Biochemical evidence for two novel enzymes in the biosynthesis of 3-dimethylsulfoniopropionate in Spartina alterniflora

Alternative dimethylsulfoniopropionate biosynthesis enzymes in diverse and abundant microorganisms

Dimethylsulfoniopropionate (DMSP) is an abundant marine organosulfur compound with roles in stress protection, chemotaxis, nutrient and sulfur cycling and climate regulation. Here we report the discovery of a bifunctional DMSP biosynthesis enzyme, DsyGD, in the transamination pathway of the rhizobacterium Gynuella sunshinyii and some filamentous cyanobacteria not previously known to produce DMSP. DsyGD produces DMSP through its N-terminal DsyG methylthiohydroxybutyrate S-methyltransferase and C-terminal DsyD dimethylsulfoniohydroxybutyrate decarboxylase domains. Phylogenetically distinct DsyG-like proteins, termed DSYE, with methylthiohydroxybutyrate S-methyltransferase activity were found in diverse and environmentally abundant algae, comprising a mix of low, high and previously unknown DMSP producers. Algae containing DSYE, particularly bloom-forming Pelagophyceae species, were globally more abundant DMSP producers than those with previously described DMSP synthesis genes. This work greatly increases the number and diversity of predicted DMSP-producing organisms and highlights the importance of Pelagophyceae and other DSYE-containing algae in global DMSP production and sulfur cycling.

The Stereochemical Course of DmdC, an Enzyme Involved in the Degradation of Dimethylsulfoniopropionate

The acyl-CoA dehydrogenase DmdC is involved in the degradation of the marine sulfur metabolite dimethylsulfonio propionate (DMSP) through the demethylation pathway. The stereochemical course of this reaction was investigated through the synthesis of four stereoselectively deuterated substrate surrogates carrying stereoselective deuterations at the α- or the β-carbon. Analysis of the products revealed a specific abstraction of the 2-pro-R proton and of the 3-pro-S hydride, establishing an anti elimination for the DmdC reaction.

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.

Molecular discoveries in microbial DMSP synthesis

Dimethylsulfoniopropionate (DMSP) is one of the Earth's most abundant organosulfur compounds because many marine algae, bacteria, corals and some plants produce it to high mM intracellular concentrations. In these organisms, DMSP acts an anti-stress molecule with purported roles to protect against salinity, temperature, oxidative stress and hydrostatic pressure, amongst many other reported functions. However, DMSP is best known for being a major precursor of the climate-active gases and signalling molecules dimethylsulfide (DMS), methanethiol (MeSH) and, potentially, methane, through microbial DMSP catabolism. DMSP catabolism has been extensively studied and the microbes, pathways and enzymes involved have largely been elucidated through the application of molecular research over the last 17 years. In contrast, the molecular biology of DMSP synthesis is a much newer field, with the first DMSP synthesis enzymes only being identified in the last 5 years. In this review, we discuss how the elucidation of key DMSP synthesis enzymes has greatly expanded our knowledge of the diversity of DMSP-producing organisms, the pathways used, and what environmental factors regulate production, as well as to inform on the physiological roles of DMSP. Importantly, the identification of key DMSP synthesis enzymes in the major groups of DMSP producers has allowed scientists to study the distribution and predict the importance of different DMSP-producing organisms to global DMSP production in diverse marine and sediment environments. Finally, we highlight key challenges for future molecular research into DMSP synthesis that need addressing to better understand the cycling of this important marine organosulfur compound, and its magnitude in the environment.

Effects of methyl halide flux characteristics following Spartina alterniflora invasion in a seaward direction in a temperate salt marsh, China

