The holdfast anchors giant kelp to the substratum. No, these are not roots but rather haptera--structures that must withstand the drag of ocean surge and the lift of the floats. The alga actually absorbs nutrients through its floating blades. In fact, giant kelp will not grow in quiet water--it requires ocean surge to sweep nutrients across the blades. This cluster of blades enveloping the holdfast constitutes the alga's reproductive structures--they meiotically release millions of spores.
Gas filled floats support giant kelp as it races from depths of over 25 m toward the surface seeking sunlight. There it spreads forming vast canopies. During upwelling, it can grow at rates up to 50 cm /day--as fast as tropical bamboo. . The green color of kelp's chlorophyll is masked by the golden brown fucozanthin, an accessory photosynthetic pigment.
