The mystery of the giant kelp forest - Luka Seamus Wright and Salomé Buglass

Kelp forests are some of the most majestic ecosystems on Earth—towering underwater structures teeming with life. But what exactly is kelp? All kelps are seaweeds, but not all seaweeds are kelp. The term "seaweed" typically refers to marine macroalgae, which are algae visible to the naked eye. Algae that are too small to see without a microscope are called microalgae, better known as phytoplankton.

Algae are a complex and diverse group of organisms that we often classify into colors for simplicity: blue-green, green, red and brown. These organisms are very different from land plants and their descendants, such as seagrasses, but fulfill the same function, so we still call them plants. The lines are blurry. For instance kelps, the fastest-growing and largest seaweeds in the world, are brown algae that belong to the heterokonts, which are as far removed from land plants as from us, but still photosynthesise and create marine forests, which is a pretty planty thing to do. According to AlgaeBase, the most comprehensive online database on algae, there are an estimated 70 thousand recognised species of algae, of which about 12 thousand are seaweeds and just over 120 are kelps.

Algae are really interesting model organisms for studying photosynthesis because they have such diverse methods of harvesting light. Green algae use the familiar pigments that land plants use, mostly chlorophylls a and b, but red algae also have a whole family of accessory pigments called phycobiliproteins, which are what turn them pink, red or purple, and brown algae use chlorophyll c instead of b and have loads of fucoxanthin which turns them ochre or chocolate brown. Algae also live in the most light-limited environments on the planet. You’ve already met the deep kelps, but there are other algae that can handle next to no light, such as crustose coralline algae which encrust the deep seafloor or build rhodolith or maërl beds and the endosymbiotic heterokont algae of mesophotic corals. This extends photoautotrophy to the tips of the last sunrays, only giving way to heterotrophy in the dark water column and chemoautotrophy at hydrothermal vents and cold seeps bathed in complete darkness. What is especially exciting is that some kelp detritus, which was presumed to be dead like the leaves falling off trees, continues to photosynthesise and different species do this to different extents.

Such live drift could reach even deeper zones than kelp forests, but we still don’t understand how photosynthesis is even possible with so little light or how quickly kelp is decomposed there, so the effect on the carbon cycle remains unclear.

Kelp forests are under increasing threat and their conservation and restoration have become important topics. They are declining globally due to ocean warming, overgrazing, pollution and habitat destruction. Marine heatwaves in particular are devastating, pushing kelp species beyond their thermal tolerance. In response, scientists and managers are seeking climate refugia—places where stable environmental conditions may allow vulnerable species to survive and recover. It is thought that the mesophotic zone could provide refuge for many species and even entire ecosystems such as kelp forests. As you have learned, mesophotic ecosystems are found between 30 and 150 meters depth and receive limited light but benefit from stable temperatures and an abundance of the essential ingredients for photosynthesis. These conditions make them promising climate refuges, not just for kelp forests but for other photosynthesising communities like the mesophotic coral reefs and rhodolith beds mentioned above.

Mesophotic ecosystems are difficult to study due to their depth. Research mostly relies on technical diving and expensive submersibles. But increasingly affordable remotely operated vehicles have recently opened new opportunities for exploration. Are you interested to know how these tools were used to discover deep kelp forests in the Galápagos? Check out The day we discovered the kelp forest in the Galápagos, The quest for kelp and diving with Dr. Sylvia Earle and Exploring the “bajos” or shallow seamounts of the Galápagos Marine Reserve. Technical diving remains an important albeit less accessible way to reach deep kelp forests. Check out these cool expeditions off France and Italy.

The study of algae is called phycology (not to be confused with psychology!). Here are some popular phycology journals: Journal of Phycology, European Journal of Phycology, Phycologia, Journal of Applied Phycology, Phycological Research, Botanica Marina, Aquatic Botany, Algal Research and Algae. If you’re interested in photosynthesis in particular, the best research is published in Photosynthesis Research, New Phytologist, Annals of Botany, Photosynthetica, Nature Plants, The Plant Journal, Plant, Cell & Environment, The Plant Cell, Plant Physiology and Physiologia Plantarum.