The Underwater Forest

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Photo by Getty/chayanan

Seaweed Chronicles(Algonquin Books of Chapel Hill) by Susan Hand Shetterly, teaches readers about the growth and harvesting of the essential organism seaweed.  Through the use of stories and descriptions, readers can experience what it is like growing up on the shores of the Gulf of Maine and walking along the shoreline learning about the organisms that live within the algae seaweeds.

When my children were small, I took them to the shore. It would be low tide, and we walked over the pebbly mud and parted the seaweed strands, the bladder wracks and the knotted wracks attached to the big rocks that the glacier had dragged with it from miles away. We peered beneath the seaweeds. The outer layers had dried in the air, but the under layers held a briny wetness that made the creatures we found within especially bright: starfish; the egg capsules of the dog whelks, small sea snails whose eggs look like tiny Greek amphora; green crabs as new and small as my children’s fingernails; young green sea urchins; limpets; sideways-swimming scuds; yellow periwinkles; sometimes a hermit crab or a sea anemone.

It seemed right somehow to be bringing young and growing children to the edge of the bay where life had evolved so far back in time that it was hardly imaginable, as if this place with its seaweeds were the proof we needed that we had come from a world of water and that everything might have looked, at one time, something like this.

When we are children, our psyches tend to become imprinted on the places we know and love, and for many of us, that edge where water and land meet is one that stays with us all our lives. I didn’t think of it then, but now I believe I was offering them exactly this: their home place to imprint upon so that they might go into the larger world with a sense of where they come from, and thus a sense of who they are.

Lifting the seaweeds and finding life beneath them reminded me of the crepe paper balls my sister and I used to find in our Easter baskets when we were young. We would unroll them across the floor of our New York City apartment, where they spilled out treasures wrapped in tissue paper: tin rings with glass stones, and little metal animals that clicked. But these saltwater surprises, sheltering beneath the seaweed at low tide, were better. They were alive. My children and I gazed into a forest, not a tree forest that stretched its branches into the air, but a forest of overlapping, protective fronds, resting like sheaves against the rocks. We were looking for treasures, not noticing then that the real treasure was probably the seaweed itself.

After we left, the tide came back and the various species of seaweeds lifted into the water. The long fronds of the knotted wrack stood at full length in a high tide. The ribbon weeds and the sea lettuce would bend in the water’s pulse. The purple laver and the Irish moss would start to glow as the sun reached them through the polish of the rising water. And the lives within them began to stir. Fish moved over them and into them, feeding. The crabs skimmed down from their low-tide hiding places and scuttled across the bottom. The ducks would come, the mergansers hunting fish and crabs, the black ducks puddle-dunking in the shallows for snails and clam worms, and the elegant female eider ducks escorting their buoyant ducklings.

Seaweeds are algae, a word of Latin origin that once implied a primal ooze, a genesis of original and elemental stuff. They are not plants, in the strict sense. They have no roots, no leaves, no stems. Not really. Only a few species have developed vascular tissues, and these are only sieve tubes that transport nutrients between them, not like the vascular tissues of land plants, with their complex transport of sugars from the leaves down, and water and minerals from the ground up. Essential to land plants is a sophisticated coordination between specialized cells that have distinct jobs to do. When cooperation and distinctions between cells are present in seaweeds, they are much simpler. Each seaweed cell uses the water in which it lives, taking from it the nutrients it needs.

They are called seaweeds, but scientists still can’t pin them down with a satisfactory all-encompassing definition because species keep slipping in or out of the categories we have constructed for them. The ancient fossil record is sparse, and little can be done to bring to the fore a clear image of their history.

Sometimes, casting a look back, it is difficult to distinguish a single-celled alga from a bacterium. That’s because they both speak to beginnings when first things—experimental, most of them evanescent — merged and separated and borrowed from one another.

Cyanobacteria were the first living things. They exist today much like their fossil ancestors: they live in water and manufacture their own food from sunlight. Some species are toxic and dangerous.

They used to be called blue-green algae because of this miraculous ability to turn sunlight, mixed with carbon dioxide and water, into food, which is the prime occupation of algae and land plants. Over time, these photosynthetic creatures floating in prehistoric seas created the biggest revolution on earth. Nothing matches it, not the Chicxulub asteroid plunging into the Yucatán Peninsula and upending the age of dinosaurs, not the wrenching apart of Pangaea, the Paleozoic mega-continent that fractured into the continents we know today.

