Host-finding behaviour and navigation capabilities of symbiotic zooxanthellae

Past studies have shown that the initiation of symbiosis between the Red-Sea soft coral Heteroxenia fuscescens and its symbiotic dinoflagellates occurs due to the chemical attraction of the motile algal cells to substances emanating from the coral polyps. However, the resulting swimming patterns of zooxanthellae have not been previously studied. This work examined algal swimming behaviour, host location and navigation capabilities under four conditions: (1) still water, (2) in still water with waterborne host attractants, (3) in flowing water, and (4) in flow with host attractants. Algae were capable of actively and effectively locating their host in still water as well as in flow. When in water containing host attractants, swimming became slower, motion patterns straighter and the direction of motion was mainly towards the host—even if this meant advancing upstream against flow velocities of up to 0.5 mm s⁻¹. Coral-algae encounter probability decreased the further downstream of the host algae were located, probably due to diffusion of the chemical signal. The results show how the chemoreceptive zooxanthellae modify their swimming pattern, direction, velocity, circuity and turning rate to accommodate efficient navigation in changing environmental conditions.     

Comparative Proteomics of Symbiotic and Aposymbiotic Juvenile Soft Corals

The symbiotic association between corals and photosynthetic unicellular algae is of great importance in coral reef ecosystems. The study of symbiotic relationships is multidisciplinary and involves research in phylogeny, physiology, biochemistry, and ecology. An intriguing phase in each symbiotic relationship is its initiation, in which the partners interact for the first time. The examination of this phase in coral–algae symbiosis from a molecular point of view is still at an early stage. In the present study we used 2-dimensional polyacrylamide gel electrophoresis to compare patterns of proteins synthesized in symbiotic and aposymbiotic primary polyps of the Red Sea soft coral Heteroxenia fuscescens. This is the first work to search for symbiosis-specific proteins during the natural onset of symbiosis in early host ontogeny. The protein profiles reveal changes in the host soft coral proteome through development, but surprisingly virtually no changes in the host proteome as a function of symbiotic state.     

Delay in migration of symbiotic algae in Hydra viridis by inhibitors of microtubule protein polymerization

An English strain of the fresh water symbiotic coelenterate Hydra viridis was experimentally "bleached" of its Chlorella algae and maintained indefinitely by feeding. The algal symbiosis could be re-established by injecting other symbiotic algae into aposymbionts. Although algal uptake and recognition were not affected by microtubule protein polymerization inhibitors, these compounds i.e., podophyllotoxin, beta-peltatin and vinblastine had delaying effects on the migration of the algae through the host digestive cells. Picropodophyllotoxin did not delay migration. The rates, the reversibility and the sensitivity of algal migration to low concentrations of drugs known to bind tubulin suggests the symbionts migrate somehow via labile polymerization of host hydra tubulin into microtubules.     

Costs,benefits, and loss of vertically transmitted symbionts affect host population dynamics

The costs and benefits of symbiotic interactions may vary with host and symbiont ontogeny. Effects of symbionts at different stages of host development or on different host demographic rates do not contribute equally to fitness. Although rarely applied, a population dynamics approach that integrates over the host life cycle is therefore necessary for capturing the net costs or benefits and, thus, the mutualistic or parasitic nature of symbioses. Using the native, disturbance‐specialist grass Agrostis hyemalis, we asked how a symbiotic endophyte affected the population dynamics of its host and how imperfect vertical transmission influenced symbiont frequency in a late successional environment. A size‐structured integral projection model (IPM) parameterized with experimental field data showed that greater rates of individual growth and reproduction for endophyte‐symbiotic (E+) hosts outweighed their lower rates of survival, leading to a net positive effect of symbiosis on equilibrium plant population growth (slower rate of extinction). Given that populations under going successional transitions are unlikely to be at an equilibrium size structure, we also conducted transient analysis that showed an initial short‐term cost to endophyte symbiosis. We used a megamatrix approach to link E? and E+ IPMs via imperfect vertical transmission and found that this parameter strongly influenced the frequency of symbiosis via complex interactions with host demographic rates. Overall, our population dynamics approach improves the ability to characterize the outcome of symbiotic interactions, and results suggest that particular attention should be paid to interactions between the rate of vertical transmission and host demography.     

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.     

