Biosynthesis and dynamics of lipids in Plasmodium-infected mature mammalian erythrocytes

The asexual development of Plasmodium within the mature mammalian erythrocyte is associated with intense membrane biogenesis, notably to ensure the increase in the size of the parasite and of the parasitophorous vacuolar membranes PVM. A considerable increase in the content of most lipids except cholesterol [namely, phospholipids PL, neutral lipids, and fatty acids FA] occurs. The PL composition and the constitutive FAs of the parasite differ markedly from the original host cell membrane. Particularly notable is the absence of cholesterol and sphingomyelin SM from the parasite membranes. How can the parasite obtain such a quantity of new lipid molecules in a host cell totally devoid of any lipid biosynthetic activity? Like the normal erythrocyte, the infected cell is unable to synthesize cholesterol or FAs. In contrast, it exhibits an intense biosynthesis of neutral lipids and a bewildering variety of PL biosyntheses. Phosphatidylcholine PC is synthesized by a de novo pathway, and also by methylation of phosphatidylethanolamine PE, which itself originates from de novo biosynthesis or from decarboxylation of phosphatidylserine PS. Hence, interference with this intense and specific PL metabolism could provide the basis for a new malaria chemotherapy. Indeed, compounds that interfere with the entry of the plasmatic precursors (FAs or polar heads) or with their metabolism are lethal to the parasite. Lastly, we focus on the structural modifications of the host cell membrane with respect to lipids, including increased fluidity and enhanced transbilayer mobility of PLs. Possible modifications in the asymmetric distribution of PLs in the host cell membrane are discussed in light of the various methods used and their limits. The capacity of infected cells to take up and metabolize large quantities of exogenous vesicles of PLs accounts for the intense dynamics of lipids in the infected erythrocytes.     

A malaria parasite-encoded vacuolar H(+)-ATPase is targeted to the host erythrocyte

The asexual development of malaria parasites inside the erythrocyte is accompanied by changes in the composition, structure, and function of the host cell membrane and cytoplasm. The parasite exports a membrane network into the host cytoplasm and several proteins that are inserted into the erythrocyte membrane, although none of these proteins has been shown to have enzymatic activity. We report here that a functional malaria parasite-encoded vacuolar (V)-H(+)-ATPase is exported to the erythrocyte and localized in membranous structures and in the plasma membrane of the infected erythrocyte. This localization was determined by separation of parasite and erythrocyte membranes and determination of enzyme marker activities and by immunofluorescence microscopy assays using antibodies against the B subunit of the malarial V-H(+)-ATPase and erythrocyte (spectrins) and parasite (merozoite surface protein 1) markers. Our results suggest that this pump has a role in the maintenance of the intracellular pH (pH(i)) of the infected erythrocyte. Our results also indicate that although the pH(i) maintained by the V-H(+)-ATPase is important for maximum uptake of small metabolites at equilibrium, it does not appear to affect transport across the erythrocyte membrane and is, therefore, not involved in the previously described phenomenon of increased permeability of infected erythrocytes that is sensitive to chloride channel inhibitors (new permeation pathway). This constitutes the first report of the presence of a functional enzyme of parasite origin in the plasma membrane of its host.     

Altered membrane permeability: a new approach to malaria chemotherapy

During its development: in the host erythrocyte, the malarial parasite causes profound alterations in the permeability of the host cell membrane. Nucleoside transport pathways, which are induced by the parasite in the host erythrocyte membrane, have properties significantly different from those of the host cell. Here, Annette Gero and Joanne Upston review the current knowledge o f the parasite-induced transporters and show that they can be used to selectively direct cytotoxic compounds into the parasite-infected cell, thereby indicating their chemotherapeutic potential.     

Phospholipid organization in monkey erythrocytes upon Plasmodium knowlesi infection

