Role of Sulfur for Algae: Acquisition, Metabolism, Ecology and Evolution

Characterization of low molecular weight sulfur species in seaweed from the Antarctic continent

Antarctic seaweeds are vital components of polar marine ecosystems, playing a crucial role in nutrient cycling and supporting diverse life forms. The sulfur content in these organisms is particularly interesting due to its implication in biogeochemical processes and potential impacts on local and global environmental systems. In this study, we present a comprehensive characterization of seaweed collected in the Antarctic in terms of their total sulfur content and its distribution among different classes of species, including thiols, using various methods and high-sensitivity techniques. The data presented in this paper are unprecedented in the scientific literature. These methods allowed for the determination of total sulfur content and the distribution of sulfur compounds in different fractions, such as water-soluble and proteins, as well as the speciation of sulfur compounds in these fractions, providing valuable insights into the chemical composition of these unique marine organisms. Our results revealed that the total sulfur concentration in Antarctic seaweeds varied widely across different species, ranging from 5.5 to 56 g kg-1 dry weight. Furthermore, our investigation into the sulfur speciation revealed the presence of various sulfur compounds, including sulfate, and some thiols, which were quantified in all ten seaweed species evaluated. The concentration of these individual sulfur species also displayed considerable variability among the studied seaweeds. This study provides the first in-depth examination of total sulfur content and sulfur speciation in brown and red Antarctic seaweeds.

The production of dimethylsulfoniopropionate by bacteria with mmtN linked to non-ribosomal peptide synthase gene

Dimethylsulfoniopropionate (DMSP) is a vital sulfur-containing compound with worldwide significance, serving as the primary precursor for dimethyl sulfide (DMS), a volatile sulfur compound that plays a role in atmospheric chemistry and influences the Earth's climate on a global scale. The study investigated the ability of four bacterial strains, namely Acidimangrovimonas sediminis MS2-2 (MS2-2), Hartmannibacter diazotrophicus E18T (E18T), Rhizobium lusitanum 22705 (22705), and Nitrospirillum iridis DSM22198 (DSM22198), to produce and degrade DMSP. These strains were assessed for their DMSP synthesis ability with the mmtN linked to non-ribosomal peptide synthase (NRPS) gene. The results showed that MS2-2, and E18T bacteria, which contained the mmtN but not linked to an NRPS gene, increased DMSP production with increasing salinity. The highest production of DMSP was achieved at 25 PSU when either methionine was added or low nitrogen conditions were present, yielding 1656.03 ± 41.04 and 265.59 ± 9.17 nmol/mg protein, respectively, and subsequently under the conditions of methionine addition or low nitrogen, both strains reached their maximum DMSP production at 25 PSU. Furthermore, the strains MS2-2, E18T, and 22705 with the mmtN gene but not linked to an NRPS gene were found to be involved in DMS production. This research contributes to the understanding of the genes involved in DMSP biosynthesis in bacteria that produce DMSP.

Genomic insights into the coupling of a Chlorella-like microeukaryote and sulfur bacteria in the chemocline of permanently stratified Lake Cadagno

Meromictic Lake Cadagno is a permanently stratified system with a persistent microbial bloom within the oxic-anoxic boundary called the chemocline. The association between oxygenic and anoxygenic photosynthesis within the chemocline has been known for at least two decades. Although anoxygenic purple and green sulfur bacteria have been well studied, reports on oxygenic phytoplankton have remained sparse since their discovery in the 1920s. Nearly a century later, this study presents the first near-complete genome of a photosynthetic microbial eukaryote from the chemocline of Lake Cadagno, provisionally named Chlorella-like MAG. The 18.9 Mbp nuclear genome displays a high GC content (71.5%), and the phylogenetic placement suggests that it is a novel species of the genus Chlorella of Chlorophytes. Functional annotation of the Chlorella-like metagenome-assembled genome predicted 10,732 protein-coding genes, with an approximate 0.6% proportion potentially involved in carbon, sulfur, and nitrogen (C, N, and S) metabolism. In addition to C4 photosynthesis, this study detected genes for heat shock proteins (HSPs) in the Chlorella-like algae, consistent with the other Chlorella species. Altogether, the genomic insights in this study suggest the cooperation of photosynthetic algae with phototrophic sulfur bacteria via C, N, and S metabolism, which may aid their collective persistence in the Lake Cadagno chemocline. Furthermore, this work additionally presents the chloroplast genome of Cryptomonas-like species, which was likely to be presumed as cyanobacteria in previous studies because of the presence of phycobilisomes.