Minute though they are, cyanobacteria changed the earth’s atmosphere by adding one ingredient: oxygen. And here’s the marvel: they floated for a few billion years, they were joined on the water’s surface by a floating one-celled alga. Two tiny, astonishing beings floating around together for another billion years or so, as the algae and the cyanobacteria, took the energy from the sun to make food and dispersed into the air their tiny gifts of oxygen, which, over huge amounts of time, grew into the matrix of all life to follow.

Marine algae, both the one-celled phytoplankton floating in the oceans today and the seaweeds anchored to our shores, and also seaweeds that float free, such as some species of Sargassum, supply the atmosphere of the earth with at least half its oxygen, which is the air we breathe. In the process of photosynthesizing, algae also disperse oxygen within the water to aquatic animals, including fish: the air they breathe.

We find one-celled algae in fresh water, on the damp trunks of trees, on wet rocks, on snow, in rain pools, and in the ocean. The phytoplankton that floats on the surface of the Gulf of Maine and all other saltwater bodies, a soup of many different species of one-celled algal life, are called microalgae because they are very small. The seaweeds that rim our shores to form underwater forests, or grow untethered and floating in deeper water, are the macro-algae. They are multicellular and gigantic by comparison.

Because seaweeds look much like land plants, one might assume that the hostas and lilies and such that we grow in our gardens evolved from them, that red, green, and brown seaweeds slowly made their way out of the sea and onto the land. That is not so. Land plants and particular green and red seaweed groups may share a possible evolutionary point of origin, but what we are witnessing in most of them is an example of parallel evolution: land plants and seaweeds came up with similar shapes as the best way to live.

They both need to anchor. Land plants have roots. A seaweed has a holdfast. Some holdfasts are shaped like disks, but others form a fist with many fingers called haptera. Holdfasts attach to rocks, wharf pilings, breakwaters, clamshells, anything rigid and stable. They are made up of thick tissue and fine hairs and a glue-like substance that sticks them in place. Seaweeds that anchor to corals secrete an acid that wears away a little carbonate chink of the coral into which they insert their attachment filaments.

While holdfasts anchor as roots do, they don’t transport water or minerals up into the algae they anchor. What they do is set the seaweed in one place and keep it there through tides and currents and storms, as water bathes the porous cells with the nutrients they need. The few seaweed species that grow in a hard sand or clay bottom use a root-like structure called a rhizoid to penetrate the substrate. These seaweeds with rhizoids are in the green algae group and are probably distantly related to land plants.

The simple parts of seaweeds begin with the stipe, a stalk that lifts up from the holdfast. It looks much like the thin trunk of a young sapling, or the stem of a large grass plant. It takes the seaweed into the light. The blade or frond or thallus (often more than one word in the seaweed lexicon can be used for the same thing) is what the stipe brings to the light. The blade is the equivalent of branches and leaves on a tree. A seaweed blade may branch, and air bladders may punctuate it, especially in seaweeds that are long or that grow in quiet waters where currents may not lift it high enough to the surface and into the light. The bladders are simple buoys carrying the blades aloft. Bathed in water and sunlight, the blades have two important jobs: photosynthesis and reproduction.

Some Sargassum species can reproduce by fragmentation, but most species of seaweed reproduce by alternating generations. They have a sporophyte phase, which sends out a drift of tiny spores, as mushrooms and ferns do. And they have a phase of sexual reproduction, as flowers do. With the diploid form of reproduction — the spore phase — a seaweed can replicate itself, but this allows for no genetic variation. Sexual reproduction — the haploid form — introduces the possibility of a genetic mix, but it’s extravagant because the microscopic male and female reproductive cells are dispersed into the tide and swept into the enormity of turbulent currents. Most of them never find each other, never join. As a result, the solution for most, but not all, seaweed species is to depend on both means, which allows for genetic change as well as a chance at abundance.

Ascophyllum nodosum, or knotted wrack, is the tough, familiar seaweed along our shore. It reproduces only through the haploid form. Its blade is multi-branched, shaped like a hardwood tree. At the tips of all these “branches” it begins its period of reproduction by growing receptacles, egg-shaped pouches that mature over the winter into male and female gametes, and when the water warms to the temperature the seaweed requires, it will release eggs and sperm in a cloudy mix into an incoming tide.

If you go to a shore with a marked incline, you will find that the native seaweeds arrange themselves neatly into bands. The green algae live closest to shore, the brown seaweeds inhabit the inshore waters and can also thrive in subtidal depths where sunlight reaches, but it is the red seaweeds that can live in the deepest water with the least light.

Excerpted from Seaweed Chronicles by Susan Hand Shetterly. © 2018 by Susan Hand Shetterly. Reprinted by permission of Algonquin Books of Chapel Hill. All rights reserved.