How light and biomass density influence the reproduction of delayed Saccharina latissima gametophytes (Phaeophyceae)

Abstract

Kelp life‐cycle transitions are complex and susceptible to various (a)biotic controls. Understanding the microscopic part of the kelp's lifecycle is of key importance, as gametophytes form a critical phase influencing, among others, the distributional limits of the species. Many environmental controls have been identified that affect kelp gametogenesis, whose interactive effects can be subtle and counterintuitive. Here we performed a fully factorial experiment on the (interactive) influences of light intensity, light quality, and the Initial Gametophyte Density (IGD) on Saccharina latissima reproduction and vegetative growth of delayed gametophytes. A total of 144 cultures were followed over a period of 21 d. The IGD was a key determinant for reproductive success, with increased IGDs (≥0.04 mg DW · mL−1) practically halting reproduction. Interestingly, the effects of IGDs were not affected by nutrient availability, suggesting a resource‐independent effect of density on reproduction. The Photosynthetically Usable Radiation (PUR), overarching the quantitative contribution of both light intensity and light quality, correlated with both reproduction and vegetative growth. The PUR furthermore specifies that the contribution of light quality, as a lifecycle control, is a matter of absorbed photon flux instead of color signaling. We hypothesize that (i) the number of photons absorbed, independent of their specific wavelength, and (ii) IGD interactions, independent of nutrient availability, are major determinants of reproduction in S. latissima gametophytes. These insights help understand kelp gametophyte development and dispersal under natural conditions, while also aiding the control of in vitro gametophyte cultures.

The role of seasonality in reproduction of multiannual delayed gametophytes ofSaccharina latissima

“…Biomass density (Ebbing et al. 2020) or in this study culture age could influence the reproduction of multiannual delayed gametophyte cultures in more profound ways than it would influence the reproduction of newly formed gametophyte cultures. This is because the prolonged periods of vegetative growth not only increase the age and biomass density of multiannual delayed gametophyte cultures, but may also change genetic diversity and sex ratio of the cultures if genotypes and sexes have different vegetative grow rates (e.g., see Destombe and Oppliger 2011).…”

“…The transitions between these life phases are controlled by abiotic and biotic environmental factors (Lüning and Dieck 1989), hereafter described as life cycle controls (Ebbing et al. 2020). Extensive studies on life cycle controls for newly formed Saccharina latissima gametophytes have identified the following as the main drivers: light intensity (Hsiao and Druehl 1971, Bolton and Levitt 1985), temperature (Lüning and Neushul 1978, Morita et al.…”

In-Culture Selection and the Potential Effects of Changing Sex Ratios on the Reproductive Success of Multiannual Delayed Gametophytes of Saccharina latissima and Alaria esculenta

“…Putative density-associated nutrient deficiency is sometimes offered as an explanation [23,30]. However, [21] showed that the interaction between gametophyte density and reproduction was not due to nutrient availability but could also directly be correlated with the density of the culture itself. The next logical step was to assess whether male or female gametophyte biomass was instrumental to this apparent correlation.…”

“…Ripe Saccharina latissima sorus of 6 individuals from Leknesund, Norway (61.212994, 4.896644 • E) and Alaria esculenta sorus of 8 individuals from Fuhreholmen, Norway (61.043292, 4.868790 • E), were collected and pooled in 2016. The release of zoospores was performed in the Netherlands, using a standard extraction protocol (protocol of [21], similar to earlier methods described by [22,23]). Zoospores were isolated from the culture to set up clean clonal cultures, separating the sexes.…”

(A)biotic lifecycle controls of mutliannual delayed gametophytes of Laminariales (brown seaweeds)

“…This results in the spatiotemporal separation of meiosis and fertilization, with somatic development occurring at both the haploid and diploid stages (Bell et al 1997). The transitions between these life phases is controlled by abiotic and biotic environmental factors (Lüning and Dieck 1989), hereafter described as life cycle controls (Ebbing et al 2020). Extensive studies on life cycle controls for newly formed S. latissima gametophytes have identified the following as the main drivers: light intensity (Hsiao and Druehl 1971;Bolton and Levitt 1985), temperature (Lüning and Neushul 1978;Morita et al 2003), nutrients (Harries 1932;Martins et al 2017), daylength (Choi et al 2005;Martins et al 2017), and initial gametophyte density (Choi et al 2005;Carney and Edwards 2010;Ebbing et al 2020).…”