Effects of depth-cycling on nutrient uptake and biomass production in the giant kelp Macrocystis pyrifera
Ignacio Navarrete, Diane Y. Kim, Cindy Wilcox, Daniel C. Reed, David W. Ginsburg, Jessica M. Dutton, John F. Heidelberg, Yubin Raut, Brian Wilcox
Abstract
Seasonal or chronic nutrient limitations in the photic zone limit large-scale cultivation of seaweed (macroalgae) in much of the world's oceans, hindering the development of macroalgae as a biofuel feedstock. One possible solution is to supply nutrients using a diel depth-cycling approach, physically moving the macroalgae between deep nutrient-rich water at night and shallow depths within the photic zone during the day. This study tested the effects of depth-cycling on the growth, morphology, and chemical composition of the giant kelp Macrocystis pyrifera, a target species for renewable biomass production. Giant kelp grown under depth-cycling conditions had an average growth rate of 5% per day and produced four times more biomass (wet weight) than individuals grown in a kelp bed without depth-cycling. Analysis of tissue from the depth-cycled kelp showed elevated levels of protein, lower C:N ratios, and distinct δ15N and δ13C values suggesting that the depth-cycled kelp were not nitrogen-deficient and assimilated nutrients from deep water. Depth-cycled kelp also exhibited smaller and thicker-walled pneumatocysts and larger blades. Overall, this study supports further investigation of depth-cycling as a macroalgal farming strategy.