Photosynthesis is undervalued. We enjoy, and in fact need, the outputs of this process every day of our lives. Even during a long Vermont winter, the process of photosynthesis is essential to our survival. We benefit from it with every breath we take and with every bite we eat. We really should be thinking about it, and appreciating it, much more often than we do.
Photosynthesis shapes the aquatic world too. Along with providing food and oxygen for other creatures, it has influenced the amazing diversity of algae found in the water. Finding enough light isn’t easy, especially when all the other photosynthesizers are looking for their own ray of sunshine. Just like the finches of Galapagos, algae have evolved to take advantage of specific photosynthetic niches in the watery world. Let’s look at the two important ways they get the light they need.
Different photosynthetic pigments
Algal pigments capture very specific wavelengths of light. The main pigment in photosynthesis, chlorophyll a, absorbs light in two areas of the spectrum – around 642 and 372 nm. All algae have chlorophyll a, and many have additional (accessory) pigments that capture light at other wavelengths. We classify algae based on their pigment composition – for example green algae have primarily chlorophyll a and c, dinoflagellates have chlorophyll a and a unique set of pigments called xanthophylls.
As sunlight penetrates the water, it breaks up and only some wavelengths reach the depths. Ice cover also limits the passage of light. Only algae with pigments able to capture light under those conditions can survive. Diatoms commonly grow attached to the bottom of the ice layer in our lakes during the winter when sunlight is already in short supply. In summer, certain cyanobacteria are able to thrive in the thermocline well below the surface because of their pigments. Cryptophyte algae can harvest a wider range of wavelengths than other algae and are common in many waters (see Smithsonian Magazine).
Some algae can move with flagella. Chlamydomonas (green algae)and Euglena (euglenoid algae) also have eyespots that help them sense light so they move in the right direction. Diatoms living on the bottom have special grooves in their silica cell walls filled with mucilage that lets them slide along surfaces a little like bumper cars at the fair (see https://www.diatoms.de/en/diatoms/introduction-to-motility). Filamentous cyanobacteria living in the sediment have special fibers that help them spiral through shifting mud. Physical adaptations like these help the cells get to and stay in the light.
A lot of algae aren’t able to move. Instead, they stay suspended in the water column by increasing their surface area to slow their sinking rate. Diatoms and green algae grow in long filaments, chains, and flat sheets of cells (see the diatoms Aulocoseira and Fragilaria, and the green algae Mougeotia and Hydrodictyon). Other algae have long fine spines and surface decorations that slow sinking. Some cyanobacteria have specialized gas vesicles called aerotopes that they can use to move up to the surface in quiet waters.
There are many other things that have contributed to algal diversity, but access to light is a big one for these photosynthetic organisms. Humans aren’t the only ones celebrating the coming of spring, longer days, and more sunlight in the north country!