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The swaying and winding of kelp in the ocean currents has an entrancing, hypnotic quality that would be the envy of the most skilled Raq Sharqi dancer, Kelp seem like the ultimate Taoists, attaching to a rock with a holdfast and simply going with the flow. Most fascinating of all are the kelp forests where each species seems to be dancing to its own choreography.
The morphology of a typical kelp is simple.  Visually it resembles a plant. A rootlike holdfast attaches to a rock.  A  stipe resembles a stem and blades resemble leaves. Floats or pneumocysts are gas filled compartments that help the kelp blades reach available light near the surface of the ocean.  However, functionally, kelp is very different.  The roots only serve as an anchor and have no role in securing nutrition.  The blades are entrusted with the responsibility of both photosynthesis and mineral absorption.
As of recently, partly in response to new genomic information,  kelp are no longer considered plants.  They are classified as Phaophyta or brown algae in the kingdom of chromists.  This reclassification may seem to be a point of interest only to academics.  But it would be an injustice to call kelp plants because  chromists have their own characters, with a unique biochemistry not found in plants..  Like plants, kelp uses chlorophyll a in photosynthesis.  However, it also uses chlorophyll c, only found to chromists.  Chlorophyll c is based on fucoxanthin, a pigment that is most efficient at utilizing the blue green light that penetrates the ocean.  This gives kelp its brown coloration. Fucoxanthin is medically interesting and is under investigation for anticancer and antiinflammatory properties.
Plants and chromists store energy differently. Plants store carbohydrates as starch consisting of polymers of glucose.  Chromists build laminarin, chains of mannitol and glucose, in organelles called pyrenoids.  The cell walls of chromists contain algin, a gum used as a thickener and emulsifier in the food and cosmetics industry. East Asian cultures have traditionally exploited kelp for food and medicinal value.  The rich mineral content of kelp makes it valuable in soil enrichment.
Certain lifestyle features are similar between plants and kelps.  One is the alternation of generations between a large diploid sporophyte and a microscopic haploid gametophyte.  However seeds are essentially a terrestrial adaptation and kelp have not evolved beyond spores. Like pants, kelp exist as perennials and annuals.   Kelps use chemical defenses against grazers as do plants  Kelp forests resemble terrestrial forests in their layered structure.  At they top is the canopy with blades that reach the surface and beyond. The understory consists of the smaller kelps that make do with the filtered light that penetrates the canopy..
On one level, the biomechanics of kelp is simple and on another so complex that it continues to elude scientists. Some kelps, called rock kelps, have short stubbly leaves and are largely immobile.   But prototypical kelps occupy a space between sessile and motile organisms.  They attach to a rock but their blades float freely, gathering light and nutrients. Kelps of course lack muscles.  Instead they rely on ocean currents to provide them with motion.
The complexity is in the details and the dynamics. Most kelps inhabit the intertidal and subtidal zones in northern temperate zones where living creatures must contend with ever changing waves, surf crash, turbulence, buoyancy and current.  When a kelp blade is stretched  to its limit it may break in the tug of war between the pull of the water flow and the grip of  the holdfast. Or it may be uprooted. A blade may be ripped apart by drag forces.  Each kelp genome and morphology encodes intimate evolutionary knowledge of tides, seasons, storm patterns, predators and ecological allies. Every species has its own mix of strategies.  Many kelp locate their meristem in the stipe.  This allows regeneration if blades are torn off or grazed by predators.  Kelp often grow near other kelps where they collectively exert a damping force on waves. The continual twisting and curling of the blade allows its 3 dimensional structure to change shape to take better advantage of streamlining.  The variations in form reflect the variations in habitat.  Common names are based on appearance, as with feather boa kelp (Egregia menziesii), bull whip kelp (Nereocystis leutkeana), winged kelp (Alaria marginata), strap kelp (Phyllospora comosa). Some kelp are shape shifters that can adapt the shape of their blades to local conditions. The blades may be wide in calm water zones and thin in exposed areas. 
A kelp forest forms the basis of an undersea ecosystem that provides food, shelter and protection for a variety of marine organisms including plankton, sea urchins, mussels, fish and sea otters.  Sadly, these algae are under threat.  One  danger is pollution. Another is global warming as it encroaches upon the cold water habitat of kelp. The loss of kelp forests would be a tragedy. Partly because of the economic losses but also because a kelp forest is one of the undersea wonders of the world.

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