Introduction

Protists and Fungi comprise two unique kingdoms of life. Protists demonstrate a robust variance in traits complicating their taxonomy. Fungi are much simpler to characterize. Fungi diverged from Protists about 1.5 billion years ago [1] an event that entailed flagellum loss during the transition from an aquatic to terrestrial habitat with concomitant development of new non-aquatic based spore dispersal mechanisms [2]. Fungi have adapted a recognizable set of characteristics that help clarify their differences from Protists.

Cellular Differences

Protists are unicellular organisms. The majority of fungi are multicellular and structured in a branching elongated filamentous system of hyphae [1]. The branching hyphae structures consist of one or (usually) more cells enclosed within a tubular cell wall [1]. Most Protists are spherical in shape, which is sub-optimal for obtaining oxygen by diffusion. Large Protists have an elongated shape to accommodate their increased need for oxygen diffusion [3].

Cell Size

The unicellular protists are mostly microscopic but rare examples have been found thousands of square meters in area [3]. Fungi are commonly large enough to be observed by the naked eye but a large number of microscopic species exist [1].

Cell Membrane

Protists can contain plant-like cell walls, animal-like cell walls and even pellicles providing protection from the external environment [3]. Many Protists do not have a cell wall [3]. In contrast to Protist cell membrane variety, a defining characteristic of fungi is the ubiquitous presence of a chitinous cell wall [14].

Intracellular Organization

Fungi are comprised of convoluted system of hyphae compartmentalized by a partitioning system of septa [1]. Septa have not been found in any Protists [3]. Fungal septa divide hyphae into permeable compartments [1]. Perforation of the septa allow translocation of organelles including ribosomes, mitochondria and nuclei between cells [3]. Protist organelles exist in a non compartmentalized cytoplasm [3].

Cellular Appendages

Unlike the mostly stationary Fungi, protists are motile [1,3] and this motility differentiates Protists morphologically from fungi by the addition of cellular appendages. Protists frequently contain appendages like cilia, flagella and pseudopodia [3]. Fungi generally do not have cellular appendages though rare examples of conidial appendages in fungi do exist [4].

Respiration

Protist Respiration

i) Protist Aerobic Respiration

Protists obtain oxygen by diffusion and this limits their capacity for cellular growth [3]. Some Protists like the phytoflagellates carry out both autotrophic and oxidative heterotrophic metabolism [3]. Protist metabolism functions optimally through a wide range of temperatures and oxygen consumption quantities. This is a by-product of the plethora of niches they inhabit, which have a vast range of temperatures and oxygen availability [3].

ii) Protist Anaerobic Respiration

Obligatory anaerobic respiration exists among parasitic Protists, a rarity for eukaryotes [3]. Many obligate anaerobe Protists lack cytochrome oxidase resulting in atypical mitochondria [3].

iii) Fungal Respiration

Most Fungi respire aerobically by utilizing branched respiratory chains to transfer electrons from NADH to oxygen [5]. Fungal NADH dehydrogenases are used to catalyze oxidation of matrix NADH and are capable of doing so even in the presence of some inhibitors like rotenone [5]. Fungi also use alternative oxidases to respire in the presence of inhibitors for ubiquinol:cytochrome c oxidoreductase and cytochrome c oxidase [5]. Alternative oxidases likely enable effective pathogenicity in the presence of nitric oxide-based host defense mechanisms [5].

Osmoregulation

Protists that inhabit aqueous environment have an amplification of cellular structures not found in fungi. This amplification enables a higher degree of osmoregulation. Contractile vacuoles are Protist organelles that enable osmoregulation and prevent swelling and cell rupture [3]. Contractile vacuoles are surrounded by a system of tubules and vesicles collectively called the spongiome that assists in expulsion of the contractile vacuoles from the cell [3]. Contractile vacuoles are significantly less abundant in Fungi [1,3].

Mitochondrial Differences

Protist Mitochondrial Genomes

Unlike Fungi, protist mitochondrial (mt) genomes have retained a number of ancestral proto-mitochondrian genomic elements. This is evident by gene reduction in Fungi mtGenomes [6]. Protist mtGenomes range in size from the 6kb genome of Plasmodium falciparum to the 77kb genome of the choanoflagellate Monosiga brevicollis, a smaller range than Fungi [6]. The average Protist mtGenome size is 40kb significantly smaller than the average Fungal mitochondrial genome size [6].

Protist mtGenomes are compact, exon rich and often comprised of overlapping coding regions [6]. Non-coding intronic space accounts for less than 10% of total Protist mtGenome size [6]. A large portion of Protist mtDNA have no group I or group II introns [6]. A+T content is higher in Protist mtGenomes compared to Fungi [6].The gene content of Protist mtGenomes resembles plant mtGenomes more so than Fungal mtGenomes [6]. Unlike Fungi, Protist mtGenomes encode for both large and small subunit RNAs [6].

