Preferential expulsion of dividing algal cells as a mechanism for regulating algal-cnidarian symbiosis

A wide range of both intrinsic and environmental factors can influence the population dynamics of algae in symbiosis with marine cnidarians. The present study shows that loss of algae by expulsion from cnidarian hosts is one of the primary regulators of symbiont population density. Because there is a significant linear correlation between the rate of algal expulsion and the rate of algal division, factors that increase division rates (e.g., elevated temperature) also increase expulsion rates. Additionally, 3H-thymidine is taken up to a greater extent by algae destined to be expelled than by algae retained in the host cnidarians. Taken together, data for rates of expulsion, rates of division at different temperatures, and uptake of 3H-thymidine suggest that dividing algal cells are preferentially expelled from their hosts. The preferential expulsion of dividing cells may be a mechanism for regulation of algal population density, where the rate of expulsion of algae may be an inverse function of the ability of host cells to accommodate new algal daughter cells. This kind of regulation is present in some cnidarian species (e.g., Aiptasia pulchella, Pocillopora damicornis), but not in all (e.g., Montipora verrucosa, Porites compressa, and Fungia scutaria).     

Stimulatory effect of aerosil on algal growth

Unicellular green alga represents not only a convenient model for its biochemical and physiological studies but also a sensitive system to test the effects of various environmental factors. Algae cells of two strains, SA-3 strain (exsymbiotic from Paramecium bursaria) and Chlorella vulgaris c-27, were asynchronously cultured in the presence of 0.01% Aerosil A-300. Aerosil effects on algae were monitored at logarithmic and stationary phases of their growth by flow cytometry and microscopic counting of algal numbers. The growth patterns of algae were evaluated by their forward light scatter versus fluorescence of endogenous chlorophyll (FL3-height) signal distributions. Although aerosil itself did not cause any direct effects on algal morphology, it affected the growth patterns and the numbers of algae of both strains. Their growth patterns were remarkably altered in the late logarithmic phase cultures (6-day cultures). However, a significant increase of cell numbers was found in the stationary phase cultures (9- and 12-day cultures). While C. vulgaris c-27 demonstrated an increase of cell numbers by approximately 11% in the 9- and 12-day cultures, the amounts of SA-3 cells in the 9- and 12-days cultures were increased by 16% and 35%, respectively. Our study shows aerosil in its colloidal form stimulates proliferation of algae mainly via an acceleration of their life cycles. The stimulatory effect of silica on the growth of algae, the mechanism of which remains to be clarified, might have a practical (e.g., ecological) interest for regulation of algal expansion.     

Expulsion of Symbiotic Algae during Feeding by the Green Hydra – a Mechanism for Regulating Symbiont Density?

Background

Algal-cnidarian symbiosis is one of the main factors contributing to the success of cnidarians, and is crucial for the maintenance of coral reefs. While loss of the symbionts (such as in coral bleaching) may cause the death of the cnidarian host, over-proliferation of the algae may also harm the host. Thus, there is a need for the host to regulate the population density of its symbionts. In the green hydra, Chlorohydra viridissima, the density of symbiotic algae may be controlled through host modulation of the algal cell cycle. Alternatively, Chlorohydra may actively expel their endosymbionts, although this phenomenon has only been observed under experimentally contrived stress conditions.

Principal Findings

We show, using light and electron microscopy, that Chlorohydra actively expel endosymbiotic algal cells during predatory feeding on Artemia. This expulsion occurs as part of the apocrine mode of secretion from the endodermal digestive cells, but may also occur via an independent exocytotic mechanism.

Significance

Our results demonstrate, for the first time, active expulsion of endosymbiotic algae from cnidarians under natural conditions. We suggest this phenomenon may represent a mechanism whereby cnidarians can expel excess symbiotic algae when an alternative form of nutrition is available in the form of prey.     

