Finding Nori: Journey into the Underwater World of Seaweeds

My understanding of marine ecosystems came, quite embarrassingly, years after I began to call myself a student of ‘wildlife biology’. I had several ‘ah-ha’ light bulb moments during my first ocean experiences and numerous very fundamental questions I thought I should have already known the answers to- why are reef fish so colourful? Did the earth always have an ocean? Corals were animals?!

I was shocked at just how ignorant I was about something as spectacular and vital as seaweed. Up until this point, I naively imagined seaweed to be tall leafy greens swaying in the background in 'The Little Mermaid' movie, and the crispy dark green bits that we rolled sushi in.

The first time I came across seaweed was during underwater coral surveys I was conducting in the Andaman Islands back in 2013 and it was like nothing I had imagined it to be. Instead of lettuce-like leafy greens, I was looking at hard crusts of bright pink and purple lining the surface of rocks. At first, I mistook these patches for coral but learned soon after that these are known as ‘crustose coralline algae’ or CCA. A photosynthetic being with a hard structure made of calcium! As I did more surveys to study coral, I came across a diversity of seaweed, some quite bizarre! From forests of sargassum to shiny single-cell ‘sailors’ eyeballs’ embedded in the reef. Thus began my slow, and continuing, journey of learning about seaweed.

For starters, I learned that the term “seaweed” is a misnomer. Agreeing upon what we should and should not call a “weed” can take us all day. By definition, weeds are plants that we perceive to be growing so profusely that they completely take over a space and use enough nutrients to stifle the growth and wellbeing of other species in the vicinity. A present-day example of a weed would be Lantana camara, the South American plant that has taken over 300,000 square kilometres of India’s forests in the 200 years it has been around in India. Seaweed, on the other hand, is a loosely used term to describe tens of thousands of extremely vital species of photosynthetic creatures we call marine algae. Interestingly, the term algae is also an overarching term that encompasses species from at least three different taxonomic kingdoms (Plantae, Protista, Chromista). Algae can range in size from microscopic phytoplankton to relatively large “macroalgae”. Phytoplankton includes but is not limited to about 25,000 diatoms and dinoflagellates, which can be free floating in the ocean but also live in partnership with animals like corals. There are about 12,000 species of macroalgae known to science, and they fall into diverse evolutionary subgroups such as red algae, green algae, and brown algae.

 

Visually seaweed, which I will henceforth call algae, can be mistaken for seagrass. Superficially many green algae and seagrass share a leafy appearance, but a closer look at both these photosynthetic marine marvels will reveal that like terrestrial plants, seagrasses have leaves, shoots, and roots that can run along under the sand, grounding the plants to the seafloor. Algae do not have roots and instead of true leaves, they have a somewhat fleshy leaflike “thalli” (“thallus” in singular) that is connected to a stable surface by an anchoring structure known as a holdfast.

Now that we know that algae are not a mass of noxious plants floating around in the water, it is also important to understand just how crucial their presence is in the ocean. Just like grasses, plants, and trees on land, micro and macroalgae are photosynthetic, primary producers that fuel food webs in all the marine ecosystems they inhabit.

Microalgae are eaten by microscopic animals in the zooplankton and larger macroalgae are favourites of numerous invertebrates like crustaceans, molluscs, herbivorous fish like the parrotfish and rabbitfish among others. They also cater to the vegetarian needs of reptiles like sea turtles.

Macroalgae might look like small clumps of turf, bunches of grapes, tropical ferns, or fuzzy films on seagrass. They can also grow in vast expanses like forests of Sargassum and meadows of Padina. Sargassum forests, which often grow attached to small boulders in sandy areas, offer shelter to schools of small fish that would otherwise be exposed to predators.

Sometimes Sargassum can get uprooted from its substrate but in the time that it takes to wash up onto the shore, this algae continues to float on the surface of the water using gas bladders on their thalli. These floating masses provide pitstops for resting sea snakes and turtles, and nesting habitats for a variety of animals including flying fish.

There is another side to algae too. In tropical oceans around the world, where suitable habitats are prime real estate, certain species of macroalgae are tough competitors of corals. Coral reefs are dynamic ecosystems balancing on this delicate relationship between coral and macroalgae. Since the late 1990s every year has seen an increase in sea surface temperatures that are resulting in unprecedented levels of coral bleaching and mass coral death. As a consequence of this, coral reefs are being gradually colonized by macroalgae in large swathes across their distribution. Slow-growing calcium-building animals like corals often do not stand a chance against fast-growing primary producers like macroalgae. These are some of the several ecological relationships that are getting disrupted by climate change.

At the end of the day, algae provide us with critical “ecosystem services” that power our daily lives. Protecting our coastlines, feeding us, and feeding the seafood that feeds us, storing and cycling nutrients, especially carbon. Waves of progress in the medical and pharmaceutical world have been possible due to pathbreaking research on algae. However, what has shockingly gone unnoticed by many humans, including some of us biologists for far too long, is how much the world relies on the ocean, on algae, for oxygen. 50-70% of the world's oxygen needs are met by marine algae. In other words, we owe every other breath we take to seaweed.