The phospholipid organization in monkey erythrocytes upon Plasmodium knowlesi infection has been studied. Parasitized and nonparasitized erythrocytes from malaria-infected blood were separated and pure erythrocyte membranes from parasitized cells were isolated using Affi-Gel beads. In this way, the phospholipid content and composition of the membrane of nonparasitized cells, the erythrocyte membrane of parasitized cells and the parasite could be determined. The phospholipid content and composition of the erythrocyte membranes of nonparasitized and parasitized cells and erythrocytes from chloroquine-treated monkeys cured from malaria, were the same as in normal erythrocytes. The phospholipid content of the parasite increased during its development, but its composition remained unchanged. Three independent techniques, i.e., treatment of intact cells with phospholipase A2 and sphingomyelinase C, fluorescamine labeling of aminophospholipids and a phosphatidylcholine-transfer protein-mediated exchange procedure have been applied to assess the disposition of phospholipids in: erythrocytes from healthy monkeys, nonparasitized and parasitized erythrocytes from monkeys infected with Plasmodium knowlesi, and erythrocytes from monkeys that had been cured from malaria by chloroquine treatment. The results obtained by these experiments do not show any abnormality in phospholipid asymmetry in the erythrocyte from malaria-infected (splenectomized) monkeys, neither in the nonparasitized cells, nor in the parasitized cells at any stage of parasite development. Nevertheless, a considerable degree of lipid bilayer destabilization in the membrane of the parasitized cells is apparent from the enhanced exchangeability of the PC from those cells, as well as from their increased permeability towards fluorescamine.     

Vesicle-mediated trafficking of parasite proteins to the host cell cytosol and erythrocyte surface membrane in Plasmodium falciparum infected erythrocytes

During the development of the asexual stage of the malaria parasite, Plasmodium falciparum, the composition, structure and function of the host cell membrane is dramatically altered, including the ability to adhere to vascular endothelium. Crucial to these changes is the transport of parasite proteins, which become associated with or inserted into the erythrocyte membrane. Protein and membrane targeting beyond the parasite plasma membrane must require unique pathways, given the parasites intracellular location within a parasitophorous vacuolar membrane and the lack of organelles and biosynthetic machinery in the host cell necessary to support a secretory system. It is not clear how these proteins cross the parasitophorous vacuolar membrane or how they traverse the erythrocyte cytosol to reach their final destinations. The identification of: (1) a P. falciparum homologue of the protein Sar1p, which is an essential component of the COPII-based secretory system in mammalian cells and yeast and (2) electron-dense, possibly coated, secretory vesicles bearing P. falciparum erythrocyte membrane protein 1 and P. falciparum erythrocyte membrane protein 3 in the host cell cytosol of P. falciparum infected erythrocytes recently provided the first direct evidence of a vesicle-mediated pathway for the trafficking of some parasite proteins to the erythrocyte membrane. The major advance in uncovering the parasite-induced secretory pathway was made by incubating infected erythrocytes with aluminium tetrafluoride, an activator of guanidine triphosphate-binding proteins, which resulted in the accumulation of the vesicles into multiple vesicle strings. These vesicle complexes were often associated with and closely abutted the erythrocyte membrane, but were apparently prevented from fusing by the aluminium fluoride treatment, making their capture by electron microscopy possible. It appears that malaria parasites export proteins into the host cell cytosol to support a vesicle-mediated protein trafficking pathway.     

Transport of fluorescent phospholipid analogues from the erythrocyte membrane to the parasite in Plasmodium falciparum-infected cells

The asexual development of the human malaria parasite Plasmodium falciparum is largely intraerythrocytic. When 1-palmitoyl-2-[6-[(7-nitro-2-1,3-benzoxadiazole-4-yl)amino]caproyl] phosphatidylcholine (NBD-PC) was incorporated into infected and uninfected erythrocyte membranes at 0 degrees C, it remained at the cell surface. At 10 degrees C, the lipid was rapidly internalized in infected erythrocytes at all stages of parasite growth. Our results indicate that the internalization of NDB-PC was not because of endocytosis but rapid transbilayer lipid flip-flop at the infected erythrocyte membrane, followed by monomer diffusion to the parasite. Internalization of the lipid was inhibited by (a) depleting cellular ATP levels; (b) pretreating the cells with N-ethyl maleimide or diethylpyrocarbonate; and (c) 10 mM L-alpha-glycerophosphorylcholine. The evidence suggests protein-mediated and energy dependent transmembrane movement of the PC analogue. The conditions for the internalization of another phospholipid analogue N-4-nitrobenzo-2-oxa-1,3-diazoledipalmitoyl phosphatidylethanolamine (N-NBD-PE) were distinct from that of NBD-PC and suggest the presence of additional mechanism(s) of parasite-mediated lipid transport in the infected host membrane. In spite of the lack of bulk, constitutive endocytosis at the red cell membrane, the uptake of Lucifer yellow by mature infected cells suggests that microdomains of pinocytotic activity are induced by the intracellular parasite. The results indicate the presence of parasite-induced mechanisms of lipid transport in infected erythrocyte membranes that modify host membrane properties and may have important implications on phospholipid asymmetry in these membranes.     