Fungal Mitochondrial Genomes

Fungi evolved from Protists and their divergence is characterized by gene reduction and intron addition [6]. Compared to the gene rich Protist mtGenomes, Fungal mtGenomes contain a plethora of intergenic regions comprised of non-coding repeats and introns that are mostly group I introns [7]. Variation in Fungal mtGenome size is mostly explained by intron regions rather than the gene based variance found in Protist mtGenomes [7]. Intergenic regions account for up to 5kb of length in Fungal mtGenomes [7].

Though Protist mtGenomes contain more genes, Fungal mtGenomes contain a significantly larger amount of tRNA coding genes [6,7]. Fungal mtGenome sizes span a larger range compared to Protist mtGenomes. The smallest known Fungal mtGenome is 19 kbp, found in Schizosaccharomyces pombe [6]. The largest known Fungal mtGenome is 100 kbp, found in Podospora anserina [6]. Unlike Protist mtGenomes, the gene content of Fungal mtDNA is relatively consistent across organisms [6].

Nutrient Sources & Nutrient Acquisition Strategies

Fungi Nutrient Acquisition

Fungi use mycelium, their collection of hyphae, to acquire and transport nutrients across the plasma membrane of their cells [2]. This process is highly dependent on the pH of the environment from which the nutrients are acquired [2]. Fungi are saprotrophs, acquiring their nutrients primarily from the dissolved organic matter of decomposing dead plants and animals [1]. Any required digestion of nutrients occurs extracellularly by the release of enzymes that break down nutrients into monomers to be ingested by facilitated diffusion [1]

Protist Nutrient Acquisition

Protists, by contrast, obtain their nutrients through a variety of strategies. An attempt to categorize Protist nutrient acquisition strategies defines six categories [3]:

Many of the aforementioned  strategies are mixotrophic. For example, the photo-autotrophic primary producers include marine based organisms that can employ varying levels of heterotrophy allowing nutrient acquisition that does not require energy input from sunlight when sunlight is unavailable [3].

Reproductive Differences

Protists and Fungi both include species that reproduce sexually and aesexually. Protists are unique in that they include organisms capable of both aesexual and sexual reproduction within the same lifetime [8]. The complexity of some Protist life-cycles results in stunning morphological variations within the organism’s lifetime which enables distinct methods of reproduction [8]. Reproductive-related morphological changes are not observed in Fungi to the extent they are in Protists.

Aesexual Reproductive Differences

Aesexual reproduction in Fungi occurs through the dispresement of spores emanating from fruit bodies found on the mycelium or through fragmentation of the mycelium or through budding [9]. Aesexual reproduction in Protists occurs through a variety of methods. Binary fission (single nuclear division) and multiple fission (multiple nuclear divisions) are two common aesexual reproductive methods among Protists [8]. Another Protist-specific reproductive strategy is Plasmotomy [8]. Plasmotomy occurs among multinucleated protists and entails cytoplasmic division without nuclear division [8].

Sexual Reproductive Differences

Sexual reproduction is more commonly implemented by Fungi [8,9]. It is also more complex than aesxual reproduction and thus requires a more detailed description to establish an understanding of how the process differs between Protists and Fungi.

Fungal Sexual Reproduction

During Fungal sexual reproduction the nuclear membrane and the nucleolus (usually) remains intact throughout the entire process [9]. Plasmogamy, karyogamy and meiosis comprise the three sequential stages of fungal sexual reproduction [9]. Plasmogamy entails protoplasmic fusion between the mating cells which brings the distinct haploid nuclei into the same cell [9]. The fusion of these haploid nuclei and formation of a diploid nucleus occurs in the karyogamy stage [9]. Near the end of karyogamy a zygote exists and meiosis proceeds by the formation of spindle fibers within the nucleus. This reestablishes the haploid state via diploid chromosome separation [9].

Fungal strategies for haploid nuclei interaction during sexual reproduction are more varied in Fungi compared to Protists. These strategies include gamete formation and release from gametangia (sex organs), gametangia interaction between two organisms and somatic hyphae interaction [9].

Protist Sexual Reproduction

Protist sexual reproduction strategies are nearly entirely dissimilar to those employed by Fungi. These strategies entail unique processes that differ as a result of the cellular structure, particularly cellular appendages available for contact with other Protists [8]. Gamete formation and release is a sexual reproductive method among the highly motile flagellated protists [8]. Conjugation is a method used by ciliated Protists which entails the fusion of gametic nuclei rather than the formation and release of independent gametes [8]. Autogamy, a process of self-fertilization that is still considered a form of sexual reproduction, produces homozygosity among the progeny of a self-fertilized parent cell [8].

Summarizing Table

As summarized above, the differences between Protists and Fungi are vast and can be observed at every level of structure and throughout all of their behavioral interactions with their environments. This review is merely a summary of differences. The references cited provide more in-depth explanations for those interested in learning more.

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