Algae

• 1. Introduction

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  Algae have no agreed boundary, and that is the first surprise. They are a large, diverse group of photosynthetic organisms, yet they exclude the land plants entirely. One alga can be a single cell too small to see. Another, a kelp, can stretch up to 50 metres in length. Some live as cyanobacteria, which are not even cells with a nucleus. Others are seaweeds anchored to rock by a holdfast. The word itself is slippery. The singular alga is the Latin word for seaweed, and its deeper etymology is described as obscure. So what holds this group together, if not a shared ancestor? What does it mean to be photosynthetic but not a plant? And why does a single online database count 50,605 living species while still admitting that nobody knows the real number? This is the story of a category that science keeps redrawing, and of organisms that feed the oceans, build coral reefs, and may one day fuel engines.

• 2. A Group With No Ancestor

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  Algae are polyphyletic, which means they do not descend from one common ancestor. That single fact unravels the idea of algae as a natural family. Instead, the group is held together by something they do, not something they are. The photosynthetic machinery in algae is ultimately derived from cyanobacteria, organisms that produce oxygen by splitting water molecules. This sets them apart from photosynthetic bacteria that do not split water. Chlorophytes, red algae, and grey algae carry primary chloroplasts derived directly from endosymbiont cyanobacteria. Other lineages took a stranger path. Diatoms, cryptomonads, euglenoids, and brown algae carry secondary chloroplasts, acquired indirectly from red or green algae that earlier protists engulfed. Because of this, eukaryotic algae cannot be traced to one hypothetical common ancestor. Their plastids arrived through separate acts of capture across deep time. The oldest undisputed fossil of a eukaryotic alga is Bangiomorpha pubescens, a red alga found in rocks around 1,047 million years old.

• 3. Bodies And Blueprints

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  Most algae are single-celled organisms without roots, leaves, or stems. From that simple starting point, a wide range of forms appears, and convergence in unrelated groups is common. The simplest algae are unicellular flagellates or amoeboids. Colonial and nonmotile forms developed independently across several groups. The organizational levels carry their own vocabulary: colonial groups of motile cells, capsoid cells embedded in mucilage, coccoid cells with walls, palmelloid clusters, filaments of connected cells, and parenchymatous thalli with partial tissue differentiation. Only three lineages reached full tissue differentiation. The brown algae include kelps that may reach 50 metres in length. The red algae and the green algae complete the trio. The most complex forms appear among the charophyte algae, which have distinct nodes separated by internode stems, with whorls of branches at the nodes reminiscent of horsetails. That charophyte lineage eventually led toward the higher land plants. The innovation that defines those nonalgal plants is a female reproductive organ with protective cell layers that shield the zygote and developing embryo, which is why land plants are called the Embryophytes.

• 4. Counting The Uncountable

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  As of January 2024, the online database AlgaeBase documents 50,605 living and 10,556 fossil algal species, sorted into 15 phyla or divisions. The number sounds precise, but the field treats it with open doubt. The Algal Collection of the US National Herbarium holds roughly 320,500 dried specimens, and even that is described as not exhaustive, because no exhaustive collection exists. Estimates clash sharply. In the British Isles, one report estimated 20,000 algal species in the UK, while another checklist reported only about 5,000. Faced with a 15,000-species gap, the text concludes that many detailed field surveys will be required before a reliable total is possible. Regional tallies repeat the pattern: around 5,000 to 5,500 species of red algae worldwide, some 1,300 in Australian seas, 669 marine species from California, and 642 in the check-list of Britain and Ireland. Most of these counts omit microscopic algae such as phytoplankton, which means the largest part of the group goes uncounted. The naming itself reaches back to Linnaeus, who in Species Plantarum in 1753 recognized 14 genera of algae, of which only four are still counted as algae today.

• 5. Theft And Symbiosis

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  Coral reefs are built from the calcareous exoskeletons of stony corals, and algae are the reason those corals thrive. Dinoflagellates often live as endosymbionts inside the cells of reef-building corals, where they generate sugar and oxygen through photosynthesis using the carbon dioxide the host produces. Reef-building stony corals require endosymbiotic algae from the genus Symbiodinium to stay healthy. When a coral loses its Symbiodinium, the result is coral bleaching, a condition that leads a reef to deteriorate. Lichens tell a parallel story of partnership. A lichen is defined as an association of a fungus and a photosynthetic symbiont that forms a stable vegetative body with a specific structure. The fungal partner, or mycobiont, comes mainly from the Ascomycota, and in nature it does not occur apart from the lichen. The photosynthetic partner may be a green alga or, alternatively, a cyanobacterium. The green alga genus Trentepohlia can grow on its own or become lichenised. Endosymbiotic green algae also live near the surface of some sponges, including breadcrumb sponges, where they can account for 50 to 80% of sponge growth in some species.

• 6. Food, Fuel, And Fertilizer

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  Traditional seaweed farming has existed for thousands of years, with strong traditions in East Asian food cultures. The eating of algae is strikingly varied by country. China consumes more than 70 species, including fat choy, a cyanobacterium treated as a vegetable. Japan eats over 20 species, among them nori and aonori. Ireland eats dulse, Chile eats cochayuyo, and Wales turns laver into laverbread, known there as bara lawr. Some algae carry the long-chain omega-3 fatty acids DHA and EPA, and they are the original source of the omega-3s found in fish oil, which fish acquire by eating microalgae. From the red alga Chondrus crispus comes carrageenan, used as a stabilizer in milk products. Agar, derived from red algae, grows bacteria and fungi because most microorganisms cannot digest it. Alginic acid, extracted from brown algae, serves as a food gelling agent and in medical dressings. Seaweed has been used as a fertilizer for centuries; George Owen of Henllys, writing in the 16th century, described South Wales farmers gathering drift weed in great heaps until it rotted, then casting it on the land so that good corn sprang up. Algae-based fuels hold promise because algae can produce more biomass per unit area in a year than any other form of biomass, with the break-even point estimated to occur by 2025.

• 7. Algae As Sentinel

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  Ancylonema nordenskioeldii was found in Greenland in areas known as the Dark Zone, where it increased the rate of ice-sheet melting. The same alga later appeared in the Italian Alps, after pink ice formed on parts of the Presena glacier. Algae register pollution as readily as they register cold. Because algal metabolism is sensitive to many pollutants, the species composition of algal populations may shift when chemical pollutants appear, and the algae can be sampled and kept in laboratories with relative ease. That sensitivity turns into a cleanup tool. Sewage can be treated with algae, reducing the toxic chemicals otherwise required. Agricultural Research Service scientists found that a horizontal algal turf scrubber, built as shallow 100-foot raceways of nylon netting, can capture 60 to 90% of nitrogen runoff and 70 to 100% of phosphorus runoff from manure effluents. They studied it for three years, then dried the nutrient-rich algae and tested it as fertilizer. Cucumber and corn seedlings grew just as well with the algal fertilizer as with commercial fertilizers. One alga, Stichococcus bacillaris, has even been seen colonizing silicone resins at archaeological sites and biodegrading the synthetic material.