What’s the Nutritional Difference Between Red, Green, and Brown Seaweeds? - Maine Coast Sea Vegetables

As they grow in the sea macroalgae are almost always found in hues of brown, red, or green. Most brown colored seaweeds are related as Phaeophytes.  Reddish hued seaweeds are related as Rhodophytes, and green seaweeds are related as Chlorophytes.  Sea vegetables within each group are related to one another based on structural and genetic similarities. Each group evolved from a separate lineage and they don’t share a common recent ancestor.

Scientists classify living organisms according to the scheme Kingdom - Phylum (animals) or Division (plants) – Class – Order – Family – Genus - Species.  The scientific name for every organism is a pairing of its genus and species names in Latin, the original language of science. Under this classification scheme Phaeophytes belong to the Kingdom Chromista, and Rhodophytes and Chlorophytes belong to the Kingdom Plantae. 

This means that only the red and green seaweeds can be considered plants. Brown seaweeds are about as distantly related to plants and red or green seaweeds as humans are to fungi!  However, even though brown seaweeds aren’t related to either plants or animals, people tend to think of all seaweeds as plants, and we like to refer to them as sea vegetables.

Why are seaweeds different colors? (Red, green, and brown)

The color comes from the dominant pigment produced by each algae species to photosynthesize energy from the sun while immersed in water.  All seaweeds produce chlorophyll a, a crucial component of the photosynthetic system that converts sunlight into metabolic fuel and that gives many land plants their familiar green color.

As sunlight passes through water different wavelengths on the color spectrum get filtered out.  This means that photosynthetic organisms living in deeper water need different pigments to efficiently carry out photosynthesis than those living in shallower water. Green seaweeds are adapted for life in relatively shallow water, while the darker pigments found in Rhodophytes and Phaeophytes are an adaptation to life in deeper water.

sea lettuce bed
Sea lettuce Ulva lactuca (MCSV stock image)

Chlorophytes produce an abundance of chlorophyll a, which is why they’re almost always green.  About 90% of all Chlorophytes live in fresh water and some are even adapted to life on land, but there are also many marine species. Sea lettuce (Ulva lactuca) is a marine species and it’s the only green sea vegetable currently offered by Maine Coast Sea Vegetables.  Caulerpa is another popular edible green marine seaweed.  Caulerpa resembles bunches of green fish eggs or grapes. In the Philippines, Caulerpa is eaten fresh as ‘green caviar’, and in Japan it’s known as ‘sea grapes’.

Dulse Palmaria palmata (Photo by Sara Redmond)

Rhodophytes are the largest group of marine macroalgae, with at least 6,200 known species. They contain pigments known as phycobilins that impart red, orange, or even bluish hues. Phycobilins dominate over chlorophyll, but they’re water soluble and chlorophyll is not.  If the plant detaches from where it grows and is cast adrift, the phycobilins leach out and the formerly red plant turns greenish. 

Alaria esculenta (Photo by Sara Redmond)

Phaeophytes, or brown seaweeds, are often collectively known as kelp, but the various species are actually quite different.  Our brown sea vegetables are Alaria (Alaria esculenta) (wild Atlantic Wakame), sugar kelp (Saccharina latissima) (wild Atlantic Kombu), rockweed (Ascophyllum nodosum), and bladderwrack (Fucus vesiculosus). 

Bladderwrack Fucus vesiculosus (MCSV stock image)

The color of a particular sea vegetable specimen isn’t always a reliable indicator of which group it belongs to.  The dominant pigments responsible for giving seaweed its usual color may fade under certain circumstances, especially when cooked, leaving the underlying pigments to show through. Some pigments are more water soluble than others, so for example, brown seaweed fronds that detach from the living plant may fade to green as the fucoxanthin leaches out.  Sun bleached red seaweeds may appear golden or pale.

 Sugar kelp Saccharina latissima (MCSV stock image)

How do brown, red, and green seaweeds differ?

This classification scheme is about more than just color, though.  At a deeper structural level, brown, red, and green sea vegetables have different evolutionary histories and biochemical traits.  These traits affect their nutritional, medicinal, and culinary properties, and they’re a big reason why we recommend that people eat more than one kind of sea vegetable.  As you become more of a connoisseur it will be helpful to understand how sea vegetables differ and what this means for how they’re used.

How does seaweed color affect nutrition?

Regardless of color, dried sea vegetables are low in calories but high in minerals and iodine.  They also contain high quality protein, various B vitamins, and vitamin C.  The combination of “low in calories/high in nutrients” makes them nutrient dense.  However, there are some noteworthy differences between the color groups when it comes to iodine, certain minerals, and protein.

Brown sea vegetables contain the most iodine

Iodine is an important nutrient and average healthy adults require about 150mcg per day. Deficiency is not uncommon, especially for people who avoid dairy, bread products, seafood, and iodized salt.  Seaweed is often recommended as a good source of iodine, but some sea vegetables contain more iodine than others. 

