Selective silicate-directed motility in diatoms

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 × 1012 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

Seasonal mixed layer depth shapes phytoplankton physiology, viral production, and accumulation in the North Atlantic

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Diaz, B. P., Knowles, B., Johns, C. T., Laber, C. P., Bondoc, K. G. V., Haramaty, L., Natale, F., Harvey, E. L., Kramer, S. J., Bolaños, L. M., Lowenstein, D. P., Fredricks, H. F., Graff, J., Westberry, T. K., Mojica, K. D. A., Haëntjens, N., Baetge, N., Gaube, P., Boss, E., Carlson, C. A., Behrenfeld, M. J., Van Mooy, B. A. S., Bidle, K. D. Seasonal mixed layer depth shapes phytoplankton physiology, viral production, and accumulation in the North Atlantic. Nature Communications, 12(1), (2021): 6634, https://doi.org/10.1038/s41467-021-26836-1.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.This work was made possible by NASA’s Earth Science Program in support of the North Atlantic Aerosol and Marine Ecosystem Study (15-RRNES15-0011 and 0NSSC18K1563 to K.D.B.; NNX15AF30G to M.J.B.), as well as with support from the National Science Foundation (OIA-2021032 to K.D.B., OCE-157943 to C.A.C., and OCE-1756254 to B.A.S.V.M.), the Gordon and Betty Moore Foundation (Award# 3789 to K.G.V.B.), and NASA’s Future Investigators in Space Science and Technology program (FINESST; grant #826380 to K.D.B.; graduate support to BD)

Seasonal Mixed Layer Depth Shapes Phytoplankton Physiology, Viral Production, and Accumulation In the North Atlantic

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

Directed motility of benthic diatoms

Diatoms are a group of highly diverse microalgae dominating aquatic systems and contributing to a quarter of the global primary production. They have a unique morphology including a biomineralized silicate-based cell wall. This rigid cell wall leads to cell size reduction during mitosis followed by size restoration through sexual reproduction. I used the model pennate diatom Seminavis robusta to determine how benthic diatoms forage for nutrients and locate their mating partners across their life cycle. A combination of video monitoring, trajectory analysis, and statistical modelling was utilized to determine how nutrient-starved or sexually-induced cells responded to the signal gradients formed by hotspot sources. I observed that cells are attracted to specific stimuli depending on their physiological conditions and needs. Nutrient foraging is consistent across the life cycle, wherein starved cells are attracted to silicate and phosphate, but not for nitrogen, and ammonium. On the other hand, once sexual, cells will also exhibit a finding mechanism for the mating pheromone, diproline. S. robusta exhibited a simultaneous tactic and kinetic mechanism towards the stimuli gradients. Despite being unicellular, diatoms can clearly prioritize between nutrient foraging and mating. It is highly possible that there is a cross-talk between dSi- and mate-sensing where motility is being regulated by the cells. The active searching behaviour can potentially contribute in shaping the aquatic environment on a microscale by influencing patchiness of biofilm communities, and on a global scale by affecting sediment-water nutrient fluxes and biogeochemical cycles.Diatomeen (Kieselalgen) bilden eine diverse Gruppe von Mikroalgen, die in aquatischen Systemen zu den dominanten Primärproduzenten zählen. Durch ihre große photosynthetische Aktivität tragen sie zu ca. einem Viertel der globalen Primärproduktion bei. Die einzigartige Morphologie der Diatomeen ist durch ihre biomineralisierten, silikatbasierten Zellwände geprägt. Aufgrund der starren Zellwand führt die mitotische Zellteilung zur Abnahme der Zellgröße und zu einer anschließenden Größenwiederherstellung durch sexuelle Reproduktion. Um zu ermitteln, wie motile pennate Diatomeen Nährstoffe und Paarungspartner finden, untersuchte ich die Modellkieselalge Seminavis robusta in verschiedenen Stadien ihres Lebenszyklus. Mittels einer Kombination aus Videobeobachtung, Zellbewegungsanalysen und statistischer Modellierung wurde untersucht, wie nährstofflimitierte oder sexuell-induzierte Zellen auf Signalstoffquellen reagierten. Dabei beobachtete ich, dass die Zellen entsprechend ihres physiologischen Zustands und Bedarfs durch spezifische Stimuli angezogen wurden. Dieses Suchverhalten zeigte sich während ihres gesamten Lebenszyklus, wobei hungernde Zellen von Silikat und Phosphat, allerdings nicht von Nitrat und Ammonium, angezogen wurden. Auf der anderen Seite wiesen sexuelle Zellen, auch ein Suchverhalten für das Paarungspheromon Diprolin auf. S. robusta zeigte dabei eine simultan chemotaktische und -kinetische Bewegung. Obwohl sie unizelluläre Organismen sind, können Diatomee klar zwischen Nahrungssuche und Paarung priorisieren. Zusätzlich scheint eine Motilitäts-regulierende Wechselwirkung zwischen Silikat- und Partnersuche sehr wahrscheinlich. Dieses aktive Suchverhalten hat Auswirkungen auf das aquatische System, sowohl im Mikromaßstab durch einen Einfluss auf die (Un)Gleichmäßigkeit des Biofilms, als auch im globalen Maßstab über eine Beeinflussung biogeochemischer Nährstoffzyklen und –flüsse in der Sediment-Wasser-Grenzschicht

The secrets of the sea: A battle between food and sex

Content: Diatom life cycle; diatom movement; sexual cells are attracted to both food and mating partners; if sexual cells will choose, sex is preferied over foo

Searching for a mate : pheromone-directed movement of the benthic diatom Seminavis robusta

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

Selective chemoattraction of the benthic diatom Seminavis robusta to phosphate but not to inorganic nitrogen sources contributes to biofilm structuring

Abstract 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

Decision-making of the benthic diatom Seminavis robusta searching for inorganic nutrients and pheromones

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 nutrientstarved 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