A green Paramecium strain with abnormal growth of symbiotic algae

Some hundred cells of Chlorella-like green algae are naturally enclosed within the cytoplasm of a single cell of green paramecia (Paramecium bursaria). Therefore, P. bursaria serves as an experimental model for studying the nature of endo-symbiosis made up through chemical communication between the symbiotic partners. For studying the mechanism of symbiotic regulations, the materials showing successful symbiosis are widely used. Apart from such successful model materials, some models for symbiotic distortion would be of great interest in order to understand the nature of successful symbiosis. Here, we describe a case of unsuccessful symbiosis causing unregulated growth of algae inside the hosting ciliates. Recently, we have screened some cell lines, from the mass of P. bursaria cells survived after paraquat treatment. The resultant cell lines (designated as KMZ series) show novel and unusual morphological features with heavily darker green colour distinguishable from the original pale green-coloured paramecia. In this type of isolates, endo-symbiotic algae are restricted within one or two dense spherical structures located at the center of the host cells' cytoplasm. Interestingly, this isolate maintains the host cells' circadian mating response which is known as an alga-dependent behaviour in the host cells. In contrast, we discuss that KMZ lacks the host-dependent regulation of algal growth, thus the algal complex often over-grows obviously exceeding the original size of the normal hosting ciliates. Additionally, possible use of this isolate as a novel model for symbiotic cell-to-cell communication is discussed.     

Growth kinetics of algal populations exsymbiotic from Paramecium bursaria by flow cytometry measurements.

BACKGROUND: The ciliate Paramecium bursaria normally exists as a green paramecium system because each animal cell carries several hundred, unicellular, green, algal cells in its cytoplasm. One of the remarkable and poorly understood pecularities of this system is the steady state in the number of algae per protozoan cell. A major point in the study of mechanisms governing the persistence of symbiont numbers is adequate understanding of the algal life cycle. METHODS: Asynchronously growing cell populations of several algal strains (SA-1, SA-3, and SA-9) exsymbiotic from P. bursaria were characterized by flow cytometry. Algal endogenous chlorophyll and DNA contents were monitored to analyze cell growth kinetics at logarithmic and stationary culture phases. Cell sorting visualized the morphology of algae corresponding to the hyperhaploid (2C and 4C) DNA peaks. RESULTS: Cell-division cycle-dependent changes in chlorophyll and DNA content distributions were most dramatic in logarithmically growing algal populations (an increase in the number of S-phase cells and cells with more chlorophyll), which are thought to be associated with accelerated DNA and chlorophyll metabolism in log-phase algal cultures. Upon reaching the stationary phase of growth, algal populations distinctly showed, in addition to one haploid (1C) DNA peak, two hyperhaploid peaks (2C and 4C) corresponding mainly to cells with two and four nuclei, respectively. CONCLUSIONS: Growth characteristics of algae exsymbiotic from P. bursaria monitored by flow cytometry provide valuable information for the analysis of the algal life cycle, which is important for understanding the regulation mechanisms of symbiont numbers.     

Endosymbiosis in Hydra and the Evolution of Internal Defense System

Certain species of Chlorella have exploited an intracellularhabitat and occur naturally as cytobionts in Hydra viridissima.The algae evoke phagocytosis by Hydra digestive cells and aresequestered in individual phagosomes that migrate to the baseof the host cells and resist fusion with lysosomes. The abilityto resist digestion is closely correlated with release of extracellularcarbohydrate (maltose) by the algae. The established populationof algae grows at an average rate equal to or greater than thatof the host and a constant population density is maintained.The host regulates algal population density by expelling ordigesting excess algae, or by controlling algal cell division.The control mechanism is unknown but can be breached by additionof inorganic ions to the Hydra culture medium with the resultthat the algae overgrow the Hydra. The Hydra-Chlorella symbiosis is probably mutually beneficial,but conditions such as prolonged darkness (with or without feeding)can reduce the competitive fitness of the host since this conditionresults in heterotrophy by the algae at the expense of selectedhost substrates. The evolution of selective permeability toorganic substrates is a major feature of the successful colonizationof the intracellular habitat by symbiotic Chlorella.     