Changes in ATP Sulfurylase Activity in Response to Altered Cyanobacteria Growth Conditions

We investigated variations in cell growth and ATP Sulfurylase (ATPS) activity when two cyanobacterial strains-Synechocystis sp. PCC6803 and Synechococcus sp. WH7803-were grown in conventional media, and media with low ammonium, low sulfate and a high CO₂/low O₂ atmosphere. In both organisms, a transition and adaptation to the reconstructed environmental media resulted in a decrease in ATPS activity. This variation appears to be decoupled from growth rate, suggesting the enzyme is not rate-limiting in S assimilation and raising questions about the role of ATPS redox regulation in cell physiology and throughout Earth history.

Metabolomics for early detection of stress in freshwater alga Poterioochromonas malhamensis exposed to silver nanoparticles

Silver nanoparticles (AgNPs) are one of the most used engineered nanomaterials. Despite progress in assessing their environmental implications, knowledge gaps exist concerning the metabolic perturbations induced by AgNPs on phytoplankton, essential organisms in global biogeochemical cycles and food-web dynamics. We combine targeted metabolomics, biouptake and physiological response studies to elucidate metabolic perturbations in alga Poterioochromonas malhamensis induced by AgNPs and dissolved Ag. We show time-dependent perturbation of the metabolism of amino acids, nucleotides, fatty acids, tricarboxylic acids, photosynthesis and photorespiration by both Ag-treatments. The results suggest that dissolved Ag ions released by AgNPs are the major toxicity driver; however, AgNPs internalized in food vacuoles contributed to the perturbation of amino acid metabolism, TCA cycle and oxidative stress. The metabolic perturbations corroborate the observed physiological responses. We highlight the potential of metabolomics as a tool for understanding the molecular basis for these metabolic and physiological changes, and for early detection of stress.

Using Simulated Flue Gas to Rapidly Grow Nutritious Microalgae with Enhanced Settleability

Favorable microalgal nutrition from waste resources and improved harvesting methods would offset costs for a process that could be scaled up to treat pollution and produce valuable animal feed in lieu of soy protein. Co-benefits include avoidance of carbon dioxide emissions, which may provide an additional revenue stream when carbon markets begin to flourish. To sustainably achieve these goals at scale, barriers to microalgal production such as tolerance for waste streams and dramatic improvement in dewatering and settleability of the microalgae must be overcome. Presently, it is largely assumed that nutritious microalgae, including Scenedesmus obliquus, would be inhibited by SO _x and NO _x in flue gases and settle slowly as discrete particles. Studies conducted with a 2 L photobioreactor, sparged with simulated coal-fired power plant flue gas, demonstrated that both biomass productivity and settling rates were increased. The average maximum biomass productivity was 700 ± 40 mg L^-1 d^-1, which significantly exceeded that of the control culture (510 ± 40 mg L^-1 d^-1). Thirty-minute trials of modeled bulk settling showed rapid coagulation, likely facilitated by extracellular polymeric substances, and compaction when the cultures were grown with simulated emissions. Control cultures, not exposed to the additional toxicants in flue gas, settled as discrete particles and did not show any settling progress within 30 min. Of the SO₂ sparged into the cultivation system, (111 ± 4)% was captured as either SO₄ ^2- in the medium or fixed in the S. obliquus biomass. The stress of simulated-emissions exposure decreased the S. obliquus protein contents and altered the amino acid profiles but did not decrease the fraction of methionine, a valuable amino acid in animal feed.

