Our understanding of the evolution of seaweeds and other algae is undergoing a revolution. Before the 1970s, the algae were classified in the plant kingdom but, over the last five decades, numerous scientific studies on these endlessly fascinating organisms have generated a wealth of new data and a new classification scheme that, in reflecting their evolutionary history, assigns various algal species to four of the six kingdoms of life on Earth—this is an unprecedented phenomenon in the living world! The Lives of Seaweeds: The Natural History of Our Planet’s Seaweeds and other Algae reveals the extraordinary story of the evolution of the algae, their morphology, reproduction, ecology and global relevance in sustaining life on Earth.
We are all familiar with seaweeds, the large algae we see at the seashore, and we hear about algal blooms in the media, but what is an alga?
JP: This is a difficult question. The algae are generally considered to be photosynthetic organisms that evolved in aquatic environments and, although this statement is true, it defines them on their biochemistry and ecology, not their evolutionary history on which the classification of living organisms must be based. Some algae are indeed plants and are classified in the plant kingdom, but most are not. A new classification scheme, proposed in 1998, which reflects the evolutionary history of all life on Earth, assigns the blue-green algae (now cyanobacteria) to the Kingdom Bacteria, the green and red algae to the Kingdom Plantae, the single-celled Euglena and its relatives to the Kingdom Protozoa and the brown seaweeds and their many relatives to the exclusively algal Kingdom Chromista. Only the Kingdoms Animalia and Fungi lack algal species.
Why has the evolutionary history of the algae been a recent discovery and how did the algae evolve this enormous diversity?
JP: Amazingly, the evolution of the main algal groups (phyla and lineages) took place inside cells with the plastids playing the central roles. Plastids are microscopic organelles inside cells, the sites for photosynthesis, of which the chloroplast, the green plastid of plants, is the best known. Unravelling the complicated evolutionary history of the algae began in the late 1950s when two newly-invented techniques—electron microscopy and DNA technology— enabled scientists for the first time to study in great detail the plastids of the algae.
In 1959, scientists were astounded when they found DNA in the large spiralled chloroplast of the freshwater green alga, Spirogyra. Back then, DNA—the genetic material—was thought only to occur in the nucleus. The race was on. Further studies revealed plastid DNA in numerous species of the algae and the land plants but even more intriguing, was the discovery that the plastid DNA of the red seaweed, Porphyra, was most closely related to the DNA of the cyanobacteria. Then came the astounding observation of the remnant of a nucleus in the red plastid of a single-celled alga—a cryptophyte. The startling discoveries continued and the evidence accumulated piece by piece until the pieces were sufficient to be collated into an overview that deepened our understanding of algal evolution.
How algal cells acquired their many different types of plastids and evolved, from an ancestral cyanobacterium, into nine distinct lineages is a remarkable story, almost in the realms of science fiction. The first algal cell evolved when a colorless cell engulfed and took into its cell, the colored photosynthetic cell of a cyanobacterium, which became its plastid. One cell engulfing another cell is unremarkable, merely a common method of feeding (called phagocytosis), still employed by single cells today. What is remarkable is that the colorless cell did not digest the cyanobacterial cell: it remained intact, functioned as plastid inside its host cell and was passed onto successive generations for a billion years, the result of the host cell and its plastid forming a mutually-beneficial association, or a symbiosis. This first algal cell was the ancestor to three algal lineages: the blue-gray algae, the red algae and the green algae, with one group of freshwater green algae (the Charophyta) giving rise to the land plants (mosses, ferns, conifers, flowering plants).
Not a one-off occurrence, it happened again, many times over, when colorless cells engulfed the cells of either the green or the red algae to give rise another six algal lineages. And the surprises did not stop there. The parasite that causes malaria is an alga that has lost its plastid! Half of the species in another algal group, the dinoflagellates, lack plastids and are predators while a few harvest their plastids from different algal species to other dinoflagellates.
Are all algae ancient?
JP: Some algae are ancient, some are less ancient while others have evolved relatively recently. There have been three main pulses of algal evolution.
The ancient cyanobacteria first appeared on Earth building dome-shaped structures called stromatolites around 3.5 billion years ago when bacteria were the only inhabitants of Earth’s inhospitable environment. Crucial to the evolution of life, the cyanobacteria, as the first photosynthetic organisms, were the source not only of the oxygen in our atmosphere but also of the plastids that enabled the evolution of the other algal groups and the land plants.
The second pulse of algal evolution occurred around 1.5 billion years ago and produced the blue-gray, red and green algae, ancient lineages that predate the Cambrian explosion of life around 600 million years ago. This pulse is marked by a very important fossil of a red seaweed—the first known multicellular plant on Earth—from rocks in Canada, dated to be around 1.047 billion years old.
