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| Version | Summary | Created by | Modification | Content Size | Created at | Operation |
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| 1 | Doris Ilicic | + 4998 word(s) | 4998 | 2022-02-10 07:08:28 | | | |
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Although aquatic and parasitic fungi have been well known for more than 100 years, they have only recently received increased awareness due to their key roles in microbial food webs and biogeochemical cycles. There is growing evidence indicating that fungi inhabit a wide range of marine habitats, from the deep sea all the way to surface waters, and recent advances in molecular tools, in particular metagenome approaches, reveal that their diversity is much greater and their ecological roles more important than previously considered. Parasitism constitutes one of the most widespread ecological interactions in nature, occurring in almost all environments. Despite that, the diversity of fungal parasites, their ecological functions, and, in particular their interactions with other microorganisms remain largely speculative, unexplored and are often missing from current theoretical concepts in marine ecology and biogeochemistry. In this review, we summarize and discuss recent research avenues on parasitic fungi and their ecological potential in marine ecosystems, e.g., the fungal shunt, and emphasize the need for further research.
Many species of heterotrophic stramenopiles such as representatives of the Oomycota (oomycetes) are common in the marine environment and known to infect several marine macroalgal species and planktonic diatoms [21]. In particular, oomycetes infecting marine phytoplankton comprise several marine representatives such as Lagenisma coscinodisci Drebes, which was reported as an endobiotic parasite of the centric diatom Coscinodiscus centralis Ehrenberg from the North Sea [111]. Furthermore, the endoparasitic, saprolegniaceous oomycete Ectrogella Zopf is a parasite in diatoms and, according to Sparrow [112], outbreaks of Ectrogella perforans Petersen may attain epidemic proportions in the marine pennate diatom Licmophora Agardh [113]. Other representatives can be found in Table 1. Oomycota, as other zoosporic fungi, reproduce asexually by means of flagellated zoospores that are produced in zoosporangia [21][113][114], and, as members of the ARM clade, they have been recognized as endoparasites [115]. Despite similarities in life cycles, it is important to highlight the fundamental difference between oomycetes and zoosporic fungi—oomycetes are stramenopiles, a heterotrophic sister group of, e.g., brown algae and diatoms [104]. Hence, oomycetes have predominantly cellulosic walls [113] in contrast to zoosporic fungi that are characterized by chitinaceous cell walls. Although oomycetes have frequently been reported in marine environments [7][21], not much is known about their biology and ecology. However, these parasitoids are known to play a significant role in breaking down blooms of their hosts, i.e., diatoms, and might also play similar roles in the marine food web as those of zoosporic true fungi [115].
| Parasite | Host | Literature | |
|---|---|---|---|
| Aphelida | Pseudaphelidium drebesii | Thalassiosira punctigera | [116] |
| Chytrids | Chytridium megastomum | Ceramium | [117] |
| Chytridium polysiphonae | Sphacelaria, Pyaiella, Centroceras | [66][104][117] | |
| Coenomyces sp | Cladophora | [66] | |
| Dinomyces arenysensis | Dinoflagellates | [99] | |
| Olpidium rostiferum | Cladophora, Pseudo-nitzschia, | [66][104][118] | |
| Rhizophydium | Nitzschia, Rhizosolenia, Chaetoceros | [119] | |
| Rhizophydium aestuarii | Codium fragile | [99] | |
| Rhizophydium globosum | - | [120] | |
| Rhizophydium littoreum | Codium, Cancer anthonyi, Bryopsis | [99][117][121] | |
| Thalassochytrium gracilariopsis | Gracilariopsis | [99][100] | |
| Microsporidia | Loma trichiuri | Trichiurus savala | [88] |
| Microsporidium aplysiae | Aplysia californica | [122] | |
| Microsporidium cerebralis | Salmo salar | [88] | |
| Nematocenator marisprofundi | Desmodora marci | [123] | |
| Nosema pariacanthi | Priacanthus boops | [88] | |
| Oogranate pervascens | Maculaura sp. | [88] | |
| Pleistophora finisterrensis | Micromesistius poutassou | [124] | |
| Pleistophora hippoglossoideos | Hippoglossoides limandoides | [88] | |
| Pleistophora littoralis | Blennius pholis | [125] | |
| Pleistophora senegalensis | Sparus aurata | [126] | |
| Sporanauta perivermis | Odontophora rectangula | [87] | |
| Thelohania butleri | Pandalus jordani | [127] | |
| Unikaryon legeri | Meiogymnophallus minutus | [128] | |
| Oomyocta | Cryothecomonas longipes | Thalassiosira, Pirsonia, Rhizosolenia | [129] |
| Diatomophthora drebesii = Olpidiopsis drebesii | Rhizosolenia imbricata | [115][130] | |
| Ectrogella eurychasmoides | Licmophora lyngbyei | [131] | |
| Ectrogella marina | Chlorodendron subsalsum | [103] | |
| Ectrogella perforans | Fragilaria, Licmophora, Podocystis, Striatella, Synedra, Thalassionema | [132] | |
| Eurychasma dicksonii | Ectocarpus, Feldmannia, Punctaria, Pylaiella, Stictyosiphon, Striaria | [23] | |
| Eurychasmidium tumefaciens | Ceramium | [117] | |
| Lagenisma coscinodisci[117] | Coscinodiscus | [111][133][134] | |
| Miracula helgolandica | Pseudo-nitzschia pungens | [130] | |
| Olpidiopsis porphyrae | Bangia, Porphyra | [117] | |