Impact of Sulfur Starvation in Autotrophic and Heterotrophic Cultures of the Extremophilic Microalga Galdieria phlegrea (Cyanidiophyceae)

In plants and algae, sulfate assimilation and cysteine synthesis are regulated by sulfur (S) accessibility from the environment. This study reports the effects of S deprivation in autotrophic and heterotrophic cultures of Galdieria phlegrea (Cyanidiophyceae), a unicellular red alga isolated in the Solfatara crater located in Campi Flegrei (Naples, Italy), where H2S is the prevalent form of gaseous S in the fumarolic fluids and S is widespread in the soils near the fumaroles. This is the first report on the effects of S deprivation on a sulfurous microalga that is also able to grow heterotrophically in the dark. The removal of S from the culture medium of illuminated cells caused a decrease in the soluble protein content and a significant decrease in the intracellular levels of glutathione. Cells from heterotrophic cultures of G. phlegrea exhibited high levels of internal proteins and high glutathione content, which did not diminish during S starvation, but rather glutathione significantly increased. The activity of O-acetylserine(thiol)lyase (OASTL), the enzyme synthesizing cysteine, was enhanced under S deprivation in a time-dependent manner in autotrophic but not in heterotrophic cells. Analysis of the transcript abundance of the OASTL gene supports the OASTL activity increase in autotrophic cultures under S deprivation.

The relevance of compartmentation for cysteine synthesis in phototrophic organisms

In the vascular plant Arabidopsis thaliana, synthesis of cysteine and its precursors O-acetylserine and sulfide is distributed between the cytosol, chloroplasts, and mitochondria. This compartmentation contributes to regulation of cysteine synthesis. In contrast to Arabidopsis, cysteine synthesis is exclusively restricted to chloroplasts in the unicellular green alga Chlamydomonas reinhardtii. Thus, the question arises, whether specification of compartmentation was driven by multicellularity and specified organs and tissues. The moss Physcomitrella patens colonizes land but is still characterized by a simple morphology compared to vascular plants. It was therefore used as model organism to study evolution of compartmented cysteine synthesis. The presence of O-acetylserine(thiol)lyase (OAS-TL) proteins, which catalyze the final step of cysteine synthesis, in different compartments was applied as criterion. Purification and characterization of native OAS-TL proteins demonstrated the presence of five OAS-TL protein species encoded by two genes in Physcomitrella. At least one of the gene products is dual targeted to plastids and cytosol, as shown by combination of GFP fusion localization studies, purification of chloroplasts, and identification of N termini from native proteins. The bulk of OAS-TL protein is targeted to plastids, whereas there is no evidence for a mitochondrial OAS-TL isoform and only a minor part of OAS-TL protein is localized in the cytosol. This demonstrates that subcellular diversification of cysteine synthesis is already initialized in Physcomitrella but appears to gain relevance later during evolution of vascular plants.

Sulfur assimilation in photosynthetic organisms: molecular functions and regulations of transporters and assimilatory enzymes

Sulfur is required for growth of all organisms and is present in a wide variety of metabolites having distinctive biological functions. Sulfur is cycled in ecosystems in nature where conversion of sulfate to organic sulfur compounds is primarily dependent on sulfate uptake and reduction pathways in photosynthetic organisms and microorganisms. In vascular plant species, transport proteins and enzymes in this pathway are functionally diversified to have distinct biochemical properties in specific cellular and subcellular compartments. Recent findings indicate regulatory processes of sulfate transport and metabolism are tightly connected through several modes of transcriptional and posttranscriptional mechanisms. This review provides up-to-date knowledge in functions and regulations of sulfur assimilation in plants and algae, focusing on sulfate transport systems and metabolic pathways for sulfate reduction and synthesis of downstream metabolites with diverse biological functions.