The third pulse which produced the diverse members of the Kingdom Chromista, occurred relatively recently, after the first appearance of many animal and land plant phyla. One chromist phylum, the diatoms, first appeared as fossils around 190 million years ago.
How much diversity in morphology is exhibited among different algal lineages?
JP: The megadiverse algae exist in every conceivable shape and size, ranging from microscopic single cells measuring just one micron in diameter to giant leathery kelps that grow to 160 feet (50 m) in length. Various algal species have bodies that are single cells, colonies of cells, multicellular, or are composed of large multinucleate cells or siphons.
Single cells are not just simple spheres; they have also evolved into the shapes of spindles, crescents, pyramids, stars, cubes, cones and even more elaborate configurations with their cell margins bearing horns, lobes, ridges, spires and winglike extensions. Colonies are composed of loosely held together cells which, like the single cells, vary in shape in different species. Some single cells and colonies can swim, propelled through the water by the beating of a whiplike appendage called a flagellum.
Multicellular algal species—predominantly the seaweeds and pondweeds—are equally morphologically diverse and range from delicate to robust filaments, broad sheets, tubes, blades, fernlike fronds, crusts, sacs, cushions or large leathery straps.
Some seaweed and pondweed species are composed of numerous very large cells, some amazingly visible to the unaided eye, that contain hundreds of nuclei and chloroplasts, while one group of tropical green seaweeds have a body that is a siphon—a long tube that can be 3 feet (about 1m) or more in length, without cross walls that would partition the protoplast—this is life without cells.
The most visible and familiar algae are seaweeds that inhabit rocky seashores. Where do the other algal species live?
JP: In nature, the megadiverse algae are almost everywhere. Amazing adaptations enable different algal species to live in most habitats on Earth. Unicellular algae float or swim in the world’s oceans and in freshwater environments. Microscopic algae form mats on the mud surfaces of mud flats and pondweeds live in lakes, streams and ponds. Terrestrial algae inhabit tree trunks, leaves, fences and the soil surface. Even small, temporary puddles, water-filled rock hollows and birdbaths are often teeming with algae too small to be seen with the unaided eye. There are also algal species that live in extreme environments such as hot springs, salt lakes, soda lakes, desert soils and snow. In each of these habitats the algae are free-living organisms, but various species form associations with a diverse array of other organisms. Some algal species may grow on other algal, plant or animal species, or they may enter into more intimate relationships as symbionts, or parasites.
Are algae important?
JP: Various algal species are important on a global scale, others are important human foods while still others are valuable sources of chemical compounds that range from biofuels to pharmaceuticals.
Algal species that maintain large populations in the vast contemporary oceans drive the global carbon, nitrogen and sulphur cycles. In the carbon cycle, photosynthesis is the only process that removes large quantities of carbon dioxide from the atmosphere, calculated for marine algae to be around 46 percent, less than the 54 percent removed by the land plants, of the global total measured in billions of tonnes of carbon each year. Also important, are the tonnes of carbon locked away from the atmosphere when dead algal cells sink to the deep ocean floor and when the calcifying algae not only lay down calcium carbonate onto their cell walls but also form, from their dead bodies, carbonate sediments and the carbonate rock, limestone.
Important in the global nitrogen cycle, cyanobacteria take nitrogen gas from the atmosphere to make ammonia which increases the fertility of aquatic environments. Marine algae ameliorate the planet’s weather by releasing volatile sulfur compounds into the seawater which rise up into the upper atmosphere where these compounds cool the climate by either scattering the sun’s rays or seeding cloud formation.
Algae have been eaten by humans and animals for millennia. Among the most widely eaten are the nutritious health food, spirulina, which is also the main food of the lesser flamingo, the delicious red seaweed nori wrap around sushi rolls and the Irish moss which, nowadays, makes a pudding reminiscent of panna cotta. Carbohydrates found only in the cell walls of red and brown seaweeds have numerous industrial applications, most notably as gels and stabilizers in the food industry.
Extracts of algae are added to pharmaceuticals, cosmeceuticals, and nutraceuticals as well as being trialled as a source of biofuels, a modern energy source comparable to petroleum that was formed by the algae that lived long ago.
Julie A. Phillips is an environmental consultant in aquatic ecosystem health, algal blooms, and seaweed communities. She has investigated a diverse range of algal projects on reproduction, life histories, ultrastructure of motile cells, chemical composition of pheromones, taxonomy, biogeography, invasive species, ecology, biodiversity, and conservation. Prior to consultancy, she was research fellow at Monash University and the University of Queensland.