(2018) Frontiers in Marine Science. 5, p. 144+ 144. Abstract
Microscopic marine phytoplankton drift freely in the ocean, harvesting sunlight through photosynthesis. These unicellular microorganisms account for half of the primary productivity on Earth and play pivotal roles in the biogeochemistry of our planet (Field et al., 1998). The major groups of microalgae that comprise the phytoplankton community are coccolithophores, diatoms and dinoflagellates. In present oceans, phytoplankton individuals and populations are forced to rapidly adjust, as key chemical and physical parameters defining marine habitats are changing globally. Here we propose that microalgal populations often display the characteristics of a multicellular-like community rather than a random collection of individuals. Evolution of multicellularity entails a continuum of events starting from single cells that go through aggregation or clonal divisions (Brunet and King, 2017). Phytoplankton may be an intermediate state between single cells and aggregates of physically attached cells that communicate and co-operate; perhaps an evolutionary snapshot toward multicellularity. In this opinion article, we journey through several studies conducted in two key phytoplankton groups, coccolithophores and diatoms, to demonstrate how observations in these studies could be interpreted in a multicellular context.
(2016) eLife. 5, e17473. Abstract
Emiliania huxleyi is a model coccolithophore micro-alga that generates vast blooms in the ocean. Bacteria are not considered among the major factors influencing coccolithophore physiology. Here we show through a laboratory model system that the bacterium Phaeobacter inhibens, a well-studied member of the Roseobacter group, intimately interacts with E. huxleyi. While attached to the algal cell, bacteria initially promote algal growth but ultimately kill their algal host. Both algal growth enhancement and algal death are driven by the bacterially-produced phytohormone indole-3-acetic acid. Bacterial production of indole-3-acetic acid and attachment to algae are significantly increased by tryptophan, which is exuded from the algal cell. Algal death triggered by bacteria involves activation of pathways unique to oxidative stress response and programmed cell death. Our observations suggest that bacteria greatly influence the physiology and metabolism of E. huxleyi. Coccolithophore-bacteria interactions should be further studied in the environment to determine whether they impact micro-algal population dynamics on a global scale.[All authors]
(2016) Algal Research-Biomass Biofuels And Bioproducts. 19, p. 370-380 Abstract
Flow cytometry (FCM) is a well-established tool in the field of aquatic phytoplankton ecology and microalgal biotechnology, which allows for rapid assessment of the viability and physiological state of individual cells in algal populations. However, the autofluorescent spectra of different types of chlorophyll and other algal pigments may overlap with fluorescent dyes and affect the resolution of algae clusters, sensitivity, and signal-to-noise ratio. Dying algal cells continue to exhibit a strong autofluorescent signal, which may affect the evaluation of algal viability. Herein, we tested two different approaches to measure algal fluorescence in the presence of a strong autofluorescent signal: 1) by separating dyes between different excitation lasers in order to reach minimal spectral overlap with the autofluorescent signal using flow and imaging cytometry and 2) through full spectrum analysis, virtual filtering and spectral unmixing of dye combinations and algal pigments' autofluorescence via spectral flow cytometry. For this purpose, we used viability dyes from the SYTOX family and lipophilic dyes. Among the dyes tested, the SYTOX Blue (SB) dye had minimal overlap with chlorophyll fluorescence and can be combined with autofluorescence assessment and lipophilic dyes (validated with Emiliania huxleyi algal monocultures). Imaging cytometry provided a detailed characterization of algal subpopulations stained with a combination of fluorescent dyes. A spectral flow cytometer allowed us to analyze environmental phytoplankton samples stained with fluorescent dyes in the presence of strong and heterogeneous autofluorescence from intrinsic algal pigments. We concluded that the multi-color staining of algal samples can be achieved in the presence of strong and diverse algal autofluorescence using dyes with minimal spectral overlap, a multi-laser approach (flow and imaging cytometry) and/or virtual filter and spectral flow cytometry instrumentation. This can open a ne
(2016) Journal of Phycology. 52, 1, p. 125-130 Abstract
The microalga Emiliania huxleyi produces alkenone lipids that are important proxies for estimating past sea surface temperatures. Field calibrations of this proxy are robust but highly variable results are obtained in culture. Here, we present results suggesting that algal-bacterial interactions may be responsible for some of this variability. Co-cultures of E.huxleyi and the bacterium Phaeobacter inhibens resulted in a 2.5-fold decrease in algal alkenone-containing lipid bodies. In addition levels of unsaturated alkenones increase in co-cultures. These changes result in an increase in the reconstructed growth temperature of up to 2 degrees C relative to axenic algal cultures.