Symbiosis between hydra and chlorella: Molecular phylogenetic analysis and experimental study provide insight into its origin and evolution

Although many physiological studies have been reported on the symbiosis between hydra and green algae, very little information from a molecular phylogenetic aspect of symbiosis is available. In order to understand the origin and evolution of symbiosis between the two organisms, we compared the phylogenetic relationships among symbiotic green algae with the phylogenetic relationships among host hydra strains. To do so, we reconstructed molecular phylogenetic trees of several strains of symbiotic chlorella harbored in the endodermal epithelial cells of viridissima group hydra strains and investigated their congruence with the molecular phylogenetic trees of the host hydra strains. To examine the species specificity between the host and the symbiont with respect to the genetic distance, we also tried to introduce chlorella strains into two aposymbiotic strains of viridissima group hydra in which symbiotic chlorella had been eliminated in advance. We discussed the origin and history of symbiosis between hydra and green algae based on the analysis.     

Photoacclimation and the effect of the symbiotic environment on the photosynthetic response of symbiotic dinoflagellates in the tropical marine hydroid Myrionema amboinense

Symbiotic dinoflagellates of the genus Symbiodinium and residing in the tropical hydroid Myrionema amboinense acclimate to low photon flux associated with low light 'shade' environments by increasing the amount of photosynthetic pigments per algal cell. The photosynthetic light intensity (PI) curves suggested that the low-light pigment response involved an increase in the number of photosynthetic units (PSU) in the chloroplast in addition to any increases in PSU size. Comparisons of light-dependent portion of the P-I curves of freshly isolated zooxanthellae (FIZ) with those from symbionts within the intact animal suggest that the host cell environment reduced average light levels reaching the symbiotic algae by more than half. This phenomenon may protect the algae from photobleaching of pigments and/or photoinhibition of photosynthesis at high light intensities present in shallow water habitats. In addition, maximum photosynthesis (P(max)) of symbionts removed from the host cell was higher than that recorded from dinoflagellates in the intact association, suggesting that the availability of carbon dioxide for photosynthesis may be limited in the intact hydroid. Shaded polyps contained fewer zooxanthellae and had less tissue biomass (measured as protein) than unshaded polyps. However symbionts from shaded polyps acclimated to the low light intensities by increasing chlorophyll levels and photosynthetic rates. The higher photosynthetic rates may have resulted from increased availability of carbon dioxide associated with lower symbiont density. Calculations of the contribution of zooxanthellae carbon to the host animal's respiratory demand (CZAR) showed that zooxanthellae from shaded polyps living in the field potentially provide about the same amount of carbon to their host as zooxanthellae from polyps living in the field in unshaded high light intensities.     

Genetics of the relationship between the ciliate Paramecium bursaria and its symbiotic algae

Abstract. Paramecium bursaria , a freshwater protozoan, typically harbors hundreds of symbiotic algae ( Chlorella sp.) in its cytoplasm. The relationship between host paramecia and symbiotic algae is stable and mutually beneficial in natural environments. We recently collected an aposymbiotic strain of P. bursaria . Infection experiments revealed that the natural aposymbiotic strain (Ysa2) showed unstable symbiosis with Chlorella sp. The algae aggregated at the posterior region of the host, resulting in aposymbiotic cell production after cell division. Cross-breeding analyses were performed to determine the heritability of the aposymbiotic condition. In crosses of Ysa2 with symbiotic strains of P. bursaria , F1 progeny were able to form stable symbioses with Chlorella sp. However, unstable symbiosis, resembling Ysa2 infection, occurred in some F2 progeny of sibling crosses between symbiotic F1 clones. Infection experiments using aposymbiotic F2 cells showed that these F2 subclones have limited ability to reestablish the symbiosis. These results indicate that the maintenance of stable symbiosis is genetically controlled and heritable, and that Ysa2 is a mutant lacking the mechanisms to establish stable symbiosis with Chlorella sp.     

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.     

Aposymbiotic Plesiastrea versipora continues to produce cell-signalling molecules that regulate the carbon metabolism of symbiotic algae

The scleractinian coral Plesiastrea versipora produces cell signals that regulate the carbon metabolism of its symbiotic algae. Host release factor (HRF) stimulates the release of photosynthate, and photosynthesis inhibiting factor (PIF) partially inhibits carbon fixation in freshly isolated symbiotic algae. Naturally occurring aposymbiotic specimens of P. versipora are rare in Port Jackson, Australia, but one that was collected contained HRF and PIF. Artificially produced aposymbiotic corals of P. versipora that had been kept in the dark for up to 23 months continued to produce both HRF and PIF in the absence of photosynthetically active algae. Aposymbiotic P. versipora from which most of the tissue had been removed, regenerated when they were kept in the dark and fed; the regenerated tissue also contained HRF and PIF. These results suggest that the presence of symbiotic algae is not required for the production of HRF and PIF in P. versipora. We suggest that these cell signals may have evolved in response to symbiosis with Symbiodinium sp. but are now always expressed in the coral P. versipora.     

