Strain isolation, maintenance, and genetic characterization are illustrated in Pelusi et al. socialis (APC1, APC2, and APC12) were isolated from the germination of spores collected in surface sediments at station LTER-MareChiara in the Gulf of Naples. Methods Strains and experimental conditions 2017) suggesting that such collective response might be widespread in nature. Intraspecific communication mediated by chemical signals is known also for unicellular eukaryotes and includes, among others, the production of sexual pheromones and the induction of programmed cell death (reviewed by Venuleo et al. Density-dependent mechanisms, including the formation of dormant stages, have been investigated in unicellular organisms like bacteria, which can perceive their population density through the production and perception of signaling molecules (Keller and Surette 2006). We tested nitrogen depletion, traditionally the most effective environmental factor, but explored also the possible role of biological cues related to cell density indicative of a coordinated response at the level of the whole population.

2019) and is abundant in many coastal areas (De Luca et al. We addressed the issue using the marine colonial diatom Chaetoceros socialis, which produces endogenous spores (Pelusi et al. This latter intriguing result suggests that formation of resting stages in diatoms may be driven by more sophisticated mechanisms than those presently assumed. 2010), or even in control conditions when nutrients were available (Oku and Kamatani 1995). 1993 McQuoid and Hobson 1996 Sugie et al. In some species, however, spores are produced at low light conditions, or at low/high temperature, or when the concentration of other nutrients is low (Kuwata et al. 1993 McQuoid and Hobson 1996 and references therein). Laboratory experiments showed that nitrogen limitation is the most effective factor inducing this life cycle transition (Kuwata et al. 2013).ĭespite all this, the triggers and the mechanisms that drive the shift from vegetative cells to resting stages are not completely understood. In addition, the high abundance of spores in sediment traps deployed in open oceanic waters demonstrates that they can considerably contribute to carbon export at depth (Rynearson et al.

Thus, resting stages assure the persistence of the species over a wide time span and is a key trait of diatom evolutionary fitness. 2013), which can remain viable, and thus capable to germinate, for decades (Härnström et al. Resting stages sink to the sediments where they accumulate forming “seed banks,” especially in coastal surface sediments (McQuoid et al. Both stages have high amounts of storage materials and reduced metabolism that allow them to survive in the absence of light and at cold temperatures, thus overcoming unfavorable environmental conditions and mortality by predators or pathogens (McQuoid and Hobson 1996 Raven and Waite 2004). Spores have thick siliceous cell walls and are morphologically different from the vegetative cells thriving in the water column, while resting cells are morphologically undistinguishable from vegetative cells (McQuoid and Hobson 1996). In many species, the end of the bloom is accompanied by the formation of spores or resting cells (Pitcher 1986 von Bodungen et al. Exploring the link between cell density and spore formation may elucidate mechanisms involved in species succession.ĭiatoms are a prominent group of unicellular microalgae and their life strategy includes periods of rapid growth during which they can form extensive blooms followed by mass sinking (Smetacek 1985 Raven and Waite 2004). Moreover, spore formation is induced by spent culture media obtained from healthy as well as from lysed cells, demonstrating that the transition from vegetative cells to spores is mediated by a chemical signal produced by the cells. By contrast, our study shows that the marine diatom Chaetoceros socialis forms spores also in nutrient replete conditions when cell concentration is high. The prevalent view is that resource availability plays a major role, and nutrient starvation is considered the major trigger for the formation of resting spores in diatoms. The interplay between environmental and biotic factors in regulating organisms' life strategies is a frequently debated issue in plankton ecology.

Spore formation