In this study, we explored the source-sink characteristics of methyl halide (CH₃X; X = Cl, Br, I) in coastal wetlands located in temperate regions, and identified key factors affecting the spatio-temporal variation of CH₃X during the invasion of Spartina alterniflora. We used static chamber-gas chromatography to monitor CH₃X fluxes in the S. alterniflora area and bare flat area of the Jiaozhou Bay salt marsh for a long time from August 2015 to May 2017. Our results indicated that CH₃X emissions showed obvious seasonal and diurnal variations. The S. alterniflora area was a source of CH₃X, with higher fluxes in the spring and autumn seasons. CH₃X fluxes were higher during the daytime than at night, and the diurnal difference in CH₃Br was the most significant (4.51 times). The bare flat area was mainly a sink for CH₃X, and the maximum absorption flux occurred in summer. At this time, the microbial activity was greater, and the consumption rate during the day was higher than that at night. Extreme linear correlations existed between the fluxes of CH₃Cl, CH₃Br, and CH₃I (P < 0.01), indicating that the production and consumption of the three gases were likely to have similar mechanisms and were affected by the same factors. S. alterniflora invasion increased CH₃X emissions and shifted the original bare flat area from a sink to a source of CH₃X. The biomass of S. alterniflora, especially the leaf, significantly affects CH₃X fluxes. Additionally, S. alterniflora increased the content of total organic carbon, total sulfur, available sulfur, and iron (III) in the soil, which were the main factors promoting the source-sink transformation of CH₃X. Based on the current invasive area of S. alterniflora in China, we estimated that the annual emissions of CH₃Cl, CH₃Br, and CH₃I from S. alterniflora into the troposphere were 9.04 × 10⁶, 2.42 × 10⁵ and 2.06 × 10⁵ mol, respectively.

Insights into methionine S-methylation in diverse organisms

Dimethylsulfoniopropionate (DMSP) is an important marine anti-stress compound, with key roles in global nutrient cycling, chemotaxis and, potentially, climate regulation. Recently, diverse marine Actinobacteria, α- and γ-proteobacteria were shown to initiate DMSP synthesis via the methionine (Met) S-methyltransferase enzyme (MmtN), generating S-methyl-Met (SMM). Here we characterize a roseobacterial MmtN, providing structural and mechanistic insights into this DMSP synthesis enzyme. We propose that MmtN uses the proximity and desolvation mechanism for Met S-methylation with two adjacent MmtN monomers comprising the Met binding site. We also identify diverse functional MmtN enzymes in potentially symbiotic archaeal Candidatus Woesearchaeota and Candidate Phyla Radiation (CPR) bacteria, and the animalcule Adineta steineri, not anticipated to produce SMM and/or DMSP. These diverse MmtN enzymes, alongside the larger plant MMT enzyme with an N-terminus homologous to MmtN, likely utilize the same proximity and desolvation mechanism. This study provides important insights into the catalytic mechanism of SMM and/or DMSP production, and proposes roles for these compounds in secondary metabolite production, and SMM cycling in diverse organisms and environments.

S-methyl methionine (SMM) is a key molecule in production of dimethylsulfoniopropionate (DMSP), an important marine anti-stress compound, with roles in global nutrient cycling. Here, the authors determine the mechanism of SMM synthesis and uncover unexpected roles for SMM in archaea, CPR bacteria and animals.

Spatiotemporal distribution of bacterial dimethylsulfoniopropionate producing and catabolic genes in the Changjiang Estuary

The osmolyte dimethylsulfoniopropionate (DMSP) is produced in petagram amounts by marine microorganisms. Estuaries provide natural gradients in salinity and nutrients, factors known to regulate DMSP production; yet there have been no molecular studies of DMSP production and cycling across these gradients. Here, we study the abundance, distribution and transcription of key DMSP synthesis (e.g. dsyB and mmtN) and catabolic (e.g. dddP and dmdA) genes along the salinity gradient of the Changjiang Estuary. DMSP levels did not correlate with Chl a across the salinity gradient. In contrast, DMSP concentration, abundance of bacterial DMSP producers and their dsyB and mmtN transcripts were lowest in the freshwater samples and increased abruptly with salinity in the transitional and seawater samples. Metagenomics analysis suggests bacterial DMSP-producers were more abundant than their algal equivalents and were more prominent in summer than winter samples. Bacterial DMSP catabolic genes and their transcripts followed the same trend of being greatly enhanced in transitional and seawater samples with higher DMSP levels than freshwater samples. DMSP cleavage was likely the dominant catabolic pathway, with DMSP lyase genes being ~4.3-fold more abundant than the demethylase gene dmdA. This is an exemplar study for future research on microbial DMSP cycling in estuary environments.

