Algae: What are they?

So in this blog I talk a lot about seaweeds, but what exactly are seaweeds? Seaweed is a common name used to refer to marine macroalgae, a type of large algae. So seaweeds are large algae that live in the sea. But this just seems to raise the bigger question: what are algae?

Most people probably know algae as the clumps of seaweed spoiling the beach or the green stuff that wraps up your sushi.


Seaweed washed up on the beach. Goleta. California. United States.

But algae can come in a vast array of shapes and sizes. The term algae actually encompasses a large range of organisms, from single celled individuals that are a similar size to bacteria, up to large multi-celled, plant-like organisms up to 53m in length in the case of giant kelp.

Diatoms, mostly Arachnoidiscus

Coloured electron microscopy of diatoms Arachnoidiscus

Kelp Forest

Kelp Forest. West coast of North America

Algae are not always alone, and can actually be found forming important symbiotic relationships with many marine animals, the most notable being that with corals, but also with jellyfish, sea anemones and even clams. These algae are provided shelter within the animal whilst in exchange providing nutrients to their animal host in a mutually beneficial relationship.

Fleet Of Jellyfish

Algae and Jellyfish symbiosis. Golden Jellyfish Mastigias sp. Jellyfish Lake. Palau

Algae are grouped together in a rather informal way, with many members not necessarily being closely related. These algae can come from multiple lineages, meaning that they are grouped together despite not sharing an immediate common ancestry. This explains why we see such a diverse range of characteristics within the group.

However though a diverse group, they do share one crucial similarity: they are photosynthetic. But then are algae plants? The answer is slightly contentious, but generally, we define them not to be. To be a plant you need a root or internal vascular systems and to produce seeds or flowers, which algae do not. Some algae can have similar looking structures, such as the vein-like midrib in bladderwrack, but these are not true vascular systems. Unlike plants, the body of an alga is relatively undifferentiated, whereby there is no division of labour within the algal body.

Bladder Wrack

Bladderwrack. St. Mary’s Island. NE coast of England.

So algae are a group of photosynthetic organisms that are corralled into a group because they do not fit well into the other more well-defined photosynthetic groups like plants, fungi and lichens. Hence it really isn’t unsurprising that they are so diverse.

The greatest anomaly within algae are the cyanobacteria. Cyanobacteria, also known as blue-green algae, are normally (but not always) included within algae and are probably most famous for forming a greeny sheen of scum on the surface of the water that stops you going in for a swim at the sea or lake side.


Cyanobacteria on the water’s surface

Cyanobacteria are actually very unlike all other algae, even residing within a different kingdom within the tree of life. Cyanobacteria reside within the kingdom of bacteria, alongside the familiar E. coli and Salmonella. Bacteria in general are primitive cells with a less complex structure and are always single-celled. These single cells can exist on their own or as several to many cells living in a colony, chain or filament.

Video: The Cyanobacteria: Oscillatoria and Gleocapsa

The Cyanobacteria: Oscillatoria and Gleocapsa.

All other algae have a complex cell structure, having many different cellular parts including those specialised for photosynthesis: the chloroplasts. Interestingly, cyanobacteria are considered the origin of chloroplasts, when a cyanobacterial cell was incorporated into the cell of an algal ancestor. Therefore, though cyanobacteria are not directly related to all other algae they are related to the photosynthetic part of them.

All these algae belong to the kingdom of protista. Of these complex algae, there are three major groups: Green, Red, and Brown. As you may guess they are named for their usual colour.

This evolutionary tree shows the hypothesised relationships among the six kingdoms. Each group branching off the tree can be thought of as a cluster of close relatives. Source:

The green group contains algal species within the scientific designation Chlorophyta, the red within Rhodophyta, and brown within Heterokonta, Haptophyta, Cryptophyta, and Alveolata. All algae within each of these groups share a set of characteristics that distinguishes them from those within another group, and thus are grouped accordingly.

Red and brown algae. Penguin Island. Western Australia.

The main characteristic difference between these groups are the types of pigment each contains. Just like plants, all these algae contain the green photosynthetic pigment chlorophyll; though the type of chlorophyll varies between them. In green algae we find chlorophyll a and b, which is the same as can be found in plants. Both red and brown algae also contain chlorophyll a, but the second is either chlorophyll d in red algae or chlorophyll c in brown algae. Green, red, and brown algae also contain other different accessory pigments that help chlorophyll in absorbing light. These pigments also contribute to the algae’s colour. 

Scenedesmus at 1000x - phase contrast

The green algae: Scenedesmus.

There are also a few other smaller groups of algae not contained within these three main groups. These are the freshwater Glaucocystophyta, the predominantly freshwater Euglenophyta, and the marine Chlorarachniophyta. They are all far smaller groups of solely single celled individuals with either 15, ~800 and 5 species respectively. Nevertheless the vast majority of algal species are grouped within the greens, reds and browns.  

