Microalgae are free-living unicelled (single-celled) organisms, that are mainly phototrophic, which means that they catch photons and CO2 and convert it into sugar as energy storage. This is known as photosynthesis. Some of these algae became, at one point in time, specialized to life on land as chlorophyll, which is the green pigment in the leaves of plants, and is essential for their ability to perform photosynthesis. Microalgae exists everywhere in the ocean, and will, under the right circumstances, bloom to gigantic proportions. This happens naturally every spring, but it can also arise from other circumstances, and sometimes it can have devastating consequences for other sea life. Microalgae are also known as phytoplankton.
Macroalgae, on the other hand, are multicellular, commonly known as kelp and seaweed. Occationally you'll hear "sea plant" being used to describe kelp and seaweed, but macroalgae and plants are very different organisms. Seaweed doesn't have a root system that absorbs nutrients and water, instead is has an organ called "holdfast" which secures the blade firmly to the seabed while the nutrients are absorbed in the blade. "Blade" and "holdfast" are macroalgae analogues to "leaf" and "root" in plants, though they function very differently. Macroalgae can be divided into three groups, red, green and brown, which are the three pigment types that exists. They all perform photosynthesis, but they differ in color. All three categories have species that are well suited for human consumption, but so far almost all of the macroalgae cultivated in Norway are brown macroalgae. There are several reasons for this, some algae have very complicated life cycles, but brown macroalgae are much larger and have much higher biomass yield than the other types.
The difference between kelp and seaweed is far less important, but often times it's part of the name, like Sugar Kelp. Seaweeds are generally smaller and live in the shallows close to shore, kelp prefers deeper waters. Even though these differences exists, we commonly use "seaweed" to describe cultivated macroalgae.
Seaweed cultivation is a subject that many people are interested in because it opens up new venues to solve many of our problems related to production of raw materials and food. Gloomy predictions about food shortages and resource depletion have all been put to shame by advances in technology, which has made agriculture ever more efficient, and pushed the limit of production and resource exploitation to new heights. Despite these advances, agricultural land is under ever more pressure, and every year huge new areas are being converted to agriculture.
Seaweed is cultivated at sea, and can be expanded and increased almost indefinitely without having to chop down a single tree. The available area is practically unlimited, but developing a whole new industry will take time. Those who wish to reduce the pressure on the world's rainforests can help by choosing seaweed wherever it exists as an alternative.
Seaweed cultivation is a brand new industry, in the west anyway, and the species are still in the primeval state. There is a vast potential for improvement and specialization, but we don't have to wait thousands of years for seaweed to start replacing plant products. Seaweed cultivation is admittedly more expensive and difficult than any plant cultivation today, but that could soon change.
It's not just as food or raw materials that seaweed can have great value, seaweed can be used in anything from medicine to energy. This huge range of application can be utilized with better technology, and this will open up countless new markets for seaweed. Many of these markets could be using seaweed today if it were not for the high costs. If people choose seaweed whenever they can, they are directing the markets to invest in better technology for both seaweed production and processing. Seaweed is the "plant" that can do anything and everything, but it needs a push.