造礁石珊瑚对低温的耐受能力及响应模式

通过实验室生态模拟,研究了低温胁迫下三亚湾5种造礁石珊瑚（十字牡丹珊瑚、佳丽鹿角珊瑚、花鹿角珊瑚、强壮鹿角珊瑚、澄黄滨珊瑚）的耐受性,分析了造礁石珊瑚对低温的响应模式.结果表明：造礁石珊瑚耐受低温能力与其骨骼类型有关,枝状珊瑚最先死亡,块状珊瑚的耐受能力明显高于枝状珊瑚;14 ℃持续3 d是三亚湾枝状造礁石珊瑚的致死低温;14 ℃持续3 d为块状澄黄滨珊瑚的致白化低温;12 ℃持续10 d为叶片状十字牡丹珊瑚的致死温度;块状澄黄滨珊瑚受到低温胁迫时表面形成粘膜,阻止了珊瑚进一步排出共生虫黄藻. 耐高温的珊瑚对低温也表现出较强的耐受能力,珊瑚对低温胁迫的响应模式与对高温的响应模式基本一致, 即珊瑚首先不伸展触手,紧接着不断释放粘液并排出共生藻,最后白化、死亡.     

Intracellular digestion and symbiosis in Paramecium bursaria

Electron microscopic cytochemical methods reveal that acid phosphatase activity appears exclusively in vacuoles containing recently ingested bacteria or inert particles such as carmine, Celkate or latex spheres, and not in the vacuoles surrounding established symbionts. Although newly ingested symbiotic algae are digested in large numbers, some remain to reestablish the symbiosis. Since symbiotic algae are able to delay the digestion of heat-killed algae when they coexist in a phagosome, we propose that symbiotic Chlorella actively interfere with an early event in the host digestive process.     

造礁石珊瑚与其共生藻(Symbiodinium)共生研究进展

对造礁石珊瑚与其共生藻共生研究现状及其在全球变化下的适应能力进行较全面的综述.造礁石珊瑚与遗传和生理功能独特的共生藻组成内共生关系是成功演化的范例.近年来对珊瑚共生体的分子系统学研究表明共生藻遗传多样性极为丰富,当前认为共生藻属至少包括8个(A-H)各自包含亚系群的世系或系群.珊瑚-共生藻共生功能体对诸如全球变化引起的海水温度上升等环境变化十分敏感.由于珊瑚以及珊瑚礁面临气候变化的严峻挑战,对珊瑚与其共生藻共生关系和共生功体适应能力的研究将是未来重要的研究领域之一.     

基于核及叶绿体基因序列探讨中国绿水螅共生单细胞绿藻系统发生地位

研究对中国绿水螅共生绿藻的核18S rRNA基因全长序列及其叶绿体9个基因(atpA、chlB、chlN、petA、psaB、psbA、psbC、psbD及rbcL)片段序列进行了克隆和测序, 并基于18S rRNA基因序列及叶绿体9个基因序列的整合数据分别通过最大似然法(Maximum-likelihood)和贝叶斯分析(Bayesian inference)对中国绿水螅(Hydra sinensis)共生单细胞绿藻的系统发生地位进行了探讨。系统发生表明: (1)中国绿水螅共生绿藻属于共球藻纲(Trebouxiophyceae)小球藻目(Chlorellales), 但不属于其中的小球藻属(Chlorella); (2)来源于草履虫、水螅、地衣及银杏的共生绿藻均在共球藻纲支系, 而来源于蛙类和蝾螈的共生绿藻属于绿藻纲(Chlorophyceae)支系。无论在共球藻纲支系还是在绿藻纲支系, 不同来源的共生藻并没有排他性地聚为单系群而在系统树中与其他自由生活的绿藻混杂排列, 来自不同宿主的共生绿藻没有共同起源。     

Endosymbionts in paramecium

Paramecium species are extremely valuable organisms to enable experiments for the reestablishment of endosymbiosis. This is investigated in two different systems, the first with Paramecium caudatum and the endonuclear symbiotic bacterium Holospora species. Although most endosymbiotic bacteria cannot grow outside the host cell as a result of their reduced genome size, Holospora species can maintain their infectivity for a limited time. We found that an 89-kDa periplasmic protein has an important function for Holospora's invasion into the target nucleus, and that Holospora alters the host gene expression; the host thereby acquires resistance against various stresses. The second system is the symbiosis between P. bursaria and symbiotic Chlorella. Alga-free P. bursaria and the algae retain the ability to grow without a partner. Consequently, endosymbiosis between the aposymbiotic host cells and the symbiotic algae can be reestablished easily by mixing them. We now found four checkpoints for the reestablishment of the endosymbiosis between P. bursaria and the algae. The findings in the two systems provide excellent opportunities for us to elucidate not only infection processes but also to assess the associations leading to eukaryotic cell evolution. This paper summarizes recent progresses on reestablishment of the primary and the secondary endosymbiosis in Paramecium.     