Lipid peroxidation in Plasmodium falciparum-parasitized human erythrocytes.

cis-Parinaric acid (PnA) was used as a fluorescent probe to study lipid peroxidation in nonparasitized and Plasmodium falciparum-parasitized erythrocytes, upon challenge by cumene hydroperoxide and tert-butyl hydroperoxide. Parasitized erythrocytes were less susceptible toward lipid peroxidation than nonparasitized erythrocytes with which they had been cultured. Furthermore, nonparasitized erythrocytes cultured together with parasitized cells, and thereafter isolated on a Percoll gradient, were less susceptible toward lipid peroxidation than erythrocytes kept under the same experimental conditions but in the absence of parasitized cells. We concluded, therefore, that the intracellular development of the parasite leads to an increase in the resistance against oxidative stress, not only of the host cell membrane of the parasitized erythrocyte, but also in the plasma membrane of the neighboring cells. The erythrocyte cytosol of parasitized cells and/or the intraerythrocytic parasite was required for the increased protection of the host cell membrane, since ghosts prepared from parasitized erythrocytes were more susceptible to lipid peroxidation than those prepared from nonparasitized ones. Vitamin E content of parasitized erythrocytes was lower than that of nonparasitized cells. However, parasitized erythrocytes promoted extracellular reduction of ferricyanide at higher rates, which might be indicative of a larger cytosolic reductive capacity. It is suggested that the improved response of intact erythrocytes is due to an increased reduction potential of the host-erythrocyte cytosol. The role of vitamin C as a mediator of this process is discussed.     

Sorting of malarial antigens into vesicular compartments within the host cell cytoplasm as demonstrated by immunoelectron microscopy

A double and triple immunogold labeling technique has been applied to demonstrate that several malarial antigens of the erythrocytic stages of Plasmodium falciparum are exported from the parasite into distinct compartments within the host cell cytoplasm. Multiple species of vesicles, each with specifically packaged contents, are consistent with a sorting function of vesicular structures in the Plasmodium infected erythrocyte. During schizogony, two parasite antigens, an S-antigen and a parasitophorous vacuole membrane antigen, QF 116, become packaged into such vesicles and are transported into the erythrocyte cytoplasm. At this stage of parasite development, host cell material is taken in through the parasitophorous vacuole membrane into the vacuolar space surrounding the parasite.     

Trafficking of malarial proteins to the host cell cytoplasm and erythrocyte surface membrane involves multiple pathways

During the asexual stage of malaria infection, the intracellular parasite exports membranes into the erythrocyte cytoplasm and lipids and proteins to the host cell membrane, essentially "transforming" the erythrocyte. To investigate lipid and protein trafficking pathways within Plasmodium falciparum-infected erythrocytes, synchronous cultures are temporally analyzed by confocal fluorescence imaging microscopy for the production, location and morphology of exported membranes (vesicles) and parasite proteins. Highly mobile vesicles are observed as early as 4 h postinvasion in the erythrocyte cytoplasm of infected erythrocytes incubated in vitro with C6-NBD-labeled phospholipids. These vesicles are most prevalent in the trophozoite stage. An immunofluorescence technique is developed to simultaneously determine the morphology and distribution of the fluorescent membranes and a number of parasite proteins within a single parasitized erythrocyte. Parasite proteins are visualized with FITC- or Texas red-labeled monoclonal antibodies. Double-label immunofluorescence reveals that of the five parasite antigens examined, only one was predominantly associated with membranes in the erythrocyte cytoplasm. Two other parasite antigens localized only in part to these vesicles, with the majority of the exported antigens present in lipid-free aggregates in the host cell cytoplasm. Another parasite antigen transported into the erythrocyte cytoplasm is localized exclusively in lipid-free aggregates. A parasite plasma membrane (PPM) and/or parasitophorous vacuolar membrane (PVM) antigen which is not exported always colocalizes with fluorescent lipids in the PPM/PVM. Visualization of two parasite proteins simultaneously using FITC- and Texas red-labeled 2 degrees antibodies reveals that some parasite proteins are constitutively transported in the same vesicles, whereas other are segregated before export. Of the four exported antigens, only one appears to cross the barriers of the PPM and PVM through membrane-mediated events, whereas the others are exported across the PPM/PVM to the host cell cytoplasm and surface membrane through lipid (vesicle)-independent pathways.     