Expression of the nucleus-encoded chloroplast division genes and proteins regulated by the algal cell cycle

Chloroplasts have evolved from a cyanobacterial endosymbiont and their continuity has been maintained by chloroplast division, which is performed by the constriction of a ring-like division complex at the division site. It is believed that the synchronization of the endosymbiotic and host cell division events was a critical step in establishing a permanent endosymbiotic relationship, such as is commonly seen in existing algae. In the majority of algal species, chloroplasts divide once per specific period of the host cell division cycle. In order to understand both the regulation of the timing of chloroplast division in algal cells and how the system evolved, we examined the expression of chloroplast division genes and proteins in the cell cycle of algae containing chloroplasts of cyanobacterial primary endosymbiotic origin (glaucophyte, red, green, and streptophyte algae). The results show that the nucleus-encoded chloroplast division genes and proteins of both cyanobacterial and eukaryotic host origin are expressed specifically during the S phase, except for FtsZ in one graucophyte alga. In this glaucophyte alga, FtsZ is persistently expressed throughout the cell cycle, whereas the expression of the nucleus-encoded MinD and MinE as well as FtsZ ring formation are regulated by the phases of the cell cycle. In contrast to the nucleus-encoded division genes, it has been shown that the expression of chloroplast-encoded division genes is not regulated by the host cell cycle. The endosymbiotic gene transfer of minE and minD from the chloroplast to the nuclear genome occurred independently on multiple occasions in distinct lineages, whereas the expression of nucleus-encoded MIND and MINE is regulated by the cell cycle in all lineages examined in this study. These results suggest that the timing of chloroplast division in algal cell cycle is restricted by the cell cycle-regulated expression of some but not all of the chloroplast division genes. In addition, it is suggested that the regulation of each division-related gene was established shortly after the endosymbiotic gene transfer, and this event occurred multiple times independently in distinct genes and in distinct lineages.     

A MODE OF ENTRY OF BLUE-GREEN ALGAE INTO THE APOGEOTROPIC ROOTS OF MACROZAMIA COMMUNIS

The entry of blue-green algae into an aerial root of Macrozamia communis via a break in the dermal layers and leading through a continuous cortical channel into the algal zone is reported. Penetration of the algae appears to be mainly in the form of filaments. Intercellular migration of the algae is found to occur after dissolution of host cell middle lamellae, but host cortical cells are also destroyed near some of the algae. Intracellular migration is also suggested to be a likely pathway for the algae as they proceed to the algal zone.     

Endosymbiosis of Chlorella species to the ciliate Paramecium bursaria alters the distribution of the host’s trichocysts beneath the host cell cortex

Each symbiotic Chlorella of the ciliate Paramecium bursaria is enclosed in a perialgal vacuole membrane derived from the host digestive vacuole membrane. Alga-free paramecia and symbiotic algae can grow independently. Mixing them experimentally can cause reinfection. Earlier, we reported that the symbiotic algae appear to push the host trichocysts aside to become fixed beneath the host cell cortex during the algal reinfection process. Indirect immunofluorescence microscopy with a monoclonal antibody against the trichocysts demonstrates that the trichocysts change their locality to form algal attachment sites and decrease their density beneath the host cell cortex through algal reinfection. Transmission electron microscopy to detect acid phosphatase activity showed that some trichocysts near the host cell cortex are digested by the host lysosomal fusion during algal reinfection. Removal of algae from the host cell using cycloheximide recovers the trichocyst's arrangement and number near the host cell cortex. These results indicate that symbiotic algae compete for their attachment sites with preexisting trichocysts and that the algae have the ability to ensure algal attachment sites beneath the host cell cortex.     

Flow cytometric studies of the host-regulated cell cycle in algae symbiotic with green paramecium

Summary. Paramecium bursaria (green paramecium) possesses endosymbiotically growing chlorella-like green algae. An aposymbiotic cell line of P. bursaria (MBw-1) was prepared from the green MB-1 strain with the herbicide paraquat. The SA-2 clone of symbiotic algae was employed to reinfect MBw-1 cells and thus a regreened cell line (MBr-1) was obtained. The regreened paramecia were used to study the impact of the hosts growth status on the life cycle of the symbiotic algae. Firstly, the relationship between the timing of algal propagation and the host cell division was investigated by counting the algal cells in single host cells during and after the host cell division and also in the stationary phase. Secondly, the changes in the endogenous chlorophyll level, DNA content, and cell size in the symbiotic algae were monitored by flow cytometry and fluorescence microscopy. The number of algae was shown to be doubled prior to or during the host cell division and the algal population in the two daughter cells is maintained at constant level until the host cell cycle reenters the cytokinesis, suggesting that algal propagation and cell cycle are dependent on the hosts cell cycle. During the hosts stationary growth, unicellular algal vegetatives with low chlorophyll content were dominant. In contrast, complexes of algal cells called sporangia (containing 1–4 autospores) were present in the logarithmically growing hosts, indicating that algal cell division leading to the formation of sporangia with multiple autospores is active in the dividing paramecia.Correspondence and reprints: Graduate School of Environmental Engineering, University of Kitakyushu, 1-1 Hibikino, Wakamatsu-ku, 808-0135 Kitakyushu, Japan.     

