Adaptive responses of free‐living and symbiotic microalgae to simulated future ocean conditions

Abstract

Marine microalgae are a diverse group of microscopic eukaryotic and prokaryotic organisms capable of photosynthesis. They are important primary producers and carbon sinks but their physiology and persistence are severely affected by global climate change. Powerful experimental evolution technologies are being used to examine the potential of microalgae to respond adaptively to current and predicted future conditions, as well as to develop resources to facilitate species conservation and restoration of ecosystem functions. This review synthesizes findings and insights from experimental evolution studies of marine microalgae in response to elevated temperature and/or pCO2. Adaptation to these environmental conditions has been observed in many studies of marine dinoflagellates, diatoms and coccolithophores. An enhancement in traits such as growth and photo‐physiological performance and an increase in upper thermal limit have been shown to be possible, although the extent and rate of change differ between microalgal taxa. Studies employing multiple monoclonal replicates showed variation in responses among replicates and revealed the stochasticity of mutations. The work to date is already providing valuable information on species’ climate sensitivity or resilience to managers and policymakers but extrapolating these insights to ecosystem‐ and community‐level impacts continues to be a challenge. We recommend future work should include in situ experiments, diurnal and seasonal fluctuations, multiple drivers and multiple starting genotypes. Fitness trade‐offs, stable versus plastic responses and the genetic bases of the changes also need investigating, and the incorporation of genome resequencing into experimental designs will be invaluable.

Individual variation in growth and physiology of symbionts in response to temperature

“…Although nitrogen is abundant in vitro, at least initially, it is likely to be more limiting in hospite, with increased nitrogen often destabilizing the mutualism (Morris et al, 2019 ; Rädecker et al, 2015 ). The broad availability of nutrients when conducting selection experiments in vitro may obscure trade‐offs with other algal traits, trade‐offs with host traits, or trade‐offs with other traits only observed in the context of the holobiont (Chan et al, 2021 ). Selection experiments on symbionts outside of the host may not yield the evolutionary rescue necessary to adapt to climate change, but rather may require selection experiments on the holobiont.…”

“…Conservation‐minded assisted evolution for coral reef organisms proposes that selection experiments in the laboratory could yield temperature‐tolerant symbiont genotypes that could be later used to seed reefs experiencing temperature stress (van Oppen et al, 2015 ). Selection experiments could be conducted on heterogenous cultures composed of standing genetic variation found within and among hosts on reefs (Chan et al, 2021 ; Pelosi et al, 2021 ). The genotypes used in this experiment are the result of a long‐term selection experiment.…”

Symbiodinium IN CORAL REEFS AND ITS ADAPTATION RESPONSES TOWARD CORAL BLEACHING EVENTS: A REVIEW

“…As in Figure 1, the mutual relationship is based on gas exchange, where coral offers carbon dioxide and water from cellular respiration while the endosymbiont provides oxygen as photosynthesis byproducts. Other than that, the phototrophic endosymbiont will provide organic carbon (energy) in the form of sugar, lipid, amino acid, glycerol, and other compounds from photosynthesis which are important for the growth and reproduction of coral (Hoegh-Guldberg et al, 2017;Petrou et al, 2018;Chan et al, 2021). The coral not only serves as a host for protection to the endosymbiont but also supplies it with inorganic nitrogen and phosphorus from its waste products (Tan et al, 2020;Reich et al, 2021).…”

“…The primary source of coral bleaching events is initiated by photoinhibition when oxidative damage induced by excess reactive oxygen species (ROS) is generated by heat stress in water, mainly by anomalous temperature and light exposure (Liu et al, 2018;Chan et al, 2021). Although the study of ROS related to coral bleaching is still sparse, studies on coral symbiotic oxidative stress associated with disparities in endosymbiont photosystems are growing in number (Solayan, 2016).…”

“…The bleaching event that occurred in 1998 not only adversely impacted global coral reefs but also our Malaysia coral reefs with up to 40% of coral mortality due to climate change (Waheed & Hoeksema, 2014;Qin et al, 2019). The coral bleaching event occurs when oxidative stress is generated by reactive oxygen species in Symbiodinium cells (Chan et al, 2021). Comprehensive understanding of the underlying cellular mechanisms is still limited (Nielsen et al, 2018;Liu et al, 2018).…”