Soft X-ray microscopy analysis of cell volume and hemoglobin content in erythrocytes infected with asexual and sexual stages of Plasmodium falciparum

Plasmodium falciparum, the most virulent agent of human malaria, undergoes both asexual cycling and sexual differentiation inside erythrocytes. As the intraerythrocytic parasite develops it increases in size and alters the permeability of the host cell plasma membrane. An intriguing question is: how is the integrity of the host erythrocyte maintained during the intraerythrocytic cycle? We have used water window cryo X-ray tomography to determine cell morphology and hemoglobin content at different stages of asexual and sexual differentiation. The cryo stabilization preserves native structure permitting accurate analyses of parasite and host cell volumes. Absorption of soft X-rays by protein adheres to Beer–Lambert’s law permitting quantitation of the concentration of hemoglobin in the host cell compartment. During asexual development the volume of the parasite reaches about 50% of the uninfected erythrocyte volume but the infected erythrocyte volume remains relatively constant. The total hemoglobin content gradually decreases during the 48 h cycle but its concentration remains constant until early trophozoite stage, decreases by 25%, then remains constant again until just prior to rupture. During early sexual development the gametocyte has a similar morphology to a trophozoite but then undergoes a dramatic shape change. Our cryo X-ray tomography analysis reveals that about 70% of the host cell hemoglobin is taken up and digested during gametocyte development and the parasite eventually occupies about 50% of the uninfected erythrocyte volume. The total volume of the infected erythrocyte remains constant, apart from some reversible shrinkage at stage IV, while the concentration of hemoglobin decreases to about 70% of that in an uninfected erythrocyte.     

A Plasmodium chabaudi chabaudi high molecular mass glycoprotein translocated to the host cell membrane by a non-classical secretory pathway

We have purified and characterized a novel high molecular mass glycoprotein of P. chabaudi chabaudi (Pc550gp) that is transported to the erythrocyte membrane during the intraerythrocytic cycle. Immuno fluorescence assays with polyclonal monospecific antibodies against Pc550gp show that the protein to be localized in the periphery of young trophozoite stages i.e., on the plasma membrane or parasitophorous vacuole membrane. However, in late trophozoites and schizonts the antigen is distributed in both parasite and host cell membranes. These results were confirmed by immunoblotting of isolated parasites and infected host cell membranes at different stages of parasite development. Moreover, alkali extraction of purified infected erythrocyte membranes at mature stages of parasite development does not solubilize Pc550gp, suggesting that it is an integral membrane protein. In addition proteinase K digestion of intact infected host cells induced the disappearance of Pc550gp. Further indicating its transmembrane nature and that it presents extracellular domains susceptible to proteolysis. Brefeldin A or low temperature (15 degrees C) treatment did not affect the translocation of Pc550gp from the parasite compartments to the erythrocyte membrane, indicating that the secretion of Pc550gp does not follow the classical transport pathway described in most eukaryotic cells.     

The Maurer's cleft protein MAHRP1 is essential for trafficking of PfEMP1 to the surface of Plasmodium falciparum-infected erythrocytes

During the intra-erythrocytic development of Plasmodium falciparum, the parasite modifies the host cell surface by exporting proteins that interact with or insert into the erythrocyte membrane. These proteins include the principal mediator of cytoadherence, P. falciparum erythrocyte membrane protein 1 (PfEMP1). To implement these changes, the parasite establishes a protein-trafficking system beyond its confines. Membrane-bound structures called Maurer's clefts are intermediate trafficking compartments for proteins destined for the host cell membrane. We disrupted the gene for the membrane-associated histidine-rich protein 1 (MAHRP1). MAHRP1 is not essential for parasite viability or Maurer's cleft formation; however, in its absence, these organelles become disorganized in permeabilized cells. Maurer's cleft-resident proteins and transit cargo are exported normally in the absence of MAHRP1; however, the virulence determinant, PfEMP1, accumulates within the parasite, is depleted from the Maurer's clefts and is not presented at the red blood cell surface. Complementation of the mutant parasites with mahrp1 led to the reappearance of PfEMP1 on the infected red blood cell surface, and binding studies show that PfEMP1-mediated binding to CD36 is restored. These data suggest an important role of MAHRP1 in the translocation of PfEMP1 from the parasite to the host cell membrane.     

