Differential transcriptomic and circadian regulation across giant kelp blades based on relative tissue age

Kelps are ecologically and economically important organisms. Kelp biomass generates habitat structure and carbon flux and is increasingly harvested as a sustainable resource. Kelp blades grow through cell proliferation at the proximal meristem and erode from the distal end, resulting in a spatiotemporal map reflecting tissue development. Diel physiology, such as photosynthesis, respiration, and carbohydrate metabolism, changes across the growing blade, yet the interaction of the circadian clock with age / blade structure has not been explicitly tested. Here, we sampled the proximal and distal ends of giant kelp (Macrocystis pyrifera) blades over 48 h under constant lab conditions to compare differential transcriptomic and circadian regulation across the blade. Gene function was differentially enriched across the blade, with proliferative and protective pathways upregulated in the younger proximal tissue and carbon acquisition and metabolic pathways upregulated in older distal tissue. We identified candidate aging-related genes based on similarities to plant, animal, and fungal senescence-associated genes, which were enriched in the older, distal tissue. While ~13% of analyzed genes displayed consistent circadian regulation across the blade, ~2% displayed altered rhythmic parameters, with consistently lower amplitudes and longer periods in older distal tissue. This is evidence of variable circadian physiology across giant kelp anatomy. The interacting developmental and circadian influences on the giant kelp transcriptome evoked here are integral for understanding the coordination of physiology important to kelp growth and health.