Phagosome-lysosome fusion inhibited by algal symbionts of Hydra viridis

Certain species of Chlorella live within the digestive cells of the fresh water cnidarian Hydra viridis. When introduced into the hydra gut, these symbiotic algae are phagocytized by digestive cells but avoid host digestion and persist at relatively constant numbers within host cells. In contrast, heat-killed symbionts are rapidly degraded after phagocytosis. Live symbionts appear to persist because host lysosomes fail to fuse with phagosomes containing live symbionts. Neither acid phosphatase nor ferritin was delivered via lysosomes into phagosomes containing live symbionts, whereas these lysosomal markers were found in 50% of the vacuoles containing heat-killed symbionts 1 h after phagocytosis. Treatment of symbiotic algae before phagocytosis with polycationic polypeptides abolishes algal persistence and perturbs the ability of these algae to control the release of photosynthate in vitro. Similarly, inhibition of photosynthesis and hence of the release of photosynthetic products as a result of prolonged darkness and 3-(3,4- dichlorophenyl)-1,1-dimethyl urea (DCMU) treatment also abolishes persistence. Symbiotic algae are not only protected from host digestive attack but are also selectively transported within host cells, moving from the apical site of phagocytosis to a basal position of permanent residence. This process too is disrupted by polycationic polypeptides, DCMU and darkness. Both algal persistence and transport may, therefore, be a function of the release of products from living, photosynthesizing symbionts. Vinblastine treatment of host animals blocked the movement of algae within host cells but did not perturb algal persistence: algal persistence and the transport of algae may be initiated by the same signal, but they are not interdependent processes.     

藻类中添加剂的应用研究进展

藻类富含多种营养元素和活性物质,具有重要的经济价值和广阔的应用前景。现阶段藻类培养多采用露天跑道池,成本低,但易受环境影响。不适宜的环境条件(温度、酸性、重金属、紫外、盐度和光强胁迫等)会造成藻类生产成本上升、产品产量及品质下降等后果,严重制约藻类养殖业及相关产业的发展。添加剂不仅能有效促进藻类生长,还能缓解环境胁迫对其带来的逆境伤害。将近年来添加剂在藻类生长及抗逆方面的应用进行系统汇总,并对已阐明的几种添加剂的作用机理进行分析整理。添加剂在藻类中的应用研究大多停留在生长及生理指标的测定,如藻细胞密度、光系统活性、渗透调节物质含量、脂质含量、过氧化氢酶和硝酸还原酶活力等,仅有少部分研究是利用分子技术测定防御基因的表达情况,尝试进一步探究藻类抗逆性分子机制。旨在为研究者进一步明确常用添加剂在藻类中的信号传导机制及改善非生物胁迫造成的产品产量及品质的问题提供理论依据,具有重要现实意义。     

Flow cytometry as a strategy to study the endosymbiosis of algae in Paramecium bursaria

BACKGROUND: The stable symbiotic association between Paramecium bursaria and algae is of interest to study such mechanisms in biology as recognition, specificity, infection, and regulation. The combination of algae-free strains of P. bursaria, which have been recently established by treating their stocks of green paramecia with herbicide paraquat (Hosoya et al.: Zool Sci 12: 807-810, 1995), with the cloned symbiotic algae isolated from P. bursaria (Nishihara et al.: Protoplasma 203: 91-99, 1998), provides an excellent clue to gain fundamental understanding of these phenomena. METHODS: Flow cytometry and light microscopy have been employed to characterize the algal cells after they have been released from the paramecia by ultrasonic treatment. Algal optical properties such as light scattering and endogenous chlorophyll fluorescence intensity have been monitored for symbiotic and free-living strains, and strains at stages of interaction with a host. RESULTS: Neither algal morphology nor chlorophyll content has been found to be altered by sonication of green paramecia. This fact allows to interpret in adequate degree changes in the optical properties of symbiont that just has been released from the association with a host (decreased forward light scatter and chlorophyll fluorescence signals). Optical characterization of both symbiotic and free-living algal strains with respect to their ability to establish symbioses with P. bursaria showed that chlorophyll content per cell volume seems to be a valuable factor for predicting a favorable symbiotic relationship between P. bursaria and algae. CONCLUSIONS: Flow cytometry combined with algae-free paramecia and cloned symbiotic algae identifies algal populations that may be recognized by host cells for the establishment of symbioses.     

