Resting stages are reported for several unicellular eukaryotes including diatoms that can produce spores or resting cells. These stages represent a repository of diversity in sediments but the factors that induce their formation are elusive. We investigated spore formation in the marine diatom Chaetoceros socialis. Our results confirm that nitrogen starvation is an effective experimental trigger for spore formation. However, we observed that spores are also produced when the external pool of nutrients is not limiting, and by using a semicontinuous setup, we could show that they appear only when cell density is high. The presence of a chemical cue mediating spore formation was supported by experiments carried out incubating cells with culture media conditioned by both healthy and lysed cells. These findings shed new light on the mechanisms that regulate the transition between life cycle stages and represent an experimental baseline for the identification of the signaling molecules.

Algal viruses are important contributors to carbon cycling, recycling nutrients and organic material through host lysis. Although viral infection has been described as a primary mechanism of phytoplankton mortality, little is known about host defense responses. We show that viral infection of the bloom-forming, planktonic diatom Chaetoceros socialis induces the mass formation of resting spores, a heavily silicified life cycle stage associated with carbon export due to rapid sinking. Although viral RNA was detected within spores, mature virions were not observed. 'Infected' spores were capable of germinating, but did not propagate or transmit infectious viruses. These results demonstrate that diatom spore formation is an effective defense strategy against viral-mediated mortality. They provide a possible mechanistic link between viral infection, bloom termination, and mass carbon export events and highlight an unappreciated role of viruses in regulating diatom life cycle transitions and ecological success.

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Several haptophyte algae produce unique neutral long-chain (C-C 40) ketones (alkenones) with two to four trans-carbon-carbon double (C=C) bonds and a keto-group. These molecules are of great biological interest and may have substantial commercial relevance as biofuel sources. Unfortunately, their detection and quantification,involving extraction from cells with organic solvents and fractionation of lipids through columns,are rather cumbersome. Hence, wedeveloped a method for the rapid and reliable quantification of alkenones in intact cells using Fourier transform infrared spectroscopy (FTIR). The method is based on theabsorption band at 962.5 cm-1 ,whichcorresponds to atrans-C=C bond that isunequivocally attributable to alkenones. This peak was exclusively observed in alkenone-producing haptophytessuch as Emiliania huxleyi NIES-837, Emiliania huxleyi CCMP 2090,Tisochrysis lutea(formerIsochrysis galbanaT-iso), Tisochrysis luteaCCMP 463, and Chrysotila lamellosa CCMP 1307,as well as in appropriate standards. We compared our FTIR method with a typical GC analysis method. The alkenone quantitydetermined by these two methods showed similar values. Nevertheless, the FTIRmethod developed in this studydoes not require complexextraction procedures and is therefore a much easier and rapid quantification method. This FTIR method can also be used for the screening of strains and the optimization of culture conditions for alkenone production.

Summary Rapid virus proliferation can exert a powerful control on phytoplankton host populations, playing a significant role in marine biogeochemistry and ecology. We explore how marine lytic viruses impact phytoplankton succession, affecting host and nonhost populations. Using an in silico food web we conducted simulation experiments under a range of different abiotic and biotic conditions, exploring virus–host–grazer interactions and manipulating competition, allometry, motility and cyst cycles. Virus‐host and predator–prey interactions, and interactions with competitors, generate bloom dynamics with a pronounced ‘boom‐and‐busted’ dynamic (BBeD) which leads to the suppression of otherwise potentially successful phytoplankton species. The BBeD is less pronounced at low nutrient loading through distancing of phytoplankton hosts, while high sediment loading and high nonhost biomass decrease the abundance of viruses through adsorption. Larger hosts are inherently more distanced, but motility increases virus attack, while cyst cycles promote spatial and temporal distancing. Virus control of phytoplankton bloom development appears more important than virus‐induced termination of those blooms. This affects plankton succession – not only the growth of species infected by the virus, but also those that compete for the same resources and are collectively subjected to common grazer control. The role of viruses in structuring plankton communities via BBeDs can thus provide an explanation for the paradox of the plankton.