Effects of Heat Waves and Light Deprivation on Giant Kelp Juveniles (Macrocystis pyrifera, Laminariales, Phaeophyceae)

Abstract

Due to climate change, the incidence of marine heat waves (MHWs) has increased, yet their effects on seaweeds are still not well understood. Adult sporophytes of Macrocystis pyrifera, the species forming the iconic giant kelp forests, can be negatively affected by thermal stress and associated environmental factors (e.g., nutrient depletion, light deprivation); however, little is known about the tolerance/vulnerability of juvenile sporophytes. Simultaneously to MHWs, juveniles can be subjected to light limitation for extended periods of time (days–weeks) due to factors causing turbidity, or even because of shading by understory canopy‐forming seaweeds. This study evaluated the effects of a simulated MHW (24°C, 7 d) in combination (or not) with light deprivation, on the photosynthetic capacities, nutrient uptake, and tissue composition, as well as oxidative stress descriptors of M. pyrifera juvenile sporophytes (single blade stage, up to 20 cm length). Maximum quantum yield (Fv/Fm) decreased in juveniles under light at 24°C, likely reflecting some damage on the photosynthetic apparatus or dynamic photoinhibition; however, no other sign of physiological alteration was found in this treatment (i.e., pigments, nutrient reserves and uptake, oxidative stress). Photosynthetic capacities were maintained or even enhanced in plants under light deprivation, likely supported by photoacclimation (pigments increment); by contrast, nitrate uptake and internal storage of carbohydrates were strongly reduced, regardless of temperature. This study indicated that light limitation can be more detrimental to juvenile survival, and therefore recruitment success of M. pyrifera forests, than episodic thermal stress from MHWs.

Short‐term stress responses and recovery of giant kelp (Macrocystis pyrifera, Laminariales, Phaeophyceae) juvenile sporophytes to a simulated marine heatwave and nitrate scarcity1

“…As in Sánchez‐Barredo et al. (2020), our results indicated that nitrate uptake rates were not altered in Macrocystis pyrifera juveniles after the exposure phase. Labeled nitrate concentrations used in this study ( 15 NO 3 − , 15 µM) are typically found in kelp stands during upwelling periods (Reed et al.…”

“…Sánchez‐Barredo et al. (2020) also found that α was unaffected in juvenile sporophytes of Macrocystis pyrifera exposed to simulated MHW conditions. Nonetheless, photosynthetic plasticity related to adjustments of α (and thus, E c ) has been demonstrated in adult and juvenile sporophytes of M. pyrifera .…”

“…2019, Sánchez‐Barredo et al. 2020). In our study, values of F v /F m appear unaltered among treatments, which indeed was consistent with the unaffected photosynthetic efficiency (α) generally used as a proxy of maximum quantum yield (Beer et al.…”

“…(2019) and Sánchez‐Barredo et al. (2020) showed that the effects of temperature on juvenile sporophytes of Macrocystis pyrifera are modulated by the interaction of temperature with light and nitrate availability. Unlike adults, juvenile sporophytes have relatively lower structural complexity and depend solely on their physiological plasticity (remodeling their physiology to compensate for environmental variation) to adapt to ambient changes (Umanzor et al.…”

Climate change challenges and opportunities for seaweed aquaculture in California, the United States

“…Offshore, growing giant kelp (Macrocystis pyrifera) on large farms is feasible. Nearshore, wild populations of giant kelp have been subject to die-offs in warm temperatures with low resilience of populations expected with continued climate change (Cavanaugh, Reed, Bell, Castorani, & Beas-Luna, 2019;Rogers-Bennett & Catton, 2019;Sanchez-Barredo et al, 2020). An adaptive solution for the benefit of the aquaculture industry in the long term is cultivation of alternative nearshore crops to supplement or replace wild harvest, using native species expected to be resilient to future environmental conditions along the California coast.…”