DMSP-Producing Bacteria Are More Abundant in the Surface Microlayer than Subsurface Seawater of the East China Sea

Microbial production and catabolism of dimethylsulfoniopropionate (DMSP), generating the climatically active gases dimethyl sulfide (DMS) and methanethiol (MeSH), have key roles in global carbon and sulfur cycling, chemotaxis, and atmospheric chemistry. Microorganisms in the sea surface microlayer (SML), the interface between seawater and atmosphere, likely play an important role in the generation of DMS and MeSH and their exchange to the atmosphere, but little is known about these SML microorganisms. Here, we investigated the differences between bacterial community structure and the distribution and transcription profiles of the key bacterial DMSP synthesis (dsyB and mmtN) and catabolic (dmdA and dddP) genes in East China Sea SML and subsurface seawater (SSW) samples. Per equivalent volume, bacteria were far more abundant (~ 7.5-fold) in SML than SSW, as were those genera predicted to produce DMSP. Indeed, dsyB (~ 7-fold) and mmtN (~ 4-fold), robust reporters for bacterial DMSP production, were also far more abundant in SML than SSW. In addition, the SML had higher dsyB transcripts (~ 3-fold) than SSW samples, which may contribute to the significantly higher DMSP level observed in SML compared with SSW. Furthermore, the abundance of bacteria with dmdA and their transcription were higher in SML than SSW samples. Bacteria with dddP and transcripts were also prominent, but less than dmdA and presented at similar levels in both layers. These data indicate that the SML might be an important hotspot for bacterial DMSP production as well as generating the climatically active gases DMS and MeSH, a portion of which are likely transferred to the atmosphere.

Sulfur metabolites in the pelagic ocean

Marine microorganisms play crucial roles in Earth's element cycles through the production and consumption of organic matter. One of the elements whose fate is governed by microbial activities is sulfur, an essential constituent of biomass and a crucial player in climate processes. With sulfur already being well studied in the ocean in its inorganic forms, organic sulfur compounds are emerging as important chemical links between marine phytoplankton and bacteria. The high concentration of inorganic sulfur in seawater, which can readily be reduced by phytoplankton, provides a freely available source of sulfur for biomolecule synthesis. Mechanisms such as exudation and cell lysis release these phytoplankton-derived sulfur metabolites into seawater, from which they are rapidly assimilated by marine bacteria and archaea. Energy-limited bacteria use scavenged sulfur metabolites as substrates or for the synthesis of vitamins, cofactors, signalling compounds and antibiotics. In this Review, we examine the current knowledge of sulfur metabolites released into and taken up from the marine dissolved organic matter pool by microorganisms, and the ecological links facilitated by their diversity in structures, oxidation states and chemistry.

Dimethylsulfoniopropionate biosynthesis in a diatom Thalassiosira pseudonana: Identification of a gene encoding MTHB-methyltransferase

Dimethylsulfoniopropionate (DMSP) is one of the most abundant molecules on earth and plays a pivotal role in the marine sulfur cycle. DMSP is believed to be synthesized from methionine by a four-step reaction pathway in marine algae. The genes responsible for biosynthesis of DMSP remain unidentified. A diatom Thalassiosira pseudonana CCMP1335 is an important component of marine ecosystems and contributes greatly to the world's primary production. In this study, through genome search, in vivo activity and functional studies of cDNA products, a gene encoding Thalassiosira methyltransferase (TpMMT) which catalyzes the key step of DMSP synthesis formation of 4-methylthio-2-hydroxybutyrate (DMSHB) from 4-methylthio-2-oxobutyrate (MTHB), was identified. The amino acid sequence of TpMMT was homologous to the methyltransferase from Phaeodactylum tricornutum CCAP 1055/1, but not the recently identified bacterium gene. High salinity and nitrogen limitation stresses caused the increase of DMSP content and TpMMT protein in Thalassiosira. In addition to TpMMT, the enzyme activities for the first three steps could be detected and enhanced under high salinity, suggesting the importance of four-step DMSP synthetic pathway in Thalassiosira.

