Fig 1. Experimental conditions of kelp mesocosm.

Kelp was grown for four weeks in four different abiotic stressors, present-day 12 ^oC 500 ppm of CO₂ ○, Elevated Temperature 15 ^oC 500 ppm of CO₂ □, Elevated pCO₂ 12 ^oC 1300 ppm of CO₂ △, and future conditions 15 ^oC 1300 ppm of CO₂ ◇. (a) Growth of kelp over the duration of the experiment. Each week, 18 fronds were sampled and weighed per incubation mesocosm. (b) Elevated pCO₂ was maintained between 1,200 and 1,400 ppm of CO₂ whereas temperature was held constant at either 12 ^oC or 15 ^oC. (c) microbiome species richness. At week 4, microbial communities were sampled from the water and kelp surfaces and sequenced. Alpha diversity, Shannon species richness, was calculated for all microbiome samples using MG-RAST (grey scale represents replicates; three for water and 2 or 1 for kelp surface).