The importance and movement of mud bacterial carbon within the symbiosis of the New Zealand sea anemone Anthopleura aureoradiata

<p>A. aureoradiata is New Zealand’s only native cnidarian to form a phototrophic symbiosis with dinoflagellate microalgae. It is of particular interest as it can be found in estuarine mudflat habitats attached to cockles, where it spends a portion of the day submerged under the mud, either partially or completely. This scenario is very different to the situation in the tropics, where comparable symbioses (e.g. those with reef-building corals) live in brightly lit, clear waters. How A. aureoradiata maintains a stable symbiosis is therefore of considerable interest, with one potential mechanism involving the acquisition of carbon from the surrounding mud to counter the reduced availability of light and hence the reduced rate of photosynthesis. In this thesis, I established the extent to which organic carbon in mud (especially bacteria) can be assimilated by A. aureoradiata and to what extent, if any, this carbon contributes to symbiosis nutrition and facilitates symbiosis stability under otherwise sub-optimal conditions. In the first instance, anemones were given access to¹³C glucose-labelled mud for 12 hours, in both the light and dark, and the extent of label incorporation (¹³C enrichment) in both the host and symbiont was measured by mass spectrometry. Subsequently, A. aureoradiata was starved of planktonic food for six weeks in the presence of differing quantities of unlabelled mud (‘no-mud’, ‘low-mud’ and ‘high-mud’), either with or without light, and a range of nutritional and biomass parameters measured. These included symbiont density, host protein content, and the accumulation of host lipid and symbiont starch stores. Both the host anemone and its symbiotic algae showed signs of ¹³C uptake from the mud. Host anemones maintained in the dark assimilated more ¹³C label from the mud than did anemones incubated in the light, while the extent of label assimilation by the symbionts was unaffected by irradiance. Enhanced heterotrophic feeding in the dark is consistent with patterns reported for other symbiotic cnidarians, such as reef corals, where the host must counter the reduced availability of photosynthate from the symbiotic algae. However, the reason for the equal labelling of the symbionts in the light and dark is less clear. Nevertheless, factors such as reverse translocation in the dark (i.e. the transfer of organic carbon from host to symbiont), dark fixation of inorganic carbon, and a higher respiration rate of symbionts in the light than dark, could act either alone or in concert to produce the labelling pattern seen. While the host and symbiont showed evidence of carbon uptake from the surrounding mud, mud quantity had no effect on either the host’s or symbiont’s storage products (% of starch in symbiont biomass, host protein content and lipid content), or on symbiont density. The lack of an effect of mud suggests that mud-derived bacteria comprise little of the host’s natural diet. In contrast, increased light availability (independent of mud availability) did lead to elevated symbiont density and symbiont starch content, consistent with the phototrophic nature of this symbiosis. More surprising was that host protein content was highest in the dark, suggesting perhaps that the symbionts were less of an energetic drain on their host when starved in the dark due to their lower population density. In summary, my thesis provides evidence that A. aureoradiata and its symbiotic algae can use organic carbon obtained from the surrounding mud for their nutrition, but that this carbon source is of only negligible importance. These results are consistent with previous findings for the uptake and role of mud-derived nitrogen in this system. Further work to establish how this symbiosis maintains its remarkable stability under apparently sub-optimal, low-light conditions is therefore needed.</p>

The importance and movement of mud bacterial carbon within the symbiosis of the New Zealand sea anemone Anthopleura aureoradiata

<p>A. aureoradiata is New Zealand’s only native cnidarian to form a phototrophic symbiosis with dinoflagellate microalgae. It is of particular interest as it can be found in estuarine mudflat habitats attached to cockles, where it spends a portion of the day submerged under the mud, either partially or completely. This scenario is very different to the situation in the tropics, where comparable symbioses (e.g. those with reef-building corals) live in brightly lit, clear waters. How A. aureoradiata maintains a stable symbiosis is therefore of considerable interest, with one potential mechanism involving the acquisition of carbon from the surrounding mud to counter the reduced availability of light and hence the reduced rate of photosynthesis. In this thesis, I established the extent to which organic carbon in mud (especially bacteria) can be assimilated by A. aureoradiata and to what extent, if any, this carbon contributes to symbiosis nutrition and facilitates symbiosis stability under otherwise sub-optimal conditions. In the first instance, anemones were given access to¹³C glucose-labelled mud for 12 hours, in both the light and dark, and the extent of label incorporation (¹³C enrichment) in both the host and symbiont was measured by mass spectrometry. Subsequently, A. aureoradiata was starved of planktonic food for six weeks in the presence of differing quantities of unlabelled mud (‘no-mud’, ‘low-mud’ and ‘high-mud’), either with or without light, and a range of nutritional and biomass parameters measured. These included symbiont density, host protein content, and the accumulation of host lipid and symbiont starch stores. Both the host anemone and its symbiotic algae showed signs of ¹³C uptake from the mud. Host anemones maintained in the dark assimilated more ¹³C label from the mud than did anemones incubated in the light, while the extent of label assimilation by the symbionts was unaffected by irradiance. Enhanced heterotrophic feeding in the dark is consistent with patterns reported for other symbiotic cnidarians, such as reef corals, where the host must counter the reduced availability of photosynthate from the symbiotic algae. However, the reason for the equal labelling of the symbionts in the light and dark is less clear. Nevertheless, factors such as reverse translocation in the dark (i.e. the transfer of organic carbon from host to symbiont), dark fixation of inorganic carbon, and a higher respiration rate of symbionts in the light than dark, could act either alone or in concert to produce the labelling pattern seen. While the host and symbiont showed evidence of carbon uptake from the surrounding mud, mud quantity had no effect on either the host’s or symbiont’s storage products (% of starch in symbiont biomass, host protein content and lipid content), or on symbiont density. The lack of an effect of mud suggests that mud-derived bacteria comprise little of the host’s natural diet. In contrast, increased light availability (independent of mud availability) did lead to elevated symbiont density and symbiont starch content, consistent with the phototrophic nature of this symbiosis. More surprising was that host protein content was highest in the dark, suggesting perhaps that the symbionts were less of an energetic drain on their host when starved in the dark due to their lower population density. In summary, my thesis provides evidence that A. aureoradiata and its symbiotic algae can use organic carbon obtained from the surrounding mud for their nutrition, but that this carbon source is of only negligible importance. These results are consistent with previous findings for the uptake and role of mud-derived nitrogen in this system. Further work to establish how this symbiosis maintains its remarkable stability under apparently sub-optimal, low-light conditions is therefore needed.</p>

