Don’t be fooled by brown algae. Kelps and their relatives could easily be mistaken for plants, with their stemlike stipes, leaflike blades, and rootlike holdfasts. Nothing could be further from the truth. You are more closely related to a shiitake than a kelp is to a kale.
Kelp, showing (from left to right) the blade, stipe, and holdfast. Jericho Beach, Vancouver, BC.
Figure 2g from Tang et al. 2020. Proterocladus antiquus. Scale bar 200 μm.
Researchers from Virginia Tech and the Chinese Academy of Sciences have described some billion-year-old fossils that they interpret as green algae. What’s interesting about these fossils, aside from being older than previously known green algal fossils, is that they appear to be fully multicellular, with differentiated cells. This is a valuable find, because it shows that at least one of the many green algal lineages that have independently evolved multicellularity did so relatively early. Sadly, the article, in Nature Ecology & Evolution, is paywalled. The best I can do is link to the article’s page on ResearchGate, which has a read-only version. I requested a full-text through that page, and the lead author sent a pdf promptly.
Imagine you’re swimming in nice, warm water, happily making your own food without a care in the world (other than zooplankton). You just need to store up enough starch before nightfall to hold you through the night so you can quit swimming, absorb your flagella, and wait for the sun’s return. You see a green light below, and you swim toward it. You can’t help yourself; your phototactic machinery is hardwired to respond.
Next thing you know, you’re captured by a giant, tentacled polyp. You look for a way out, but there is none. You’re stuck there for the rest of your life, forced to work and have the food you produce stolen by your coral overlord. Resigned to your fate, you absorb your flagella and get down to photosynthesizing.
Figure 2A from Aihara et al. 2019. the coral Echinophyllia aspera and its algal captives under natural light conditions (Scale bar, 1 cm.).