Light is a key factor in triggering sexual reproduction in the pennate diatom Haslea ostrearia

Abstract

Sexual reproduction is an obligatory phase in the life cycle of most diatoms, as cell size decreases with successive vegetative divisions and the maximal cell size is only restored by a specialized cell, the auxospore, which follows zygote formation as a result of sexual reproduction. While in pennate diatoms the induction of sexual reproduction depends primarily on cell-cell interactions, the importance of different external factors for the induction of sexual reproduction is less well known. Here, we investigated the effects of light on sexualization in the marine benthic pennate diatom Haslea ostrearia (Gaillon) R. Simonsen. Compatible clones were crossed and exposed to different combinations of light levels, qualities, and photoperiods. Light was found to be a key factor for sexualization, and to a certain extent, to control auxosporulation in H. ostrearia. The light conditions most favorable for sexual reproduction were low irradiances (<50 micromolphotons m(-2) s(-1)) and short photoperiods (6-10 h), conditions that prevail during winter, and to a lesser extent, the higher irradiances and longer photoperiods that correspond to the spring and fall, when blooms of this organism form in the natural environment. Auxospore formation was very rare in continuous light, and maximum in presence of red radiation, while it was never observed in darkness or in radiation other than red.

Effect of different light spectra on the growth and biochemical composition of Tisochrysis lutea

“…Changes in the quantity (irradiance) and spectral characteristics (quality or color) of light can alter biochemical composition of microalgal cultures (Flaak and Epifanio 1978;Rivkin 1989;Muller-Feuga et al 2003a;Mercado et al 2004;Liang et al 2006;Mouget et al 2009). Blue light improves growth and carbon metabolism and increases the protein content in plants (Voskresenskaya 1972).…”

AUREOCHROME1a-Mediated Induction of the Diatom-Specific Cyclin dsCYC2 Controls the Onset of Cell Division in Diatoms (Phaeodactylum tricornutum)

“…Diatoms can grow and photosynthesize over a wide range of different light intensities and wavelengths (Holdsworth, 1985;Mercado et al, 2004), and they possess specific light sensing and acclimation strategies (Nymark et al, 2009;Bailleul et al, 2010;Park et al, 2010;Zhu and Green, 2010;Lepetit et al, 2012). Light quality and intensity not only determine the photosynthetic capacity of diatoms, but it also affects different cellular processes, including motility, sexual reproduction, and cell division (Brzezinski et al, 1990;Chen et al, 2004;Cohn et al, 2004;McLachlan et al, 2009;Mouget et al, 2009). Analogous to C. reinhardtii, the cell cycle of diatoms consists of light-dependent and -independent segments .…”

Haslea karadagensis(Bacillariophyta): a second blue diatom, recorded from the Black Sea and producing a novel blue pigment

“…Within each species, crossing experiments between compatible clones under optimal conditions (see Mouget et al, 2009) always resulted in auxospore formation, including the positive control cross between the sexually compatible H. ostrearia clones NCC 158.1 and NCC 234. However, in mixtures of the two species, H. karadagensis never bred with H. ostrearia clones of either mating type, and no sexual stages (gametes, zygotes, auxospores, initial cells) were ever observed in mixtures of the two organisms.…”

“…Crossing experiments between strains were performed following Davidovich et al (2009) and Mouget et al (2009), a protocol designed to trigger maximal auxosporulation. According to this protocol, mating mixtures were prepared in sterile Petri dishes, by diluting parental cultures in exponential phase of growth at a final concentration of 2000 cells ml À1 and then placed in a culture box under low irradiance (28 AE 2 m À2 s À1 illumination) and a short photoperiod (8:16 h L:D cycle).…”

“…Some of these cells were isolated and clones were cultured in controlled conditions. A protocol previously used for the induction of sexual reproduction in H. ostrearia Mouget et al, 2009) was successfully applied to this diatom and auxospores and an F 1 generation was obtained (Davidovich et al, 2012). The discovery of the Ukrainian Haslea raises concerns about the identity of other 'blue' diatoms.…”