Abstract
Green macroalgae of the genus Ulva play a key role in coastal ecosystems and are of increasing commercial importance. However, physiological differences between strains and species have yet to be described in detail. Furthermore, the strains of Ulva used in aquaculture usually originate from opportunistic collection in the wild without prior selection of best performing strains. Hence, efforts are required to detect the potential variability in growth and metabolic accumulation between Ulva strains and ultimately select the best performing strains under given environmental conditions. Here, the growth, physiological, and metabolic characteristics of 49 laminar Ulva spp. strains were investigated using a custom-made high-throughput phenotyping platform, enzymatic assays, and gas chromatography-mass spectrometry. We found large natural variation for a wide range of growth and metabolic characteristics, with growth rates varying from 0.09 to 0.37 mg.mg-1d-1 among strains. Ulva spp. possess a unique diurnal growth pattern and primary metabolism compared with land plants, with higher growth rates during the night than during the light period. Starch and sucrose only contributed on average 35% of the carbon required to sustain Ulva's night growth. Nitrates accumulated during the night in Ulva tissues, and nitrate accumulation and consumption was positively correlated with growth. In addition, we identified six amino acids as possible biomarkers for high growth in Ulva The large variability in growth and metabolite accumulation recorded among morphologically similar Ulva strains justifies future efforts in strain selection for increasing biomass, metabolite yields, and nutrient removal in the growing aquaculture industry.
Highlights
Green macroalgae of the genus Ulva play a key role in coastal ecosystems and are of increasing commercial importance
Growth responses in laminar Ulva spp. to varying environmental conditions have been well documented in laboratory studies (Björnsäter and Wheeler, 1990; Gordillo et al, 2001; Ale et al, 2011; Angell et al, 2014), less attention has been given to the contribution of genetic factors at the exception of, for example, Lawton et al (2013), which compared eight laminar Ulva strains belonging to two species
The computerization of plant growth monitoring has allowed for an increased throughput in the number of plant species/strains/ecotypes investigated for growth differences (Fahlgren et al, 2015; Fort et al, 2016)
Summary
Green macroalgae of the genus Ulva play a key role in coastal ecosystems and are of increasing commercial importance. Those photoassimilates are degraded to supply the cells with the carbon and energy required for metabolism and growth (Sulpice et al, 2014) This basic mechanism is conserved among the breadth of autotrophic organisms, the types of photoassimilates used for short and/or long-term storage varies extensively between species. Among these strategies, cyanobacteria accumulate carbon for their night requirements in the form of glycogen (Schwarz et al, 2013) and brown algae (Phaeophyceae) are likely to use mannitol and laminarin (Gravot et al, 2010; Michel et al, 2010), whereas land plants can store their carbon reserves as sucrose (Sturm and Tang, 1999), starch (Sulpice et al, 2009), and/or fructans (Vijn and Smeekens, 1999), depending on the species considered. An efficient carbon storage and remobilization mechanism is essential to sustain the growth of autotrophic organisms during the night period (Graf et al, 2010), and to avoid carbon starvation (Graf and Smith, 2011; Sulpice et al, 2014), especially in the face of changing photoperiods during the growing season
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