| Petersenia lobata | Aglaothamnion, Callithamnion, Ceramium, Gymnothamnion, Herposiphonia, Polysiphonia, Pylaiella, Seirospora, Spermothamnion | [117] | |
| Petersenia palmariae | Palmaria mollis[117] | [135] | |
| Petersenia pollagaster | Chondrus | [117] | |
| Pontisma antithamnionis | Antithamnion | [117] | |
| Pontisma feldmannii | Falkenbergia, Trailliella | [117] | |
| Pontisma lagenidioides | Ceramium, Chaetomorpha, Valoniopsis | [66] | |
| Pythium marinum | - | [117] | |
| Pythium porphyrae | Porphyra | [117] | |
| Sirolpidium andreei | Acrosiphonia, Ceramium, Ectocarpus, Spongomorpha | [117] | |
| Sirolpidium bryopsidis | Cladophora, Rhizoclonium | [117] | |
| Rozellomycota | Rozella marina | Chytridium polysiphoniae | [136] |
Numbers of studies addressing diversity and abundances of Chytridiomycota are increasing with the improvement of molecular tools, in particular metagenomics approaches. Yet, information on their ecological role remains scarce. Efforts to retrieve such information due to the limited availability of model systems in culture [39][93][147] have been mainly focused on freshwater systems with a few, more recent, marine studies. Studies using next-generation sequencing technologies have revealed the prevalence of Chytridiomycota in both arctic and temperate marine habitats. Masana et al. [148] reported that Chytridimycota accounted for more than 60% of the rDNA sequences sampled in six near-shore sites in Europe, and were the most abundant fungal group in Arctic and sub-Arctic coastal habitats [6][80][89]. Although there are large numbers of described species of parasitic chytrids [20], only a few parasitic chytrid species have been genome sequenced and their phylogenetic positions clarified: Rhizophydium littoreum [120], Thalassochytrium gracilariopsis and Chytridium polysiphoniae (=Algochytrops polysiphoniae) [15][104]. Gaps in the reference databases relating to taxonomic coverage and marker coverage and the general lack of high quality, long sequence data are some of the major constraints for Chytridiomycota taxonomy [149][150]. Since fungi are phylogenetically diverse, DNA metabarcoding studies typically use markers that vary depending on the taxonomic group of interest and the resolution desired [149]. To overcome these drawbacks, Heeger et al. [150] created a long-read (ca. 4500 bp) bioinformatics pipeline that results in rates of sequencing error and chimera detection that are comparable to typical short-read analyses. The approach thus enabled the use of three different rRNA gene reference databases, thereby providing significant improvements in taxonomic classification over any single gene marker approach. Nevertheless, it is difficult to determine species composition and function (e.g., parasitic or saprotrophic) by analyzing environmental DNA alone [15]. Culturing, single cell PCR methods and whole genome sequencing [39][151] will presumably improve the representation of chytrids in future sequence databases.
Besides their ubiquitous nature, the importance of parasites lies also in the ecological roles they hold. By regulating host populations and mediating interspecific competition between hosts and other species [152], they affect community structure [153], but can also alter biochemical cycles, change productivity, increase trophic chain length and number of links [52], and cause changes in the topology of the trophic network and functioning of the ecosystem [154]. Thus, fungal parasites, infecting phytoplankton as primary producers, change the flow of carbon (C) in aquatic ecosystems. This process, named “mycoloop” [155], proposes that fungal infections of phytoplankton hosts transfer once inaccessible organic matter (OM) from large, inedible hosts to zooplankton by producing zoospores [51][55][154]. Zoospores are rich in polyunsaturated fatty acids (PUFAs) and cholesterol [34][45][50][52][55][156], they have relatively low carbon to nutrient ratios [54][157] and synthesize sterols de novo [53], which makes them a nutrient rich food source for grazers such as zooplankton. Hence, the presence of chytrids may not only affect the quantity of food that is being transferred, but also its quality [54][55]. It has been suggested that fungal infections may also modulate algal-bacterial interactions [158]. By utilizing their phytoplankton hosts and causing massive cell lysis, fungi provide C substrates [110] in terms of dissolved organic matter (DOM) and therefore activate the microbial loop [38][159]. In contrast, Klawonn et al. [160] have recently demonstrated that this may not be the case since fungal parasites very efficiently utilize their hosts’ cellular contents and that C and N compounds are most efficiently transferred to attached sporangia, and the developed zoospores therein. Consequently, the overall photosynthetically active biomass is reduced, as is the phytoplankton-derived contribution to the dissolved organic carbon (DOC) pool. This process, where photosynthetic carbon is bypassed from the microbial loop to fungal parasites has been called “fungal shunt” and it promotes zooplankton-mediated over microbe-mediated remineralization [160]. Multiple lines of evidence suggest that fungi play important ecological roles in marine ecosystems, yet our knowledge is still scarce. Current research is mostly focused on freshwater habitats, and we can only speculate how these results apply for the fungal parasites inhabiting marine waters.