Evolution of DMSP (dimethylsulfoniopropionate) biosynthesis pathway: Origin and phylogenetic distribution in polyploid Spartina (Poaceae, Chloridoideae)

DMSP (dimethylsulfoniopropionate) is an ecologically important sulfur metabolite commonly produced by marine algae and by some higher plant lineages, including the polyploid salt marsh genus Spartina (Poaceae). The molecular mechanisms and genes involved in the DMSP biosynthesis pathways are still unknown. In this study, we performed comparative analyses of DMSP amounts and molecular phylogenetic analyses to decipher the origin of DMSP in Spartina that represents one of the major source of terrestrial DMSP in coastal marshes. DMSP content was explored in 14 Spartina species using ¹H Nuclear Magnetic Resonance (NMR) spectroscopy and Ultra Performance Liquid Chromatography-Mass Spectrometry (UPLC-MS). Putative genes encoding the four enzymatic steps of the DMSP biosynthesis pathway in Spartina were examined and their evolutionary dynamics were studied. We found that the hexaploid lineage containing S. alterniflora, S. foliosa and S. maritima and their derived hybrids and allopolyploids are all able to produce DMSP, in contrast to species in the tetraploid clade. Thus, examination of DMSP synthesis in a phylogenetic context implicated a single origin of this physiological innovation, which occurred in the ancestor of the hexaploid Spartina lineage, 3-6MYA. Candidate genes specific to the Spartina DMSP biosynthesis pathway were also retrieved from Spartina transcriptomes, and provide a framework for future investigations to decipher the molecular mechanisms involved in this plant phenotypic novelty that has major ecological impacts in saltmarsh ecosystems.

Evolution of Dimethylsulfoniopropionate Metabolism in Marine Phytoplankton and Bacteria

The elucidation of the pathways for dimethylsulfoniopropionate (DMSP) synthesis and metabolism and the ecological impact of DMSP have been studied for nearly 70 years. Much of this interest stems from the fact that DMSP metabolism produces the climatically active gas dimethyl sulfide (DMS), the primary natural source of sulfur to the atmosphere. DMSP plays many important roles for marine life, including use as an osmolyte, antioxidant, predator deterrent, and cryoprotectant for phytoplankton and as a reduced carbon and sulfur source for marine bacteria. DMSP is hypothesized to have become abundant in oceans approximately 250 million years ago with the diversification of the strong DMSP producers, the dinoflagellates. This event coincides with the first genome expansion of the Roseobacter clade, known DMSP degraders. Structural and mechanistic studies of the enzymes of the bacterial DMSP demethylation and cleavage pathways suggest that exposure to DMSP led to the recruitment of enzymes from preexisting metabolic pathways. In some cases, such as DmdA, DmdD, and DddP, these enzymes appear to have evolved to become more specific for DMSP metabolism. By contrast, many of the other enzymes, DmdB, DmdC, and the acrylate utilization hydratase AcuH, have maintained broad functionality and substrate specificities, allowing them to carry out a range of reactions within the cell. This review will cover the experimental evidence supporting the hypothesis that, as DMSP became more readily available in the marine environment, marine bacteria adapted enzymes already encoded in their genomes to utilize this new compound.

Dimethylsulfoniopropionate biosynthesis in marine bacteria and identification of the key gene in this process

Dimethylsulfoniopropionate (DMSP) is one of the Earth's most abundant organosulfur molecules, a signalling molecule¹, a key nutrient for marine microorganisms^2,3 and the major precursor for gaseous dimethyl sulfide (DMS). DMS, another infochemical in signalling pathways⁴, is important in global sulfur cycling² and affects the Earth's albedo, and potentially climate, via sulfate aerosol and cloud condensation nuclei production^5,6. It was thought that only eukaryotes produce significant amounts of DMSP^7-9, but here we demonstrate that many marine heterotrophic bacteria also produce DMSP, probably using the same methionine (Met) transamination pathway as macroalgae and phytoplankton¹⁰. We identify the first DMSP synthesis gene in any organism, dsyB, which encodes the key methyltransferase enzyme of this pathway and is a reliable reporter for bacterial DMSP synthesis in marine Alphaproteobacteria. DMSP production and dsyB transcription are upregulated by increased salinity, nitrogen limitation and lower temperatures in our model DMSP-producing bacterium Labrenzia aggregata LZB033. With significant numbers of dsyB homologues in marine metagenomes, we propose that bacteria probably make a significant contribution to oceanic DMSP production. Furthermore, because DMSP production is not solely associated with obligate phototrophs, the process need not be confined to the photic zones of marine environments and, as such, may have been underestimated.

