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Diatoms frequently dominate marine and freshwater biofilms as major primary producers. Nutrient resources in these biofilms are patchily distributed and fluctuate dynamically over time. We recently reported that this spatially and temporally structured environment can be exploited by motile diatoms that use chemoattraction to dissolved silicate (dSi) under Si starvation. Here, we show that the behavioral response of diatoms is more complex and selective as cells are also responding to gradients of dissolved phosphate (dP) when starved in this nutrient. In contrast, neither nitrate nor ammonium (dN) triggers an attractive response under nitrogen limitation. Video monitoring and movement pattern analysis of the model diatom Seminavis robusta revealed that dP attraction is mediated by a combined chemokinetic and chemotactic response. After locating nutrient hotspots, the microalgae slow down and recover from the limitation. The fastest recovery in terms of growth was observed after dSi limitation. In agreement with the lack of directional response, recovery from dN limitation was slowest, indicating that no short‐term benefit would be drawn by the algae from the location of transient hotspots of this resource. Our results highlight the ability of diatoms to adapt to nutrient limitation by active foraging and might explain their success in patchy benthic environments. We report on the chemoreception and chemoattraction of diatoms toward inorganic nutrients and identify directed diatom motility as an unexpected reason for biofilm patchiness. We develop a picture of cells that can find their way according to their needs within a complex environment structured by chemical signals.

Diatoms are highly abundant unicellular algae that often dominate pelagic as well as benthic primary production in the oceans and inland waters. Being strictly dependent on silica to build their biomineralized cell walls, marine diatoms precipitate 240 × 10 12  mol Si per year, which makes them the major sink in the global Si cycle. Dissolved silicic acid (dSi) availability frequently limits diatom productivity and influences species composition of communities. We show that benthic diatoms selectively perceive and behaviourally react to gradients of dSi. Cell speed increases under dSi-limited conditions in a chemokinetic response and, if gradients of this resource are present, increased directionality of cell movement promotes chemotaxis. The ability to exploit local and short-lived dSi hotspots using a specific search behaviour likely contributes to micro-scale patch dynamics in biofilm communities. On a global scale this behaviour might affect sediment–water dSi fluxes and biogeochemical cycling. Diatoms often dominate production in aquatic communities, but the amount of available dissolved silicic acid (dSi) limits their growth. Here, Bondoc et al. , show that diatoms perceive gradients in dSi, and can increase the encounter with this resource by chemotaxis toward high concentrations under resource-limited conditions.

Seasonal shifts in phytoplankton accumulation and loss largely follow changes in mixed layer depth, but the impact of mixed layer depth on cell physiology remains unexplored. Here, we investigate the physiological state of phytoplankton populations associated with distinct bloom phases and mixing regimes in the North Atlantic. Stratification and deep mixing alter community physiology and viral production, effectively shaping accumulation rates. Communities in relatively deep, early-spring mixed layers are characterized by low levels of stress and high accumulation rates, while those in the recently shallowed mixed layers in late-spring have high levels of oxidative stress. Prolonged stratification into early autumn manifests in negative accumulation rates, along with pronounced signatures of compromised membranes, death-related protease activity, virus production, nutrient drawdown, and lipid markers indicative of nutrient stress. Positive accumulation renews during mixed layer deepening with transition into winter, concomitant with enhanced nutrient supply and lessened viral pressure. Phytoplankton are important primary producers. Here the authors investigate phytoplankton physiological changes associated with bloom phases and mixing regimes in the North Atlantic, finding that stratification and deep mixing shape accumulation rates by altering physiology and viral production.

Diatoms are species-rich microalgae that often have a unique life cycle with vegetative cell size reduction followed by size restoration through sexual reproduction of two mating types (MT⁺ and MT⁻). In the marine benthic diatom Seminavis robusta, mate-finding is mediated by an L-proline-derived diketopiperazine, a pheromone produced by the attracting mating type (MT⁻). Here, we investigate the movement patterns of cells of the opposite mating type (MT⁺) exposed to a pheromone gradient, using video monitoring and statistical modeling. We report that cells of the migrating mating type (MT⁺) respond to pheromone gradients by simultaneous chemotaxis and chemokinesis. Changes in movement behavior enable MT⁺ cells to locate the direction of the pheromone source and to maximize their encounter rate towards it.

Microorganisms encounter a diversity of chemical stimuli that trigger individual responses and influence population dynamics. However, microbial behavior under the influence of different incentives and microbial decision-making is poorly understood. Benthic marine diatoms that react to sexual attractants as well as to nutrient gradients face such multiple constraints. Here, we document and model behavioral complexity and context-sensitive responses of these motile unicellular algae to sex pheromones and the nutrient silicate. Throughout the life cycle of the model diatom Seminavis robusta nutrient-starved cells localize sources of silicate by combined chemokinetic and chemotactic motility. However, with an increasing need for sex to restore the initial cell size, a change in behavior favoring the attraction-pheromone-guided search for a mating partner takes place. When sex becomes inevitable to prevent cell death, safeguard mechanisms are abandoned, and cells prioritize the search for mating partners. Such selection processes help to explain biofilm organization and to understand species interactions in complex communities.