The biological carbon pump comprises phytoplankton cells, their consumers and the bacteria that assimilate their waste, which play a central role in the global carbon cycle by delivering carbon from the atmosphere to the deep sea, where it is concentrated and sequestered for centuries [161][162]. Photosynthetically active phytoplankton in the euphotic zone (0–200 m depth) transforms dissolved inorganic carbon (DIC) to organic carbon, both dissolved and particulate forms [162]. The dissolved fraction is mainly respired by bacteria and the remaining refractory matter (RDOM) sinks and is mixed into the deep sea via the so-called microbial carbon pump (MCP) [163]. Considering the high efficiency of fungal parasites in utilizing OM from their hosts through the fungal shunt [160], infections can lead to substantially lower the contribution of phytoplankton derived OM to the DOM pool, which would consequently decrease the efficiency of the microbial carbon pump. Nevertheless, the particulate fraction is more significant when it comes to carbon export to the deeper layers of the ocean [139]. It is known that phytoplankton releases transparent polymeric particles (TEP)—gel-like, sticky particles predominantly composed of acidic polysaccharides [164]. Due to their high abundance in seawater and their surface reactivity, TEP scavenge trace elements (e.g., Fe and Th) and are the key agents for increasing the coagulation efficiency of physical aggregation processes [165]. Thus, TEP have an important role in biogeochemical fluxes and consequently, the efficiency of the biological carbon pump [166]. Mass flocculation and subsequent sedimentation of phytoplankton, especially diatoms, as large, rapidly sinking aggregates occur oceanwide and represent a major global sink for carbon. It has been shown that these larger, recalcitrant phytoplankton cells serve as preferential hosts for fungal parasites such as chytrids [26][144][161]. Grossart et al. [39] introduced the term “mycoflux” which refers to any fungal interaction leading to aggregation or disintegration of organic matter. On the one hand, by taking up the nutrients from their hosts, chytrids may decrease the exudation of TEP and thus, indirectly affect aggregation processes leading to a decreased efficiency of carbon sequestration. On the other hand, fungal infections are often lethal and lead to fragmentation of large phytoplankton cells [36][45][167], making them more edible to zooplankton which then substantially contribute to carbon pump efficiency by excretion of fast-sinking fecal pellets. The overall transfer efficiency of the biological carbon pump is therefore determined by a combination of different factors: seasonality; composition of phytoplankton species; fragmentation of particles by zooplankton; solubilization of particles by microbes [168]; and presumably presence of fungal parasites on phytoplankton species [39][157] (Figure 1).

Figure 1. Fungal parasites are key components of the biological carbon pump. Fungal parasites take up phytoplankton-derived photosynthetic carbon (‘fungal shunt’) and thus lower the contribution to the DOM and POM pools. Through ‘mycoflux’, fungi control POM aggregation process, either decreasing (parasitic fungi), or increasing (saprotrophic fungi) the aggregation rate by promoting active aggregation via hyphae growth. Via fragmentation of large phytoplankton cells and redirecting carbon directly to zooplankton (‘mycoloop’) they can also indirectly modulate carbon sequestration via sinking zooplankton faeces.
As shown, fungal parasites significantly change the fate of photosynthetically derived carbon. Infecting large phytoplankton cells, they potentially circumvent the microbial carbon pump and consequently direct the carbon from phytoplankton to zooplankton, either via (1) mycoloop, mycoflux, and fungal shunt, or by (2) active fragmentation of their hosts. Thus, it is important to include fungal parasites into general biogeochemical concepts, e.g., the biological carbon pump and C and N cycling, since they significantly affect the efficiency of carbon sequestration, i.e., controlling the magnitude of the overall organic matter sinking flux. Our understanding of ocean biogeochemistry is of pivotal importance for climate and human-induced changes in food web dynamics, biogeochemical cycles and their feedbacks to future climate. Thus, incorporating fungal parasites into concepts and models is necessary for developing a more integrated view of the ocean carbon cycle, in particular the biological carbon pump efficiency, to better predicting and thus mitigating the negative effects of current global change.