Diversity, distribution and roles of osmoprotective compounds accumulated in halophytes under abiotic stress

BACKGROUND AND AIMS

Osmolytes are low-molecular-weight organic solutes, a broad group that encompasses a variety of compounds such as amino acids, tertiary sulphonium and quaternary ammonium compounds, sugars and polyhydric alcohols. Osmolytes are accumulated in the cytoplasm of halophytic species in order to balance the osmotic potential of the Na(+) and Cl(-) accumulated in the vacuole. The advantages of the accumulation of osmolytes are that they keep the main physiological functions of the cell active, the induction of their biosynthesis is controlled by environmental cues, and they can be synthesized at all developmental stages. In addition to their role in osmoregulation, osmolytes have crucial functions in protecting subcellular structures and in scavenging reactive oxygen species.

SCOPE

This review discusses the diversity of osmolytes among halophytes and their distribution within taxonomic groups, the intrinsic and extrinsic factors that influence their accumulation, and their role in osmoregulation and osmoprotection. Increasing the osmolyte content in plants is an interesting strategy to improve the growth and yield of crops upon exposure to salinity. Examples of transgenic plants as well as exogenous applications of some osmolytes are also discussed. Finally, the potential use of osmolytes in protein stabilization and solvation in biotechnology, including the pharmaceutical industry and medicine, are considered.

The chemical biology of dimethylsulfoniopropionate

This review addresses synthesis, biosynthesis, transport and degradation of dimethylsulfoniopropionate and its derivatives.

Bacterial Catabolism of Dimethylsulfoniopropionate (DMSP)

Dimethylsulfoniopropionate (DMSP) is a metabolite produced primarily by marine phytoplankton and is the main precursor to the climatically important gas dimethylsulfide (DMS). DMS is released upon bacterial catabolism of DMSP, but it is not the only possible fate of DMSP sulfur. An alternative demethylation/demethiolation pathway results in the eventual release of methanethiol, a highly reactive volatile sulfur compound that contributes little to the atmospheric sulfur flux. The activity of these pathways control the natural flux of sulfur released to the atmosphere. Although these biochemical pathways and the factors that regulate them are of great interest, they are poorly understood. Only recently have some of the genes and pathways responsible for DMSP catabolism been elucidated. Thus far, six different enzymes have been identified that catalyze the cleavage of DMSP, resulting in the release of DMS. In addition, five of these enzymes appear to produce acrylate, while one produces 3-hydroxypropionate. In contrast, only one enzyme, designated DmdA, has been identified that catalyzes the demethylation reaction producing methylmercaptopropionate (MMPA). The metabolism of MMPA is performed by a series of three coenzyme-A mediated reactions catalyzed by DmdB, DmdC, and DmdD. Interestingly, CandidatusPelagibacter ubique, a member of the SAR11 clade of Alphaproteobacteria that is highly abundant in marine surface waters, possessed functional DmdA, DmdB, and DmdC enzymes. Microbially mediated transformations of both DMS and methanethiol are also possible, although many of the biochemical and molecular genetic details are still unknown. This review will focus on the recent discoveries in the biochemical pathways that mineralize and assimilate DMSP carbon and sulfur, as well as the areas for which a comprehensive understanding is still lacking.

Plant phenylacetaldehyde synthase is a bifunctional homotetrameric enzyme that catalyzes phenylalanine decarboxylation and oxidation

We have isolated and characterized Petunia hybrida cv. Mitchell phenylacetaldehyde synthase (PAAS), which catalyzes the formation of phenylacetaldehyde, a constituent of floral scent. PAAS is a cytosolic homotetrameric enzyme that belongs to group II pyridoxal 5'-phosphate-dependent amino-acid decarboxylases and shares extensive amino acid identity (approximately 65%) with plant L-tyrosine/3,4-dihydroxy-L-phenylalanine and L-tryptophan decarboxylases. It displays a strict specificity for phenylalanine with an apparent Km of 1.2 mM. PAAS is a bifunctional enzyme that catalyzes the unprecedented efficient coupling of phenylalanine decarboxylation to oxidation, generating phenylacetaldehyde, CO2, ammonia, and hydrogen peroxide in stoichiometric amounts.

