Photoacclimation and Photoprotection of Juvenile Sporophytes of Macrocystis pyrifera (Laminariales, Phaeophyceae) Under High‐light Conditions During Short‐term Shallow‐water Cultivation¹

Abstract

This study was designed to understand better if and how juvenile sporophytes of Macrocystis pyrifera can photoacclimate to high‐light conditions when transplanted from 10 to 3 meters over 7 d. Acclimation of adult sporophytes to light regimes in the bathymetric gradient has been extensively documented. It primarily depends on photoacclimation and translocation of resources among blades. Among other physiological differences, juvenile sporophytes of M. pyrifera lack the structural complexity shown by adults. As such, juveniles may primarily depend on their photoacclimation capacities to maintain productivity and even avoid mortality under changing light regimes. However, little is known about how these mechanisms operate in young individuals. The capacity of sporophytes to photoacclimate was assessed by examining changes in their photosynthetic performance, pigment content, and bio‐optical properties of the blade. Sporophytes nutritional status and oxidative damage were also determined. Results showed that juvenile sporophytes transplanted to shallow water were able to regulate light harvesting by reducing pigment concentration, and thus, absorptance and photosynthetic efficiency. Also, shallow‐water sporophytes notably enhanced the dissipation of light energy as heat (NPQ) as a photoprotective mechanism. Generally, these adjustments allowed sporophytes to manage the absorption and utilization of light energy, hence reducing the potential for photo‐oxidative damage. Furthermore, no substantial changes were found in the internal reserves (i.e., soluble carbohydrates and nitrogen) of these sporophytes. To our knowledge, these results are the first to provide robust evidence of photoprotective and photoacclimation strategies in juveniles of M. pyrifera, allowing them to restrict or avoid photodamage during shallow‐water cultivation.

Solar Radiation as an Isolated Environmental Factor in an Experimental Mesocosm Approach for Studying Photosynthetic Acclimation of Macrocystis pyrifera (Ochrophyta)

“…Nevertheless, algal photosynthetic capacity estimated as in situ ETR was reduced by between 57 and 79% in the middle zone blades and from 79 to 93% in the bottom blades. A lesser reduction of 20% of the juvenile sporophytes has been found in another field study (Umanzor et al, 2020). This effect is only a consequence of light reaching the bottom of the tank because nutrients, temperature, pH, and salinity were the same at all depths in the experimental tanks.…”

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

“…This has been shown for juveniles of Macrocystis pyrifera adapted to "deep" farming conditions, which increased their respiratory activity during short‐term shallow water cultivation, likely because of the enhancement of metabolic carbon processes and other stress protection mechanisms associated with light over‐exposure (Umanzor et al. 2020).…”

“…2017, Umanzor et al. 2020). The availability of higher irradiance necessary for photosynthesis to compensate for respiratory activity can be critical for reaching positive net productivity and survival of juvenile sporophytes.…”

“…This suggests either that the photosynthetic efficiency was unable to recover or that it was down‐regulated to limit light capture under diminished photosynthetic and carbon fixation capabilities (Hanelt and López‐Figueroa 2012, Umanzor et al. 2020). The ratio between photosynthesis and respiration (P:R ratio), as a proxy of plant carbon balance, was conditioned by the increase in respiration (R) after the exposure phase, while the alteration of gross‐P mainly conditioned the P:R ratio in recovered sporophytes.…”

“…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. 2020). As such, different ontogenic stages may respond differently to the same environmental stressor, similar to what occurs in other seaweed species (Dean and Jacobsen 1986, Muñoz et al.…”