Localization of attachment area of the symbiotic Chlorella variabilis of the ciliate Paramecium bursaria during the algal removal and reinfection

Chlorella spp. and ciliate Paramecium bursaria share a mutual symbiosis. However, both alga-removed P. bursaria and isolated symbiotic algae can grow independently. Additionally, mixing them experimentally can cause algal reinfection through host phagocytosis. Although the symbiotic algal localization beneath the host cell cortex is a prerequisite phenomenon for maintenance of the relationship of their endosymbiosis, how and where the algae locate beneath the host cell cortex remains unknown. To elucidate this phenomenon, algal distribution patterns during algal removal and reinfection were observed. During algal removal, algae at the host anterior cortex were easier to remove than at the posterior and ventral or dorsal cortex areas. During algal reinfection, the algae after separation from the host digestive vacuoles tended to localize beneath the host ventral or dorsal cortex more readily than that at other cortices. Algae that reinfected trichocyst-removed paramecia didn’t show this localization. Trichocyst-discharge experiments clarified that trichocysts of the anterior cortex are difficult to remove. In 14 strains of P. bursaria, some of the paramecia lacked their symbiotic algae at the anterior cortex. These observations demonstrate that symbiotic algae of P. bursaria are difficult to localize at the anterior cortex and that they are easy to remove from the area.     

Discovery of an endophytic alga in Ginkgo biloba

Although intracellular associations with mycorrhizal fungi are known for Ginkgo biloba, no other endosymbiotic relationships have ever been reported for this "living fossil." A protoplast culture derived from haploid explants has now revealed the existence of a green alga in vitro, whose eukaryotic status was confirmed by transmission electron microscopic studies. Phylogenetic 18S rDNA sequence analyses showed this alga to be closely related to the lichen photobiont Coccomyxa. Algae, which in host cells exist as more or less undifferentiated "precursor" forms, proliferated within necrosing G. biloba cells of a subculture derived from a zygotic embryo and were finally released into the medium. Light and electron microscopic observations showed that G. biloba cells rapidly filled up with countless green particles whose number increased up to the bursting of the hypertrophic host cells. At the beginning of reproduction no algae were visible in the nutritive medium, demonstrating that the proliferation started inside the G. biloba cells and excluding the possibility of an exogenous contamination. Occasionally, mature algae together with their precursor forms were detected by transmission electron microscopy in intact host cells of a green callus. The algae were easily identified by their similarity to the cultured algae. Eukaryotic algae have never been reported to date to reside inside higher plant cells, whereas several algal associations are well known from the animal kingdom.     

From extracellular to intracellular: the establishment of mitochondria and chloroplasts.

Paracoccus and Rhodopseudomonas are unusual among bacteria in having a majority of the biochemical features of mitochondria; blue-green algae have many of the features of chloroplasts. The theory of serial endosymbiosis proposes that a primitive eukaryote successively took up bacteria and blue-green algae to yield mitochondria and chloroplasts respectively. Possible characteristics of transitional forms are indicated both by the primitive amoeba, Pelomyxa, which lacks mitochondria but contains a permanent population of endosymbiotic bacteria, and by several anomalous eukaryotic algae, e.g. Cyanophora, which contain cyanelles instead of chloroplasts. Blue-green algae appear to be obvious precursors of red algal chloroplasts but the ancestry of other chloroplasts is less certain, though the epizoic symbiont, Prochloron, may resemble the ancestral green algal chloroplast. We speculate that the chloroplasts of the remaining algae may have been a eukaryotic origin. The evolution or organelles from endosymbiotic precursors would involve their integration with the host cell biochemically, structurally and numerically.     