(2015) PLoS ONE. 10, 11, e0141300. Abstract
The Roseobacter clade is a key group of bacteria in the ocean exhibiting diverse metabolic repertoires and a wide range of symbiotic life-styles. Many Roseobacters possess remarkable capabilities of attachment to both biotic and abiotic surfaces. When attached to each other, these bacteria form multi-cellular structures called rosettes. Phaeobacter inhibens, a well-studied Roseobacter, exhibits various cell sizes and morphologies that are either associated with rosettes or occur as single cells. Here we describe the distribution of P. inhibens morphologies and rosettes within a population. We detect an N-acetylglucosamine-containing polysaccharide on the poles of some cells and at the center of all rosettes. We demonstrate that rosettes are formed by the attachment of individual cells at the polysaccharide-containing pole rather than by cell division. Finally, we show that P. inhibens attachment to abiotic surfaces is hindered by the presence of DNA from itself, but not from other bacteria. Taken together, our findings demonstrate that cell adhesiveness is likely to play a significant role in the life cycle of P. inhibens as well as other Roseobacters.
(2015) Molecular Cell. 57, 4, p. 695-707 Abstract
The bacterial spore can rapidly convert from a dormant to a fully active cell. Here we study this remarkable cellular transition in Bacillus subtilis and reveal the identity of the newly synthesized proteins throughout spore revival. Our analysis uncovers a highly ordered developmental program that correlates with the spore morphological changes and reveals the spatial and temporal molecular events fundamental to reconstruct a cell. As opposed to current knowledge, we found that translation takes place during the earliest revival event, termed germination, a process hitherto considered to occur without the need for any macromolecule synthesis. Furthermore, we demonstrate that translation is required for execution of germination and relies on the bona fide translational factors RpmE and Tig. Our study sheds light on the spore revival process and on the vital building blocks underlying cellular awakening, thereby paving the way for designing new antimicrobial agents to eradicate spore-forming pathogens.
(2013) Journal of Bacteriology. 195, 9, p. 1875-1882 Abstract
Bacterial spores can remain dormant for years, yet they possess a remarkable potential to rapidly resume a vegetative life form. Here, we identified a distinct phase at the onset of spore outgrowth, designated the ripening period. This transition phase is exploited by the germinating spore for molecular reorganization toward elongation and subsequent cell division. We have previously shown that spores of different ages, kept under various temperatures, harbor dissimilar molecular reservoirs (E. Segev, Y. Smith, and S. Ben-Yehuda, Cell 148:139-149, 2012). Utilizing this phenomenon, we observed that the length of the ripening period can vary according to the spore molecular content. Importantly, the duration of the ripening period was found to correlate with the initial spore rRNA content and the kinetics of rRNA accumulation upon exiting dormancy. Further, the synthesis of the ribosomal protein Rp1A and the degradation of the spore-specific protein SspA also correlated with the duration of the ripening period. Our data suggest that the spore molecular cargo determines the extent of the ripening period, a potentially crucial phase for a germinating spore in obtaining limited resources during revival.
(2012) Cell. 148, 1-2, p. 139-149 Abstract
Upon starvation, the bacterium Bacillus subtilis enters the process of sporulation, lasting several hours and culminating in formation of a spore, the most resilient cell type known. We show that a few days following sporulation, the RNA profile of spores is highly dynamic. In aging spores incubated at high temperatures, RNA content is globally decreased by degradation over several days. This degradation might be a strategy utilized by the spore to facilitate its dormancy. However, spores kept at low temperature exhibit a different RNA profile with evidence supporting transcription. Further, we demonstrate that germination is affected by spore age, incubation temperature, and RNA state, implying that spores can acquire dissimilar characteristics at a time they are considered dormant. We propose that, in contrast to current thinking, entering dormancy lasts a few days, during which spores are affected by the environment and undergo corresponding molecular changes influencing their emergence from quiescence.
(2006) GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS. 7, Q02P09. Abstract
 The ratio of magnesium to calcium (Mg/Ca) in CaCO3 shells of foraminifera is widely used to determine paleotemperatures. However, Mg/Ca is highly variable within and between species, suggesting a strong physiological influence on the incorporation of Mg2+ into the shells. While most field and laboratory calibrations have focused on the effect of temperature, we chose to study the effect of ambient Mg/Ca on the calcification process and the final shell composition. We cultured two species of symbiont-bearing benthic foraminifera, Amphistegina lobifera and Amphistegina lessonii, in seawater with different Mg/Ca ratios. Electron probe analysis of the shell Mg/Ca revealed a positive ( but not entirely linear) correlation with Mg/Ca in the culturing media with slightly different curves for each species. Partition coefficients of Mg2+ (D-Mg) in the calcite shells showed a decrease by a factor of roughly 2 between the lowest and highest Mg/Ca in the ambient water. This was previously demonstrated in inorganic calcite precipitation experiments. However, the biogenic DMg was significantly lower than the inorganic one, suggesting a physiological mechanism that reduces Mg/Ca at the calcification site. Unlike inorganic experiments that display a dependence of DMg on the kinetics of precipitation, the biogenic DMg is not correlated with the rate of calcification. Both DMg and calcification rates in our experiment were sensitive to the Mg/Ca ratio rather than the concentration of either Ca2+ or Mg2+. The largest addition of CaCO3 was obtained at Mg/Ca of 1, and not at present-day seawater ratio (Mg/Ca = 5). This may reflect the Mg/Ca that prevailed during the Eocene (Mg/Ca similar to 1.5), when this genus evolved.