All of our sea vegetables are great sources of dietary iodine, but the browns pack the biggest punch.  Brown sea vegetables typically contain much more iodine than reds or greens, though the amounts can vary by quite a lot.  Alaria has about 295mcg per gram (ppm), whereas sugar kelp is an iodine superstar, containing about 2,700ppm.  Bladderwrack and rockweed fall in-between, containing about 390ppm and 700ppm respectively.

The iodine content of red sea vegetables tends to be intermediate between the browns and the greens.  Irish moss contains about 295ppm, similar to Alaria.  Dulse contains about 140ppm iodine and laver is lowest with just 75ppm.

Green species usually have less iodine than browns or reds. Sea lettuce only contains about 45ppm, but even at this ‘low’ level a 3-gram serving provides about 85% of the adult RDI of iodine.

The Tolerable Upper Intake Level (UL) for daily iodine intake is considered to be about 1,100 mcg per day for a healthy adult and much less for children.   An occasional seaweed salad containing iodine above these levels is unlikely to cause harm, but regular consumption could cause thyroid dysfunction in certain sensitive individuals.  Fortunately, this is usually quickly remedied by simply ingesting less iodine. If you want the benefits of sea vegetables but with less iodine, than red or green species are for you.  You should always consult a health practitioner before embarking on a sustained iodine fortification program with seaweed. 

Green sea vegetables are mineral rich

Sea vegetables are renowned for their mineral content, and most of them contain high levels of all five essential minerals and the ten trace elements required for human health.  For the most part there aren’t large differences in mineral content between browns, reds, and greens, with the exceptions of calcium, iron, and magnesium.

Sea lettuce and other chlorophytes can be especially rich in these three minerals.  A 3-gram serving of sea lettuce contributes about 23% of the RDI for iron and 19% of the RDI for magnesium.  Only dulse contains more iron than sea lettuce, while the other two reds (laver and Irish moss) contain relatively modest (but still respectable) levels.

Red sea vegetables often contain the most protein

All sea vegetables contain protein and some have more on a dry weight basis than many other foods.  Red and green sea vegetables usually have more protein than browns.  Brown sea vegetables are about 8-12% protein on a dry weight basis; red seaweeds range from 13% to 40% protein; and sea lettuce is about 15% protein.   Laver and the closely related nori species usually contain the most protein…up to 40%. Laver and other red seaweeds are good choices for those wanting to include vegetarian sources of balanced protein in their diet.

Because sea vegetables are generally eaten in small portions, they don’t usually contribute more than 5% of the recommended total daily protein intake of 50g. However, seaweed protein is considered high quality because it has a well-balanced mix of amino acids, the building blocks of protein.

Amino acid composition varies between color groups.  Although all seaweeds are rich in glutamic and aspartic acids (giving them their savory umami flavor), browns usually contain the most.  This makes brown sea vegetables an especially good choice for flavoring broths or soups.  Red seaweeds tend to be richer in lysine than browns or greens.  Lysine is known as a limiting amino acid because humans are unable to make it on their own and must get it from their diet.  Plant based foods are low in lysine, so red sea vegetables are good for vegans trying to incorporate lysine rich foods into their diet.

Protein content can considerably vary in all seaweeds throughout the season.  It tends to be highest for sea vegetables harvested in the winter and lowest in summer/fall.  Dulse, for example, is about 20% protein in the winter and about 12% protein in summer/fall.  Most of our sea vegetables are harvested in late spring and through the summer until late fall.

Colorful Sea Vegetables for Health

All three sea vegetable color groups contain bioactive compounds that can help promote good health, often in similar ways. However, the nature and structure of these compounds is unique within each group.  The two most important types of bioactive compounds are the pigments that give each sea vegetable its typical color, and the structural polysaccharides that form the plant's cell walls.

Brown and red seaweeds contain bioactive pigments

The pigments responsible for the deep brown or rich red colors of many sea vegetables are known as carotenoids.  Many people know carotenoids are powerful antioxidants with health promoting properties.  Beta carotene, responsible for giving many fruits and vegetables their orange pigment, is perhaps the most well-known carotenoid.  Beta carotene has been found to protect against cancer and aging, and humans convert it into Vitamin A, needed for healthy skin, immune system, and eyes.  Orange foods such as sweet potatoes, carrots, and squash are abundant in beta carotene, as are leafy green vegetables.

Researchers have found that seaweed carotenoids are just as beneficial as beta carotene.  Fucoxanthin, the orange pigment found in brown seaweeds, has specific functionalities not found in other carotenoids.  This led some scientists to refer to fucoxanthin as a treasure from the sea.  One of its most well studied properties is its potential to address metabolic syndrome and obesity.  In one study, the abdominal fat weight of mice was significantly reduced after they were fed diets containing fucoxanthin.  Although scientists haven’t gone so far as to say that eating brown sea vegetables can cure a beer belly, they do believe it could help control diabetes and obesity.