Evolution and functional diversification of fructose bisphosphate aldolase genes in photosynthetic marine diatoms

Diatoms and other chlorophyll-c containing, or chromalveolate, algae are among the most productive and diverse phytoplankton in the ocean. Evolutionarily, chlorophyll-c algae are linked through common, although not necessarily monophyletic, acquisition of plastid endosymbionts of red as well as most likely green algal origin. There is also strong evidence for a relatively high level of lineage-specific bacterial gene acquisition within chromalveolates. Therefore, analyses of gene content and derivation in chromalveolate taxa have indicated particularly diverse origins of their overall gene repertoire. As a single group of functionally related enzymes spanning two distinct gene families, fructose 1,6-bisphosphate aldolases (FBAs) illustrate the influence on core biochemical pathways of specific evolutionary associations among diatoms and other chromalveolates with various plastid-bearing and bacterial endosymbionts. Protein localization and activity, gene expression, and phylogenetic analyses indicate that the pennate diatom Phaeodactylum tricornutum contains five FBA genes with very little overall functional overlap. Three P. tricornutum FBAs, one class I and two class II, are plastid localized, and each appears to have a distinct evolutionary origin as well as function. Class I plastid FBA appears to have been acquired by chromalveolates from a red algal endosymbiont, whereas one copy of class II plastid FBA is likely to have originated from an ancient green algal endosymbiont. The other copy appears to be the result of a chromalveolate-specific gene duplication. Plastid FBA I and chromalveolate-specific class II plastid FBA are localized in the pyrenoid region of the chloroplast where they are associated with β-carbonic anhydrase, which is known to play a significant role in regulation of the diatom carbon concentrating mechanism. The two pyrenoid-associated FBAs are distinguished by contrasting gene expression profiles under nutrient limiting compared with optimal CO2 fixation conditions, suggestive of a distinct specialized function for each. Cytosolically localized FBAs in P. tricornutum likely play a role in glycolysis and cytoskeleton function and seem to have originated from the stramenopile host cell and from diatom-specific bacterial gene transfer, respectively.     

Phylogenetic Analysis of Algal Symbionts Associated with Four North American Amphibian Egg Masses

Egg masses of the yellow-spotted salamander Ambystoma maculatum form an association with the green alga “Oophila amblystomatis” (Lambert ex Wille), which, in addition to growing within individual egg capsules, has recently been reported to invade embryonic tissues and cells. The binomial O. amblystomatis refers to the algae that occur in A. maculatum egg capsules, but it is unknown whether this population of symbionts constitutes one or several different algal taxa. Moreover, it is unknown whether egg masses across the geographic range of A. maculatum, or other amphibians, associate with one or multiple algal taxa. To address these questions, we conducted a phylogeographic study of algae sampled from egg capsules of A. maculatum, its allopatric congener A. gracile, and two frogs: Lithobates sylvatica and L. aurora. All of these North American amphibians form associations with algae in their egg capsules. We sampled algae from egg capsules of these four amphibians from localities across North America, established representative algal cultures, and amplified and sequenced a region of 18S rDNA for phylogenetic analysis. Our combined analysis shows that symbiotic algae found in egg masses of four North American amphibians are closely related to each other, and form a well-supported clade that also contains three strains of free-living chlamydomonads. We designate this group as the ‘Oophila’ clade, within which the symbiotic algae are further divided into four distinct subclades. Phylogenies of the host amphibians and their algal symbionts are only partially congruent, suggesting that host-switching and co-speciation both play roles in their associations. We also established conditions for isolating and rearing algal symbionts from amphibian egg capsules, which should facilitate further study of these egg mass specialist algae.     

