If you have ever walked along a beach or rocky shore, there is a high possibility that you would’ve encountered the brown coloured seaweed, like the one imaged in figure 1. Seaweed is the general term given to marine, macroscopic, multicellular green, brown, and red algae, but there are many other varieties of algae. Algae are aquatic photosynthetic species that lack the true roots, stems, leaves, or vascular and reproductive systems of higher terrestrial plants, but have replaced them with specialised organs. Typically, green, brown, and red algal species are found attached to the substrate by a holdfast in the shallow, light rich coastal waters, otherwise known as the littoral zone. But there are some extreme and abnormal deviations from this generalised rule, such as the Cyanidiophyceae– a group of acidophilic unicellular red algae that are able to grow within acidic hot springs, the following are further examples of algal species that have deviated from the generalised norm.

Figure 1: An image of the brown algal species, Fucus serratus and F. vesiculosus during the low tide on a shore in Wales (Photo Credit: M J Richardson).

Deep-Sea Red Algae

On a submersible dive in 1983, on deep seamounts near San Salvador, Bahamas, scientist found the deepest know photosynthetic eukaryote (unicellular or multicellular organism, who have a cell nucleus and other organelles enclosed within a cell membrane). This un-named red crust-like coralline algal species was found at the incredible depth of 268 m. This species is capable of photosynthesising on light irradiance that is 0.00005 – 0.00009 % of the maximum light irradiance that is found at the surface. To put that into perspective, typically a red alga in the littoral zone to reach its compensation point (the amount of light intensity on the light curve where the rate of photosynthesis matches the rate of respiration) needs a light intensity between 4 and 56 μmol.m-2.s-1, even with the aid of the crystal clear blue waters of the Bahamas, the deep-sea red alga is able to survive on light intensities that never exceed 0.0015 – 0.027 μmol.m-2.s-1. An astonishing 99.325 % lower than the lowest compensation points in the littoral zone, in the best-case scenario. The alga demonstrates the importance and resilience of algal species around the world for their productivity and important in the marine food web.

Figure 2: A coloured diagram of the Sargassum fish, which spend its entire life cycle living in and around the floating mats of Sargassum seaweed (Photo Credit: A. H. Baldwin)

Pelagic Brown Alga

Sargassum is the genus name of approximately 150 species of brown algae. Typically, like the vast majority of algae, Sargassum spp. are found attached to the substrate along temporal coasts. However, there are two exceptions to this generalised rule, S. natans and S. fluitans, both of which have left the substrate behind and are only found floating in masses pelagically in the Sargasso Sea. Like others in their genus, the pelagic species have leaf-like blades, branch-like stipes, and oxygen-filled berry-lie structures called pneumatocysts which aid in buoyancy, but lack the anchoring holdfast. These unique algal species provide food, shelter, and a breeding ground for a wide range of animal species, ranging from fish to sea turtle and birds. It is a primary nursery area for important commercial fish such as amberjacks and dorado. The Sargassum fish, which is imaged in figure 2 is a species of frogfish (typically a truly benthic family) that spends its entire life cycle inhabiting floating masses of Sargassum algae. The unique algal species is so rich in biodiversity, that in 2005 when scientists analysed a tow of 82 kg of Sargassum, they identified 3000 individual fish living within the drift of algae. This pelagic species of algae therefore has very high ecological importance to the borderless Sargasso Sea, but is coming under threat, and is suffering from a reduction in biodiversity, possibly as a result of ocean acidification and warming. As suggested in the video from MBARI below.

Video Credit: Monterey Bay Aquarium Research Institute (MBARI)

Sea Ice Algae

In the Southern Ocean surrounding Antarctica there is brown ice. This phenomenon was first recorded by the sealers and whalers who would know to look for this marker, as there would be a great density of marine mammals in that area. The reason behind this is that the brown colouration came from extremely high densities of unicellular photosynthetic macroalgae called diatoms. These diatoms are one of the primary producers in the Antarctic marine environment, but are trapped within chambers and brine channels in the sea ice during the winter months. Within the brine channels, these diatoms can photosynthesise using the sunlight that makes it through the snow and ice layers, or a single gene induces an adjustment to the metabolism of the organism, that it can then allow the diatom to grow on glucose in the absence of light. This strategy is so effective that the photosynthetic pigments within the diatoms create the brown colouration of the sea ice, which when it melts sparks the beginning of the entire food web.

Like the threats facing the Sargassum algae, ocean warming is causing a reduction in sea ice cover, as well as the length the sea ice is formed, which is causing a reduction in the densities of diatoms produced during the winter months, which would greatly affect the wider Antarctic marine community, from krill, to penguins, to cetaceans.

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