Biology subjects

Graff van Creveld, S.

Publications and source records attributed to Graff van Creveld, S..

6 recordsLinked to original sources

bioRxiv2026

Defining traits of low-light adapted Prochlorococcus inhabiting surface waters of the Equatorial Pacific Ocean

A diverse array of photosynthetic phytoplankton drives primary production in equatorial surface waters. Among these, the cyanobacterium Prochlorococcus is an important contributor to net primary production in these typically iron-limited, high-nutrient and low-chlorophyll (HNLC) regions. Here, we explore the diversity of these organisms, in part, through targeted enrichment of Prochlorococcus cells using field-based high-speed cell sorting techniques. We demonstrate that the genomes of Prochlorococcus belonging to the low-light adapted LLI clade, and isolated from the surface of the Equatorial Pacific Ocean, are depleted in functions related to the assimilation of urea, nitrite, and amino acids. These are the first examples of LLI Prochlorococcus that have lost the ability to use nitrite, a trait considered to be a core feature of this clade. All new equatorial cultures of LLI Prochlorococcus appear to use a distinct isoform of protoporphyrinogen IX oxidase (HemG), for the biosynthesis of a chlorophyll precursor, that does not require the use of iron-containing heme. In contrast, the heme-dependent HemJ isoform is typically used by Prochlorococcus found outside equatorial HNLC waters. Together, these findings suggest that low-light adapted Prochlorococcus in the equatorial ocean possess accessory gene content that reflects adaptation to the generally iron-limited but nitrogen-replete conditions of surface waters.

microbiology↗

bioRxiv2025

Induced pathogenicity toward open-ocean diatoms by a newly isolated filterable bacterium Ekhidna algicida sp. nov.

Phytoplankton are the base of marine food webs. They form intricate interactions with heterotrophic bacteria ranging from mutualistic to pathogenic that together impact oceanic carbon and nutrient cycling. Our understanding of these interactions in marine environments remains primarily limited to laboratory-based studies of model organisms. Here, we report the discovery and characterization of Ekhidna algicida sp. nov. strain To15, isolated from the oligotrophic Pacific Ocean (16{degrees}N, 140{degrees}W) based on its algicidal effect on the pelagic diatom Thalassiosira oceanica. Subsequent co-culture experiments demonstrate that E. algicida is lethal within days to a diverse array of diatoms, with the effect mediated by bacterial exudates that remain algicidal on their own against axenic T. oceanica cultures. Twenty additional algicidal Ekhidna strains were subsequently isolated from the Pacific Ocean. Our findings reveal E. algicida as a potentially widespread pathogen of diatoms, that can alter microbial community composition dynamics in pelagic ecosystems. TeaserA newly discovered Pacific Ocean bacterium can kill diatoms, revealing a hidden pathogenic role in open-ocean ecosystems.

ecology↗

bioRxiv2025

Metacaspases mediate thermotolerance of diatoms following marine heat waves

O_LIMarine heat waves (MHW) are extreme climate events characterized by elevated ocean temperatures lasting from days to months and spanning thousands of kilometers. Recent predictions show an increment in MHW frequency, duration and intensity worldwide along with climate change. Nevertheless, little is known about MHW impacts on marine microbial life and, specifically, on algal blooms. Recent studies in yeast and green algae suggested that thermotolerance is mediated by metacaspases (MC), cysteine proteases, structurally similar to caspases known to mediate programmed cell death. C_LIO_LIInitially we created a heatwave model of 72-hours that exposes the model diatom Phaeodactylum tricornutum to elevated temperature with a recovery phase that enables to capture the mechanisms underpinning such acclimation. We generated triple knock-out mutants of the PtMCA genes and identified a vital role for MC in heat-stress acclimation. C_LIO_LIPtMCA triple mutants exhibited increased sensitivity to the heatwave treatment, which induced cell death that peaked days after returning to initial temperatures. We further revealed that heatwave treatment induced accumulation of reactive oxygen species and the PtMCA mutants were hypersensitive to oxidative stress as compared to WT cells. C_LIO_LIWe propose that metacaspases have a pivotal role in diatoms acclimation to elevated temperatures a trait vital for algal survival considering climate change. C_LI

microbiology↗

bioRxiv2022

Divergent functions of two clades of flavodoxin in diatoms mitigate oxidative stress and iron limitation

Phytoplankton rely on diverse mechanisms to adapt to the decreased iron bioavailability and oxidative stress-inducing conditions of todays oxygenated oceans, including replacement of the iron-requiring ferredoxin electron shuttle protein with a less-efficient iron-free flavodoxin under iron limiting conditions. And yet, diatoms transcribe flavodoxins in high-iron regions in contrast to other phytoplankton. Here, we show that the two clades of flavodoxins present within diatoms exhibit a functional divergence, with only clade II flavodoxins displaying the canonical role in adaptation to iron limitation. We created CRISPR/Cas9 knock-outs of the clade I flavodoxin from the model diatom Thalassiosira pseudonana and found these cell lines are hypersensitive to oxidative stress, while maintaining a wild-type response to iron limitation. Within natural diatom communities, clade I flavodoxin transcript abundance is regulated over the diel cycle rather than in response to iron availability, whereas clade II transcript abundances increase either in iron-limiting regions or under artificially induced iron-limitation. The observed functional specialization of two flavodoxin variants within diatoms reiterates two major stressors associated with contemporary oceans and illustrates diatom strategies to flourish in diverse aquatic ecosystems.

evolutionary biology↗

bioRxiv2018

A redox-regulated type III metacaspase controls cell death in a marine diatom

Programmed cell death (PCD) in marine phytoplankton was suggested as one of the mechanisms that facilitates large scale bloom demise. Yet, the molecular basis for algal PCD machinery is rudimentary. Metacaspases are considered ancestral proteases that regulate cell death, but their activity and role in algae are still elusive. Here we biochemically characterized a recombinant metacaspase 5 from the model diatom Phaeodactylum tricornutum (PtMC5), revealing calcium-dependent protease activity. This activity includes auto-processing and cleavage following arginine. PtMC5 overexpressing cells exhibited higher metacaspase activity and were more sensitive to a diatom-specific infochemical compared to WT cells. Mutagenesis of potential disulfide-forming cysteines decreased PtMC5 activity, suggesting redox regulation. This cysteine pair is widespread in diatom type III metacaspases, but was not found in any other taxa. The characterization of a cell death associated protein in marine phytoplankton will enable deeper understanding of the ecological significance of PCD in bloom dynamics.

microbiology↗

bioRxiv2018

Phenotypic variability in chloroplast redox state predicts cell fate in a marine diatom

Diatoms are photosynthetic microorganisms of great ecological and biogeochemical importance, forming vast blooms in diverse aquatic ecosystems. Current understanding of phytoplankton acclimation to stress is based on population-level analysis, masking cell-to-cell variability. Here we investigated heterogeneity within Phaeodactylum tricornutum populations in response to oxidative stress, which is induced by environmental stress conditions. We combined flow cytometry and a microfluidics system for live imaging to measure redox dynamics at the single-cell level using the roGFP sensor. Chloroplast-targeted roGFP exhibited a light-dependent, bi-stable oxidation pattern in response to H2O2, revealing distinct subpopulations of sensitive oxidized cells and resilient reduced cells. Subpopulation proportions depended on growth phase, linking the bi-stable phenotype to proliferation. Oxidation of chloroplast-targeted roGFP preceded commitment to cell death and was used as a novel cell fate predictor. We propose that light-dependent metabolic heterogeneity results in differential stress responses that regulate cell fate within diatom populations.

microbiology↗