Green algae are the most closely related algae to plants, sharing a common recent ancestor. They can be found in both marine and freshwater environments. Examples of green algae include sea lettuce (Ulva spp) and Dead man’s fingers (Codium fragile).

Sea lettuce - Ulva sp

Sea lettuce, Ulva spp. Bluefish Point. Manly. Australia

Red algae are mostly marine, and can be found living at greater depths than green and brown algae. This is because they can absorb blue light, which penetrates deeper than other light waves do, and thus they are still able to photosynthesise where other algae cannot.

Some red algae can even be reef builders, similar to that of the corals. Coralline algae are encased within calcareous deposits and form spiky underwater carpets known as maerl beds. These maerl beds are ecologically important, but unfortunately due to their fragility and slow growth are also under threat.

Fal and Helford Maerl Beds

Maerl. Fal and Helford Maerl Beds. UK.

And finally brown algae. As you may guess from the larger number of subgroups included within the main group, brown algae are rather loosely gathered together. Many do not share common recent ancestors. They are mostly marine and also include the largest of the individual algae. Yes that is the kelps, and also bladderwrack and its relatives belong in this group too. But brown algae are not only large, there are also small single celled ones too, including the diatoms. Diatoms are little cells encased in glasshouses, and despite their small size play an important ecological role on a global scale.

Coscinodiscus (light micrograph)

The diatom: Coscinodiscus.

So to conclude, algae are the misfits of the tree of life, they do not quite fit anywhere else so are instead lumped together in one big group. There are many differences within the group, but there are also similarities as well. Diversity of algae is vast, from tiny to giant, from simple to complex, but all need light to live. And the number of algae isn’t small either. Currently we have described 50,000 species of algae (Guiry & Guiry, 2020). That’s a lot of algae!


Guiry, M.D. & Guiry, G.M. 2020. AlgaeBase. World-wide electronic publication, National University of Ireland, Galway.; searched on 28 August 2020.

Are all individuals of a species the same?

The definition of a species states that all individuals within a species will have the same main characteristics (Collins Dictionary, 2020). These Burchell’s zebras all share the same characteristic striped appearance for example.


A group of Burchell’s zebras. Maasai Mara National Reserve, Kenya

So each individual within a species must share similar characteristics. But does this mean each will respond to environmental change in the same way? Traditionally a species is assumed to be one homogenous unit, assuming that all individuals within the species have similar environmental tolerances irrespective of their origin within the species range. Accordingly all individuals should be able to exist anywhere within the species’ range.

But is it that simple? Do all individuals respond the same way or do some individuals have the ability to respond differently? And more pressingly: Will all individuals within a species be affected by climate change in the same way?

Kohlekraftwerk Lünen

Coal Powerplant; Kohlekraftwerk. Lünen, Germany

Greenhouse gas emissions caused by human activity have enhanced the greenhouse effect; resulting in additional warming of the Earth’s surface (Houghton et al., 1990). Of this extra heat, 93% has been absorbed by the ocean leading to surface water warming of 0.11°C per decade in the Indian Ocean, 0.07°C per decade in the Atlantic and 0.05°C per decade in the Pacific (Hoegh-Guldberg et al., 2014).

Increased mean temperatures are redistributing species across the globe with many cold-water species moving polewards; including brown seaweeds. In Europe the cold-water cuvie kelp is already showing range shifts toward northern regions and local extinctions in its southern range (Müller et al., 2009) and by 2200 two common rockweeds will be lost from European shores south of Bordeaux (Jueterbock et al., 2013).

Forest kelp or cuvie (Laminaria hyperborea)

Forest kelp or cuvie (Laminaria hyperborea). Shetland, UK

If all individuals of a species respond the same way to warming then those at the edge of the species’ range will be more likely to be subjected to temperatures outside of their tolerance. These edge populations will therefore be most susceptible to range shifts and local extinctions. Those individuals within the central range should have tolerance to the warming since there is a greater margin before they are exposed to intolerable temperatures. Thus the species should be maintained within the central range and lost at the edges.

But what if all individuals do not respond the same way? In that case central populations could be in just as vulnerable a state.

If individuals respond differently, than those from different locations might be able to tolerate temperature changes to different degrees. Therefore those in central populations may also be exposed to temperatures outside of their tolerance.

Consequently if all individuals do not respond to warming in the same way then the species could be vulnerable to climate change throughout its whole range.

So either all individuals are the same and the edge populations are most vulnerable or individuals can be different and then all populations could be vulnerable.

evil sargassum

Gulfweed (Sargassum), Rockweed (Fucus) and Sea lettuce (Ulva). Llangennith, Wales, United Kingdom

Well, a systematic review of literature relating to marine plants and seaweeds appears to help answer this question. A striking 90% of studies observed within species differences to temperature change, meaning that individuals do in fact respond differently to temperature (King et al., 2018). Therefore all populations could be vulnerable. We could see local extinctions from both the edge and the central range of many important seaweed species.