Plasmodium lipid rafts contain proteins implicated in vesicular trafficking and signalling as well as members of the PIR superfamily, potentially implicated in host immune system interactions

Plasmodium parasites, the causal agents of malaria, dramatically modify the infected erythrocyte by exporting parasite proteins into one or multiple erythrocyte compartments, the cytoplasm and the plasma membrane or beyond. Despite advances in defining signals and specific cellular compartments implicated in protein trafficking in Plasmodium-infected erythrocytes, the contribution of lipid-mediated sorting to this cellular process has been poorly investigated. In this study, we examined the proteome of cholesterol-rich membrane microdomains or lipid rafts, purified from erythrocytes infected by the rodent parasite Plasmodium berghei. Besides structural proteins associated with invasive forms, we detected chaperones, proteins implicated in vesicular trafficking, membrane fusion events and signalling. Interestingly, the raft proteome of mixed P. berghei blood stages included proteins encoded by members of a large family (bir) of putative variant antigens potentially implicated in host immune system interactions and targeted to the surface of the host erythrocytes. The generation of transgenic parasites expressing BIR/GFP fusions confirmed the dynamic association of members of this protein family with membrane microdomains. Our results indicated that lipid rafts in Plasmodium-infected erythrocytes might constitute a route to sort and fold parasite proteins directed to various host cell compartments including the cell surface.     

An erythrocyte vesicle protein exported by the malaria parasite promotes tubovesicular lipid import from the host cell surface

Plasmodium falciparum is the protozoan parasite that causes the most virulent of human malarias. The blood stage parasites export several hundred proteins into their host erythrocyte that underlie modifications linked to major pathologies of the disease and parasite survival in the blood. Unfortunately, most are 'hypothetical' proteins of unknown function, and those that are essential for parasitization of the erythrocyte cannot be 'knocked out'. Here, we combined bioinformatics and genome-wide expression analyses with a new series of transgenic and cellular assays to show for the first time in malaria parasites that microarray read out from a chemical perturbation can have predictive value. We thereby identified and characterized an exported P. falciparum protein resident in a new vesicular compartment induced by the parasite in the erythrocyte. This protein, named Erythrocyte Vesicle Protein 1 (EVP1), shows novel dynamics of distribution in the parasite and intraerythrocytic membranes. Evidence is presented that its expression results in a change in TVN-mediated lipid import at the host membrane and that it is required for intracellular parasite growth, but not invasion. This exported protein appears to be needed for the maintenance of an essential tubovesicular nutrient import pathway induced by the pathogen in the host cell. Our approach may be generalized to the analysis of hundreds of 'hypothetical' P. falciparum proteins to understand their role in parasite entry and/or growth in erythrocytes as well as phenotypic contributions to either antigen export or tubovesicular import. By functionally validating these unknowns, one may identify new targets in host-microbial interactions for prophylaxis against this major human pathogen.     

Protein phosphatase 1, a Plasmodium falciparum essential enzyme, is exported to the host cell and implicated in the release of infectious merozoites

The malarial parasite Plasmodium falciparum transposes a Golgi-like compartment, referred to as Maurer's clefts, into the cytoplasm of its host cell, the erythrocyte, and delivering parasite molecules to the host cell surface. We report here a novel role of the Maurer's clefts implicating a parasite protein phosphatase 1 (PP1) and related to the phosphorylation status of P. falciparum skeleton-binding protein 1 (PfSBP1), a trans-membrane protein of the clefts interacting with the host cell membrane via its carboxy-terminal domain. Based on co-immunoprecipitation and inhibition studies, we show that the parasite PP1 type phosphatase modulates the phosphorylation status of the amino-terminal domain of PfSBP1 in the lumen of Maurer's clefts. Importantly, the addition of a PP1 inhibitor, calyculin A, to late schizonts results in the hyperphosphorylation of PfSBP1 and prevents parasite release from the host cell. We propose that the hyperphosphorylation of PfSBP1 interferes with the release of merozoites, the invasive blood stage of the parasite, by increasing the red cell membrane stability. Moreover, the parasite PP1 phosphatase is the first enzyme essential for the parasite development detected in the Maurer's clefts.     

Contribution of host lipids to Toxoplasma pathogenesis

As an actively dividing organism, the intracellular parasite Toxoplasma gondii must adjust the size and composition of its membranes in order to accommodate changes due to housekeeping activities, to commit division and in fine to produce new viable progenies. Lipid inventory of T. gondii reveals that the biological membranes of this parasite are composed of a complex mixture of neutral and polar lipids. After examination of the origin of T. gondii membrane lipids, three categories of lipids can be described: (i) lipids scavenged by T. gondii from the host cell; (ii) lipids synthesized in large amounts by the parasite, independently from its host cell; and (iii) lipids produced de novo by the parasite, but whose synthesis does not come close to satisfying the entire parasite's needs. These latter must be adeptly acquired from the host environment. To this end, T. gondii diverts a large variety of lipid precursors from host cytoplasm and efficiently manufacture them into complex lipids. This rather remarkable reliance on host lipid resources for parasite survival opens new avenues to restrict parasite growth. Indeed, parasite starvation can be induced upon deprivation from essential host lipids. Lipid analogues with anti-proliferative properties are voraciously taken up by the parasites, which results in parasite membrane defects, and ultimately death.     