Environmental VOSCs--formation and degradation of dimethyl sulfide, methanethiol and related materials

Volatile organic sulfur compounds (VOSCs) play a major role in the global sulfur cycle. Two components, dimethyl sulfide (DMS) and methanethiol (MT) are formed in large amounts by living systems (e.g. algae, bacteria, plants), particularly in marine environments. A major route to DMS is by action of a lyase enzyme on dimethylsulfoniopropionate (DMSP). DMSP has other roles, for instance as an osmoprotectant and cryoprotectant. Demethiolation of DMSP and other materials leads to MT. A major transport process is release of DMS from the oceans to the atmosphere. Oxidation of DMS in the atmosphere by hydroxyl and nitrate radicals produces many degradation products including CO2, COS, dimethyl sulfoxide, dimethyl sulfone, organic oxyacids of sulfur, and sulfate. These materials also have roles in biotic processes and there are complex metabolic interrelationships between some of them. This review emphasizes the chemical reactions of the organic sulfur cycle. For biotic reactions, details of relevant enzymes are provided when possible.

Plants synthesize ethanolamine by direct decarboxylation of serine using a pyridoxal phosphate enzyme

The established pathways from serine to ethanolamine are indirect and involve decarboxylation of phosphatidylserine. Here we show that plants can decarboxylate serine directly. Using a radioassay based on ethanolamine (Etn) formation, pyridoxal 5'-phosphate-dependent l-serine decarboxylase (SDC) activity was readily detected in soluble extracts from leaves of diverse species, including spinach, Arabidopsis, and rapeseed. A putative Arabidopsis SDC cDNA was identified by searching GenBank for sequences homologous to other amino acid decarboxylases and shown by expression in Escherichia coli to encode a soluble protein with SDC activity. This cDNA was further authenticated by complementing the Etn requirement of a yeast psd1 psd2 mutant. In a parallel approach, a cDNA was isolated from a rapeseed library by its ability to complement the Etn requirement of a yeast cho1 mutant and shown by expression in E. coli to specify SDC. The deduced Arabidopsis and rapeseed SDC polypeptides are 90% identical, lack obvious targeting signals, and belong to amino acid decarboxylase group II. Recombinant Arabidopsis SDC was shown to exist as a tetramer and to contain pyridoxal 5'-phosphate. It does not attack d-serine, l-phosphoserine, other l-amino acids, or phosphatidylserine and is not inhibited by Etn, choline, or their phosphoesters. As a soluble, pyridoxal 5'-phosphate enzyme, SDC contrasts sharply with phosphatidylserine decarboxylases, which are membrane proteins that have a pyruvoyl cofactor.

MACRONUTRIENT UTILIZATION BY PHOTOSYNTHETIC EUKARYOTES AND THE FABRIC OF INTERACTIONS

Organisms acclimate to a continually fluctuating nutrient environment. Acclimation involves responses specific for the limiting nutrient as well as responses that are more general and occur when an organism experiences different stress conditions. Specific responses enable organisms to efficiently scavenge the limiting nutrient and may involve the induction of high-affinity transport systems and the synthesis of hydrolytic enzymes that facilitate the release of the nutrient from extracellular organic molecules or from internal reserves. General responses include changes in cell division rates and global alterations in metabolic activities. In photosynthetic organisms there must be precise regulation of photosynthetic activity since when severe nutrient limitation prevents continued cell growth, excitation of photosynthetic pigments could result in the formation of reactive oxygen species, which can severely damage structural and functional features of the cell. This review focuses on ways that photosynthetic eukaryotes assimilate the macronutrients nitrogen, sulfur, and phosphorus, and the mechanisms that govern assimilatory activities. Also discussed are molecular responses to macronutrient limitation and the elicitation of those responses through integration of environmental and cellular cues.