To add to that, many seaweeds have limited dispersal. This results in highly structured distributions with closely related individuals remaining in close proximity. These individuals of close relation will possess similar characteristics, including temperature tolerance. Accordingly most individuals within the population will respond similarly to warming.

09192014 Sardine Run (1)

Sardine run. Tonga

In a highly mobile species with high dispersal, for example many fish species, migrant fish can move polewards to track the tolerable temperature and replace previously resident fish that can no longer tolerate the temperature in their previous habitat. These migrants can also contribute vital genetic material to the populations allowing the population to adapt to warming.

But seaweeds are not mobile; they cannot migrate since they are generally permanently attached to a hard surface. Consequently migration cannot readily contribute to the population. This means that struggling resident seaweeds cannot be replaced by migrant seaweeds. No migration and limited dispersal also means that no new genetic material is added to the population, therefore hindering adaptation to warming.

Figure 1 from King et al., 2018
Diagrammatic representation of how low and high dispersing species will respond to warming at the edge and centre of their range. Ovals represent the experienced warming within the edge and central range. Under warming conditions low dispersing species, such as seaweeds, with individuals that tolerate different temperatures will not be able to disperse into future suitable habitats and will become locally extinct.

It seems then that whether a population is at the edge or in the centre of the species range warming can be disastrous for seaweeds.

But individuals having different tolerances might also help protect a species, given a human little help. If individuals tolerant to warming are relocated to population susceptible to warming we may be able to boost local temperature tolerances. This concept of ‘Assisted migration’ is not new, having already been developed in terrestrial forests (McLachlan et al., 2007; Aitken & Whit­lock 2013; Williams and Dumroese, 2013) and seagrass meadows (Katwijk et al., 2016).

There are far more inherent logistical problems relocating seaweeds compared to the far simpler reseeding of terrestrial trees and seagrasses, however there is growing indication that this may become a feasible option in the face of climate change.

EUO © OCEANA Carlos Minguell 34111

Rhizomes of seagrass (Posidonia oceanica). Pecorini a Mare, Filicudi island, Eolian islands, Italy. © OCEANA

So we can conclude that just because all individuals from a species must share similar characteristics this does not mean that they are all exactly the same. We need to stop considering species as a single homogenous unit and instead see all individuals as unique in their own small way.

This post was based off the research of King et al., (2018).

If you enjoyed this post you can find the article here:

King, N.G., McKeown, N.J., Smale, D.A. and Moore, P.J., 2018. The importance of phenotypic plasticity and local adaptation in driving intraspecific variability in thermal niches of marine macrophytes. Ecography41(9) 1469-1484.


Aitken, S. N. and Whitlock, M. C. 2013. Assisted gene flow to facilitate local adaptation to climate change. Annual Review of Ecology, Evolution, and Systematics. 44: 367–388.

Collins Dictionary (2020) Definition of ’species’. HarperCollins Publishers. Date accessed 25/06/2020;

Houghton, J.T., Jenkins, G.J. and Ephraums, J.J., (1990) Climate change: the IPCC scientific assessment. Intergovernmental Panel on Climate change (IPCC), Cambridge University Press; Cambridge, United Kingdom

Hoegh-Guldberg, O., R. Cai, E.S. Poloczanska, P.G. Brewer, S. Sundby, K. Hilmi, V.J. Fabry, and S. Jung, 2014: The Ocean. In: Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part B: Regional Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Barros, V.R., C.B. Field, D.J. Dokken, M.D. Mastrandrea, K.J. Mach, T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea, and L.L.White (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1655-1731.

Katwijk, M. M. et al. 2016. Global analysis of seagrass restoration: the importance of large-scale planting. Journal of Applied Ecology. 53: 567–578.

King, N.G., McKeown, N.J., Smale, D.A. and Moore, P.J., 2018. The importance of phenotypic plasticity and local adaptation in driving intraspecific variability in thermal niches of marine macrophytes. Ecography41(9): 1469-1484.

McLachlan, J. S. et al. 2007. A framework for debate of assisted migration in an era of climate change. Conservation Biology 21: 297–302.

Müller, R., Laepple, T., Bartsch, I. and Wiencke, C., 2009. Impact of oceanic warming on the distribution of seaweeds in polar and cold-temperate waters. Botanica Marina52(6): 617-638.

Jueterbock, A., Tyberghein, L., Verbruggen, H., Coyer, J.A., Olsen, J.L. and Hoarau, G., 2013. Climate change impact on seaweed meadow distribution in the North Atlantic rocky intertidal. Ecology and evolution3(5):1356-1373.

Williams, M. I. and Dumroese, R. K. 2013. Growing assisted migration: synthesis of a climate change adaptation strategy. – In: Haase, D. L. et al. (tech. coord.), National Proceedings: Forest and Conservation Nursery Associations – 2012. Proceedings RMRS-P-69. Fort Collins, CO, USA Dept of Agriculture, Forest Service, Rocky Mountain Research Station, 90–96.