CARBOHYDRATE MOVEMENT FROM AUTOTROPHS TO HETEROTROPHS IN PARASITIC and MUTUALISTIC SYMBIOSIS

1. The bulk of the fixed carbon which moves from autotroph to heterotroph in most symbiotic associations is in a single compound, a carbohydrate. Techniques employing ¹⁴C have been most valuable for investigating this movement. 2. Most ‘zoochlorellae’ belong to the Chlorococcales, and they release carbohydrate to the animal tissue as either glucose or maltose. In some molluscs, the ‘zoochlorellae’ are actually chloroplasts, possibly derived from siphonaceous algae. Although it is known that these chloroplasts supply photosynthetically fixed carbon to the animal tissue, the form of the carbon compounds which move is not known. In Convoluta roscoffensis the ‘zoochlorellae’ belong to the Pyramimonadales, but carbohydrate movement has not yet been directly studied in this association. 3. Most ‘zooxanthellae’ belong to the Dinophyceae. In associations involving co-elenterates and molluscs, glycerol is the main carbohydrate moving to the animal. Homogenates of the host animal tissue stimulate excretion by isolated zooxanthellae. 4. In lichens, symbiotic blue-green algae release glucose to the fungus, but the various genera of green algae that have been studied all release polyols (either ery-thritol, ribitol or sorbitol). Lichen fungi rapidly synthesize mannitol from all these compounds. When lichen algae are isolated into pure culture, they soon lose the ability to excrete carbohydrate, and intracellular production of the carbohydrate that is excreted either becomes much reduced, or ceases altogether. 5. Mostly indirect evidence indicates that sucrose is the main carbohydrate moving from flowering plants to their associated symbiotic fungi. Diversion of the translocation stream towards the site of the association occurs. The fungi convert host sugars to their own carbohydrates, principally trehalose and polyols. 6. ‘Saprophytic’ higher plants are all obligately mycotrophic and receive carbohydrate from their associated fungi. In at least some associations, the fungus is simultaneously associated with an autotrophic higher plant, which is the ultimate source of carbohydrate for the association. 7. Some parasitic higher plants possess chlorophyll, but the extent to which they depend on their host for carbohydrate varies with different species. Green mistletoes evidently derive negligible carbon from their hosts, but other green parasites derive at least some. There is no evidence that any of the chlorophyll-containing parasites export carbohydrate back to their hosts. Parasitic higher plants which lack chlorophyll presumably derive all their carbohydrates from their hosts, but experimental investigations of this are scarce. 8. Comparison between different types of symbiotic association show that a number of common features emerge. 9. The algal symbionts of both invertebrates and lichens have, in comparison to free-living forms, reduced growth rates and greater incorporation of fixed carbon into soluble carbohydrates. They excrete a much greater proportion of their fixed carbon than free-living forms, and most of it is usually as a single carbohydrate. Particularly striking is the fact that the excreted carbohydrate is one which is either not the major intracellular carbohydrate, or one which ceases or nearly ceases to be produced in culture. 10. The translocation stream of autotrophic higher plants is diverted towards the site of association with either fungi or parasitic higher plants, but it is not known how this is achieved. 11. In all associations, the cell walls of the autotroph become reduced or modified at the site of contact with the heterotroph, but it seems likely that this is not directly connected with the mechanism of carbohydrate transfer between the symbionts. 12. In many associations, the heterotroph rapidly converts host sugars into other compounds (frequently into its own carbohydrates which are usually different from those of the host). This may serve to maintain a concentration gradient and so ensure a continued flow from the host. 13. Polyols feature prominently in symbiotic and parasitic associations, not only as the carbohydrates of many plant heterotrophs, but also as the form of carbohydrate released by both zooxanthellae and the green algae of lichens to their heterotrophic partners.     

Evolution of Protein Targeting into “Complex” Plastids: The “Secretory Transport Hypothesis”

Abstract: In algae different types of plastids are known, which vary in pigment content and ultrastructure, providing an opportunity to study their evolutionary origin. One interesting feature is the number of envelope membranes surrounding the plastids. Red algae, green algae and glaucophytes have plastids with two membranes. They are thought to originate from a primary endocytobiosis event, a process in which a prokaryotic cyanobacterium was engulfed by a eukaryotic host cell and transformed into a plastid. Several other algal groups, like euglenophytes and heterokont algae (diatoms, brown algae, etc.), have plastids with three or four surrounding membranes, respectively, probably reflecting the evolution of these organisms by so‐called secondary endocytobiosis, which is the uptake of a eukaryotic alga by a eukaryotic host cell. A prerequisite for the successful establishment of primary or secondary endocytobiosis must be the development of suitable protein targeting machineries to allow the transport of nucleus‐encoded plastid proteins across the various plastid envelope membranes. Here, we discuss the possible evolution of such protein transport systems. We propose that the secretory system of the respective host cell might have been the essential tool to establish protein transport into primary as well as into secondary plastids.