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Diatoms are the most recent major algal lineage added to the geological record, appearing more than 200 million years ago. They are stramenopile protists resulting from a secondary endosymbiotic event that yielded the only photosynthetic protistan lineage expressing external siliceous cell wall structures called frustules. Many diatoms also have large internal vacuoles, and a common assumption in the literature is that success of the diatoms is largely attributable to these two morphological inventions: the frustule for defense and vacuole for luxury nutrient uptake. Here, we revisit the evolution of these inventions, propose sequential steps in frustule development, replace luxury nutrient uptake with predator defense and buoyancy control as the driver of vacuole expansion, and suggest that perhaps the greatest significance of the frustule for diatom evolution is the secondary consequence of enhancing sexual reproduction. In this synthesis, we emphasize a distinction between the "general" success of diatoms and the success of "bloom‐forming" species, as the physiological and morphological drivers of these successes differ. Importantly, the bloom‐forming species are responsible for the major role of diatoms in aquatic biogeochemical cycles. The bloom‐forming habit we ascribe to specific physiological attributes that, at their core, revolve around influencing the balance between diatom growth and losses to predators. We propose that these physiological adaptations are linked to size‐dependent maximum division rates in bloom‐forming diatoms, because of size scaling of predator–prey interactions. The existence of these bloom‐forming species yields an apparent allometric relationship that has previously been interpreted in terms of nutrient acquisition. Our analysis yields insights into species successions during blooms, considers the fundamental benefit of blooming (and subsequent sinking) from a reproductive standpoint, and provides some reinterpretation of diatoms success over geologic time and in the modern ocean. [ABSTRACT FROM AUTHOR]

Episodic deep mixing events are one component of the biological carbon pump that physically transports organic carbon into the mesopelagic. Episodic deep mixing also disrupts summertime thermal stratification thereby changing the light field and nutrient concentrations available for phytoplankton growth. Phytoplankton survival and growth below the mixed layer following restratification depends on how rapidly cells can employ a variety of photoacclimation processes in response to the environmental changes. To compare the relative timescales of summertime episodic deep mixing events with the timescales of phytoplankton photoacclimation processes, we first analyzed autonomous float data to survey the frequency and magnitude of deep mixing events in the western North Atlantic Ocean. Next, we simulated a sustained deep mixing event in the laboratory and measured rates of acclimation processes ranging from light harvesting to growth in a model diatom and green alga. In both algae increases in chlorophyll (Chl) were coupled to growth, but growth of the green alga lagged the diatom by about a day. In float profiles, significant increases in Chl and phytoplankton carbon (Cphyto) were detected below the mixed layer following episodic deep mixing events. These events pose a previously unrecognized source of new production below the mixed layer that can significantly boost the amount of carbon available for export to the deep ocean. [ABSTRACT FROM AUTHOR]

Marine phytoplankton, and in particular diatoms, are responsible for almost half of all primary production on Earth. Diatom species thrive from polar to tropical waters and across light environments that are highly complex to relatively benign, and so have evolved highly divergent strategies for regulating light capture and utilization. It is increasingly well established that diatoms have achieved such successful ecosystem dominance by regulating excitation energy available for generating photosynthetic energy via highly flexible light harvesting strategies. However, how different light harvesting strategies and downstream pathways for oxygen production and consumption interact to balance excitation pressure remains unknown. We therefore examined the responses of three diatom taxa adapted to inherently different light climates (estuarine Thalassioisira weissflogii, coastal Thalassiosira pseudonana and oceanic Thalassiosira oceanica) during transient shifts from a moderate to high growth irradiance (85 to 1200 μmol photons m-2 s-1). Transient high light exposure caused T. weissflogii to rapidly downregulate PSII with substantial nonphotochemical quenching, protecting PSII from inactivation or damage, and obviating the need for induction of O2 consuming (light-dependent respiration, LDR) pathways. In contrast, T. oceanica retained high excitation pressure on PSII, but with little change in RCII photochemical turnover, thereby requiring moderate repair activity and greater reliance on LDR. T. pseudonana exhibited an intermediate response compared to the other two diatom species, exhibiting some downregulation and inactivation of PSII, but high repair of PSII and induction of reversible PSII nonphotochemical quenching, with some LDR. Together, these data demonstrate a range of strategies for balancing light harvesting and utilization across diatom species, which reflect their adaptation to sustain photosynthesis under environments with inherently different light regimes. [ABSTRACT FROM AUTHOR]

The reproductive strategy of diatoms includes asexual and sexual phases, but in many species, including the model centric diatom Thalassiosira pseudonana, sexual reproduction has never been observed. Furthermore, the environmental factors that trigger sexual reproduction in diatoms are not understood. Although genome sequences of a few diatoms are available, little is known about the molecular basis for sexual reproduction. Here we show that ammonium reliably induces the key sexual morphologies, including oogonia, auxospores, and spermatogonia, in two strains of T. pseudonana, T. weissflogii, and Cyclotella cryptica. RNA sequencing revealed 1,274 genes whose expression patterns changed when T. pseudonana was induced into sexual reproduction by ammonium. Some of the induced genes are linked to meiosis or encode flagellar structures of heterokont and cryptophyte algae. The identification of ammonium as an environmental trigger suggests an unexpected link between diatom bloom dynamics and strategies for enhancing population genetic diversity. [ABSTRACT FROM AUTHOR]