Phycobilins, found in red seaweeds and other photosynthetic organisms, are mostly red or blue depending on molecular structure, though yellowish and purple colored forms also exist.  The phycobilins found in red sea vegetables such as dulse, nori, and laver are known as phycoerythrins, and in common with other carotenoids they have antioxidant properties.  They’re also being investigated for their anti-inflammatory properties.

Sulfated polysaccharides in brown, red, and green sea vegetables

In addition to bioactive pigments, sea vegetables contain unique, structurally complex polysaccharides with bioactive properties not found in land plants.  These polysaccharides help build cell walls and are a major component of the dietary fiber found in sea vegetables. Their structure and bioactive properties   differ between the three color groups of algae.

Phaeophytes (brown seaweeds) are a rich source of fucoidan, a sulfated polysaccharide widely investigated for having anti-viral, anti-bacterial, anti-cancer, and anti-inflammatory properties.  Like fucoxanthin, fucoidan has received intense study for its potential to treat diabetes and metabolic syndrome.  It’s thought that fucoidan acts as a pre-biotic that promotes growth of beneficial bacteria in the intestine, which in turn have positive effects on metabolism.  Alaria, sugar kelp, and fucus are all good sources of fucoidans.

Other important bioactive polysaccharides found in brown seaweeds include alginates and laminarin.  Sodium alginate has antibacterial properties and it’s been shown to lessen glucose absorption when ingested with a meal.  Laminarin has been shown to be anti‐inflammatory, with immunostimulatory properties as well.

Carrageenan and agar are the two predominate polysaccharides found in Rhodophytes (red seaweeds). These polysaccharides have a strong capacity to form gels and they’re both widely used as vegetarian food thickeners and gelling agents.  Irish moss has a long history of being used for this purpose. Carrageenan and agar also have bioactive properties.  Carrageenan has been shown in lab studies to inhibit sexually transmitted viruses such as herpes simplex, human papillomaviruses (which can cause cervical cancer), and HIV.  Agar has been shown to decrease blood glucose levels.

Porphyran is another polysaccharide found in red Porphyra species such as laver and nori.  Porphyra species have long been used for medicinal purposes in Traditional Chinese Medicine.  Recent scientific research shows that some porphyrans are powerful antioxidants.

Green sea vegetables from the Ulva and Enteromorpha genera are widely known as sea lettuce, and they contain bioactive polysaccharides known as ulvans.  Ulvans are sulfated like other bioactive seaweed polysaccharides, and as an extract they have anticancer and antioxidant properties in lab studies.   However, ulvans aren’t digested by bacteria in the human colon and they have water-retention capacities, so they pass through the intestine largely intact.   This makes ulvans a beneficial dietary fiber, but their bioactive potential may require extracting them from the plant and formulating biologics that can be administered as capsules or along with other medicines.

To a varying extent, this may be true of other seaweed bioactive polysaccharides as well.  Because they’re often bound within dietary fiber, they may be poorly absorbed, which limits their bioactive potential to processes occurring only within the gut.  Scientists are investigating ways to develop the full potential of sea vegetable bioactive substances to promote well-being.

How to enjoy all three seaweed color groups in your diet

Variety is the spice of life, and the same is true of sea vegetables.  Quite literally, as it turns out, because like spices, sea vegetables are often used to enhance the flavor of food.  Our Sea Seasonings are a line of six shakers of ground sea vegetables and spices, from garlicky to peppery to salty.  Every color group is represented and some contain more than one.  Triple Blend, for example, is a colorful mix of red dulse, black laver and green sea lettuce flakes. The zingy flavor of dulse nicely complements the more pungent sea lettuce and nutty laver (wild nori).  Sea Seasonings can be enjoyed in any dish or recipe that ordinarily benefits from salt, and even some that don’t.

We also usually offer most of our sea vegetables in both milled and leaf forms, with every color group represented.  For example, Alaria, dulse, and sea lettuce can all be purchased as both whole leaf and milled flakes or powder.  This creates boundless opportunity to incorporate all three sea vegetable colors into your daily diet.  Sea lettuce is mineral rich but somewhat bitter, so it’s best used in small portions in cooked recipes such as soups.  Dulse leaf is excellent eaten straight from the bag as an anytime snack, and the flakes add color and flavor to scrambled eggs at breakfast or salads at lunch and dinner.  Alaria adds delicious umami flavor to any recipe because it’s rich in glutamate, but it’s especially good in miso soup.

Our dried sea vegetables have a long shelf life when properly stored because they contain very little available water and they have natural antimicrobial properties.  This makes it easy to keep samples of all three color groups on hand in the larder without fear they will go bad.  Our cookbook Sea Vegetable Celebration has 105 vegetarian recipes featuring sea vegetables, and the internet and other cookbooks have hundreds more.  We don’t see any good reason to limit yourself to just one variety of sea vegetable when there’s so much goodness on offer from all of them.  Mix, match, and enjoy!

Nutrition & health