Resistance to thermal stress in corals without changes in symbiont composition

Discovering how corals can adjust their thermal sensitivity in the context of global climate change is important in understanding the long-term persistence of coral reefs. In this study, we showed that short-term preconditioning to higher temperatures, 3°C below the experimentally determined bleaching threshold, for a period of 10 days provides thermal tolerance for the symbiosis stability between the scleractinian coral, Acropora millepora and Symbiodinium. Based on genotypic analysis, our results indicate that the acclimatization of this coral species to thermal stress does not come down to simple changes in Symbiodinium and/or the bacterial communities that associate with reef-building corals. This suggests that the physiological plasticity of the host and/or symbiotic components appears to play an important role in responding to ocean warming. The further study of host and symbiont physiology, both of Symbiodinium and prokaryotes, is of paramount importance in the context of global climate change, as mechanisms for rapid holobiont acclimatization will become increasingly important to the long-standing persistence of coral reefs.     

Diversity of symbiotic algae of the genus Symbiodinium in scleractinian corals of the Xisha Islands in the South China Sea

Symbiotic algae （Symbiodinium sp.） in scleractinian corals are important in understanding how coral reefs will respond to global climate change. The present paper reports on the diversity of Symbiodinium sp. in 48 scleractinian coral species from 25 genera and 10 families sampled from the Xisha Islands in the South China Sea, which were identified with the use of restriction fragment length polymorphism （RFLP） of the nuclear ribosomal DNA large subunit gene （rDNA）. The results showed that： （i） Symbiodinium Clade C was the dominant zooxanthellae in scleractinian corals in the Xisha Islands; （ii） Symbiodinium Clade D was found in the corals Montipora aequituberculata, Galaxea fascicularis, and Plerogyra sinuosa; and （iii） both Symbiodinium Clades C and D were found simultaneously in Montipora digitata, Psammocora contigua, and Galaxeafascicularis. A poor capacity for symbiosis polymorphism, as uncovered by RFLP, in the Xisha Islands indicates that the scleractinian corals have low adaptability to environmental changes. Further studies are needed to investigate zooxanthellae diversity using other molecular markers.     

Symbioses between salamander embryos and green algae

The symbiosis between Ambystoma maculatum (spotted salamander) embryos and green algae was initially described over 120 years ago. Algae populate the egg capsules that surround individual A. maculatum embryos, giving the intracapsular fluid a characteristic green hue. Early work established this symbiosis to be a mutualism, while subsequent studies sought to identify the material benefits of this association to both symbiont and host. These studies have shown that salamander embryos benefit from increased oxygen concentrations provided by their symbiotic algae. The algae, in turn, may benefit from ammonia excreted by the embryos. All of these early studies considered the association to be an ectosymbiotic mutualism. However our recent work has shown that algae invade both embryonic salamander cells and tissues during development. The unexpected invasion of algal cells into a salamander host changes our understanding of this symbiosis. This review will summarize the earlier research on this association in the context of these recent findings. It will also emphasize gaps in our understanding of this and other amphibian embryo-algal interactions and suggest various research avenues to address these unanswered questions.     

Allometric scaling in small colonies of the scleractinian coral Siderastrea siderea (Ellis and Solander)

Although most physiological traits scale allometrically in unitary organisms, it has been hypothesized that modularity allows for isometric scaling in colonial modular taxa. Isometry would allow increases in size without functional constraints, and is thought to be of central importance to the success of a modular design. Yet, despite its potential importance, scaling in these organisms has received little attention. To determine whether scleractinian corals are free of allometric constraints, we quantified metabolic scaling, measured as aerobic respiration, in small colonies (< or =40 mm in diam.) of the scleractinian Siderastrea siderea. We also quantified the scaling of colony surface area with biomass, since the proposed isometry is contingent upon maintaining a constant ratio of surface area to biomass (or volume) with size. Contrary to the predicted isometry, aerobic respiration scaled allometrically on biomass with a slope (b) of 0.176, and colony surface area scaled allometrically on biomass with a slope of 0.730. These findings indicate that small colonies of S. siderea have disproportionately high metabolic rates and SA:B ratios compared to their larger counterparts. The most probable explanations for the allometric scaling of aerobic respiration are (1) a decline in the SA:B ratio with size such that more surface area is available per unit of biomass for mass transfer in the smallest colonies, and (2) the small size, young age, and disproportionately high growth rates of the corals examined. This allometric scaling also demonstrates that modularity, alone, does not allow small colonies of S. siderea to overcome allometric constraints. Further studies are required to determine whether allometric scaling is characteristic of the full size range of colonies of S. siderea.