid mass. They cement loose reef pieces together and protect the reef from erosion and breakage.23Aquatic Botany. Contribution of crustose coralline algae to carbonate budget in coral formations of Tayrona National Natural Park, Colombian Caribbean24Department of Land and Natural Resources State of Hawaii. Crustose Coralline Algae
Their surface also gives drifting coral larvae a place to settle and start new colonies.25Aquatic Botany. Contribution of crustose coralline algae to carbonate budget in coral formations of Tayrona National Natural Park, Colombian Caribbean A reef with fewer of these crusts can struggle to rebuild after storms. They grow slowly, so damage takes years to recover.
They live far beyond the tropics too. Coralline red algae also grow in cold seas, including Arctic fjords, where they photosynthesize slowly in very dim light.26Journal of Phycology. PRIMARY PRODUCTION OF CRUSTOSE CORALLINE RED ALGAE IN A HIGH ARCTIC FJORD
Cutting Cattle Methane and Helping Crops
Cattle release methane, a strong heat-trapping gas, mainly through burping as they digest grass. They have a large first stomach where microbes ferment tough grass and produce methane as a byproduct.
Adding the red seaweed Asparagopsis to feed cut methane by up to about 75 to 80% in beef cattle eating mixed feed.27PubMed Central. Red seaweed (Asparagopsis taxiformis) supplementation reduces enteric methane by over 80 percent in beef steers It contains bromoform, a compound that blocks the gut microbes that make methane.28PubMed Central. Mitigating methane emissions in grazing beef cattle with a seaweed-based feed additive: Implications for climate-smart agriculture In grazing cattle given a pelleted seaweed product, the reduction was smaller, about 38%, without hurting weight gain.29PubMed Central. Mitigating methane emissions in grazing beef cattle with a seaweed-based feed additive: Implications for climate-smart agriculture So results depend on how the seaweed is fed, and it is not yet a routine farm product.
Several questions remain open. Growing enough Asparagopsis for billions of cattle is a major challenge. Bromoform must be kept low so it doesn’t end up in meat or milk. Grazing animals are harder to dose than feedlot animals, and long-term effects over many years are still being studied.
Red seaweed may help crops too. Sprays made from red seaweed improved soybean yield and drought tolerance when water was short.30PubMed Central. Biostimulant red seaweed (Gracilaria tenuistipitata var. liui) extracts spray improves yield and drought tolerance in soybean A red and brown seaweed blend helped young bean seedlings grow under salty conditions in lab tests.31Journal of Plant Nutrition. Evaluation of biostimulant and salt-mediated stress alleviating activities of tropical red and brown seaweeds based biostimulant through green gram seedling cut assay technique Red seaweed extracts reduced stress damage in rice plants exposed to fungicide.32PubMed Central. Red-seaweed biostimulants differentially alleviate the impact of fungicidal stress in rice (Oryza sativa L.)
Medicine and Science Uses
Red algae contain sugars with sulfur groups attached, including carrageenan.33PubMed Central. The Potential of Algal Biotechnology to Produce Antiviral Compounds and Biopharmaceuticals In research these sugars show antiviral, anti-inflammatory and antioxidant activity.34PubMed Central. Bioactive Compounds from Red Microalgae with Therapeutic and Nutritional Value35PubMed Central. The Potential of Algal Biotechnology to Produce Antiviral Compounds and Biopharmaceuticals Carrageenans can stop some viruses from attaching to cells.36PubMed Central. The Potential of Algal Biotechnology to Produce Antiviral Compounds and Biopharmaceuticals
Most of this is lab work, not proven treatments, so seaweed supplements are not cures. Lab results on cells don’t show that eating seaweed protects against viruses, and supplement labels often stretch early research. Ask a doctor before using seaweed supplements, especially with thyroid conditions.
The red pigment is a lab tool as well. Purified, it glows brightly, and in early lab studies it worked as a dye for imaging cells.37PubMed. Single-step purified R-phycoerythrin transmits cellular imaging functionalities in vitro
When Red Algae Become a Problem
In remote Hawaiian reefs, a red alga found in 2016 grew to cover 115 times more area between 2015 and 2021.38ECO Magazine. New Research Identifies Potential Source Populations of Invasive Red Alga It forms thick mats that smother coral, spreading about 45 square kilometers a year.39ECO Magazine. New Research Identifies Potential Source Populations of Invasive Red Alga In the Mediterranean’s Aegean Sea, two invasive red algae do the most harm to seagrass meadows.40PubMed Central. Cumulative Negative Impacts of Invasive Alien Species on Marine Ecosystems of the Aegean Sea Along North Carolina’s coast, floating mats of a red alga crowd out native species and can lower oxygen in the water.41Public Radio East. Red Algae A Problem Along Southeastern North Carolina Coast Many red seaweeds can also regrow from small broken pieces, which makes them hard to stop once established.
Red algae are not the cause of red tide, which is usually caused by tiny plankton. Red seaweed mats on beaches are usually not toxic to touch, though rotting mats can smell bad and lower oxygen in shallow water.
You can help limit the spread by rinsing boats, gear and anchors before moving between waters. Don’t dump aquarium plants or water into the sea or streams. Report unusual seaweed mats to local wildlife agencies.
Threats From a Changing Ocean
As oceans absorb carbon dioxide, seawater becomes more acidic and holds less of the mineral reef algae build with.42Journal of Experimental Marine Biology and Ecology. Ocean acidification and warming decrease calcification in the crustose coralline alga Hydrolithon onkodes and increase susceptibility to grazing The mineral becomes harder to find in a usable form, so crusty reef algae grow and harden more slowly.43PubMed. Effects of ocean acidification on population dynamics and community structure of crustose coralline algae They must spend more effort building their crusts and may produce thinner, less dense ones. Weaker crusts are easier for grazing animals to eat away,44Journal of Experimental Marine Biology and Ecology. Ocean acidification and warming decrease calcification in the crustose coralline alga Hydrolithon onkodes and increase susceptibility to grazing and the reef loses part of the cement that holds it together. Warming adds stress, and acidification may lower how well these algae tolerate heat.45Frontiers in Marine Science. Impacts of Ocean Warming on Coralline Algal Calcification: Meta-Analysis, Knowledge Gaps, and Key Recommendations for Future Research
Seaweed farming is adapting. Farms can rotate species by season, growing nori in cool water and heat-tolerant red seaweeds in summer.46Journal of the World Aquaculture Society. Temperature and nutrient responses of three red algae: Implications for species rotation in land‐based aquaculture Breeding and helpful microbes may make farmed seaweeds tougher against heat and disease.47Frontiers in Marine Science. Sustainable seaweed aquaculture and climate change in the North Atlantic: challenges and opportunities