Protein transport across the parasitophorous vacuole of Plasmodium falciparum: into the great wide open

The human malaria parasite Plasmodium falciparum resides and multiplies within a membrane-bound vacuole in the cytosol of its host cell, the mature human erythrocyte. To enable the parasite to complete its intraerythrocytic life cycle, a large number of parasite proteins are synthesized and transported from the parasite to the infected cell. To gain access to the erythrocyte, parasite proteins must first cross the membrane of the parasitophorous vacuole (PVM), a process that is not well understood at the mechanistic level. Here, we review past and current literature on this topic, and make tentative predictions about the nature of the transport machinery required for transport of proteins across the PVM, and the molecular factors involved.     

Secretion of Plasmodium falciparum rhoptry protein into the plasma membrane of host erythrocytes

The rhoptry is an organelle of the malarial merozoite which has been suggested to play a role in parasite invasion of its host cell, the erythrocyte. A monoclonal antibody selected for reactivity with this organelle identifies a parasite synthesized protein of 110 kD. From biosynthetic labeling experiments it was demonstrated that the protein is synthesized midway through the erythrocytic cycle (the trophozoite stage) but immunofluorescence indicates the protein is not localized in the organelle until the final stage (segmenter stage) of intraerythrocytic development. Immunoelectron microscopy shows that the protein is localized in the matrix of the rhoptry organelle and on membranous whorls secreted from the merozoite. mAb recognition of the protein is dithiothreitol (DTT) labile, indicating that the conformation of the epitope is dependent on a disulfide linkage. During erythrocyte reinvasion by the extracellular merozoite, immunofluorescence shows the rhoptry protein discharging from the merozoite and spreading around the surface of the erythrocyte. The protein is located in the plasma membrane of the newly invaded erythrocyte. These studies suggest that the 110-kD rhoptry protein is inserted into the membrane of the host erythrocyte during merozoite invasion.     

Export of proteins via a novel secretory pathway.

The intraerythrocytic location of the malaria parasite necessitates modification of the host cell. These alterations are mediated either directly or indirectly by parasite proteins exported to specific compartments within the host cell. However, little is known about how the parasite specifically targets proteins to locations beyond its plasma membrane. Mark Wiser, Norbert Lanners and Richard Bafford here propose an alternative secretory pathway for the export of parasite proteins into the host erythrocyte. The first step of this pathway is probably an endoplasmic reticulum (ER)-like organelle that is distinct from the normal ER. Possible mechanisms of protein trafficking in the infected erythrocyte are also discussed. The proposed ER-like organelle and alternative secretory pathway raise many questions about the cell biology of protein export and trafficking in Plasmodium.     

Lipid Class and Fatty Acid Composition of the Protozoan Parasite of Oysters, Perkinsus marinus Cultivated in Two Different Media

The meront stage of the oyster protozoan parasite, Perkinsus marinus, cultivated in two media with different fatty acid profiles was analyzed for its fatty acid and lipid class composition. The composition of fatty acids in the prezoosporangium stage of the parasite as well as that of the host oyster were investigated. Although the lipid class composition of meronts was dominated by phospholipids and triacylglycerol, there was no triaclgycerol detected in either culture medium. Despite the difference in fatty acid composition of the two media, the fatty acid composition of meronts in each medium was dominated by 14:0, 16:0, 18:0, 18:1(n-9), 20: (n-9), 18:2(n-6) and 20:4(n-6), a profile that differed from its host. The quantities of total lipids and fatty acids in meronts increased as the number of meronts increased and far exceeded the initial amounts in the media and in the initial cell inoculum. The meronts harvested 25 d post-inoculation, had about 3 to 6 times higher total lipids and 4 to 13 times higher fatty acids than the amounts contained in the media. The fatty acid profiles of both prezoosporangia and oysters resembled each other and consisted primarily of 16:0, 20:4(n-6), 20:5(n-3), 22:2delta7,15, and 22:6(n-3). These results indicate that during meront proliferation, the parasite synthesizes certain fatty acids and lipid classes. For development from meront to prezoosporangium, the parasite may rely on its host for lipid resources.