Feeding of a Scleractinian Coral, Goniopora columna, on Microalgae, Yeast, and Artificial Feed in Captivity

Nutritional requirements are critical in the process of coral aquaculture. In addition to energy from symbiotic algae, corals obtain sufficient nutrition through heterotrophic feeding. Microalgae and yeast are commonly used as nutritional supplements for many aquaculture organisms. In addition, if artificial feed can match or improve upon the nutritional supplementation provided by microalgae and yeast in the case of G. columna, then feeding this coral would be markedly easier. Hence, this article preliminarily discusses feeds suitable for G. columna. In this study, artificial PUFA rich in animal protein (R), Saccharomyces cerevisiae, Isochrysis galbana tml, and Nannochloropsis oculate were fed to G. columna at quantities of 5% and 10% of body weight. Growth, survival, body composition, and digestive enzymes were assessed. Regarding body composition, the coral’s protein content is higher than that of carbohydrate or fat; thus, evaluating the heterotrophic nutrition of G. columna by using protein absorption is appropriate. The protease content is also high in digestive enzymes. Protein content, protease activity, and specific growth rate were significantly higher in the R group than in other groups. The number of polyps in the groups fed R at 5% and 10% of body weight increased by 40.00 ± 2.43 and 47.33 ± 0.89 number, respectively, significantly greater increases than those achieved in the other groups (p < 0.05). Changes in body composition and digestive enzymes over a 24-h period were compared to determine the optimal feeding time. Protein content and protease activity increased markedly between 6:00 and 12:00. The experimental results suggest that R can improve the activity of G. columna digestive enzymes and their protein and lipid content in body tissue, shorten the cultivation time, and enhance the profitability of coral aquaculture.

The Endosymbiotic Coral Algae Symbiodiniaceae Are Sensitive to a Sensory Pollutant: Artificial Light at Night, ALAN

Artificial Light at Night, ALAN, is a major emerging issue in biodiversity conservation, which can negatively impact both terrestrial and marine environments. Therefore, it should be taken into serious consideration in strategic planning for urban development. While the lion’s share of research has dealt with terrestrial organisms, only a handful of studies have focused on the marine milieu. To determine if ALAN impacts the coral reef symbiotic algae, that are fundamental for sustainable coral reefs, we conducted a short experiment over a period of one-month by illuminating isolated Symbiodiniaceae cell cultures from the genera Cladocopium (formerly Clade C) and Durusdinium (formerly Clade D) with LED light. Cell cultures were exposed nightly to ALAN levels of 0.15 μmol quanta m–2 s–1 (∼4–5 lux) with three light spectra: blue, yellow and white. Our findings showed that even in very low levels of light at night, the photo-physiology of the algae’s Electron Transport Rate (ETR), Non-Photochemical Quenching, (NPQ), total chlorophyll, and meiotic index presented significantly lower values under ALAN, primarily, but not exclusively, in Cladocopium cell cultures. The findings also showed that diverse Symbiodiniaceae types have different photo-physiology and photosynthesis performances under ALAN. We believe that our results sound an alarm for the probable detrimental effects of an increasing sensory pollutant, ALAN, on the eco-physiology of symbiotic corals. The results of this study point to the potential effects of ALAN on other organisms in marine ecosystem such as fish, zooplankton, and phytoplankton in which their biorhythms is entrained by natural light and dark cycles.

8. Global scale pressures on reefs—climate change

Ocean temperatures are rising. This is critical for corals and other reef organisms because most live very close to their thermal limits already. The rise is caused by the greenhouse effect from increasing CO2 emissions. Superimposed on a general background rise caused by the general increase in heat content of the world are pulses—ocean heatwaves—caused by vagaries in ocean circulation. Globally, this is now the greatest threat to reefs. Warming pulses cause mass coral bleaching and mortality when the overstressed symbiotic algae are expelled from the corals, showing the white limestone beneath the now transparent coral tissue. All coral reef areas of the world now exhibit mass bleaching events. Recovery of a reef is possible, but only if given some decades of stable temperatures, and predictions are that warming events are occurring increasingly frequently and are of increasing severity. Coral cover on reefs in all reef areas is declining sharply. Seawater also becomes increasingly acidic, which impedes coral calcification. Added to this, there is a lag of 20–40 years for carbon dioxide in the air to equilibrate with the ocean, so even were there to be a cessation in the rise in the atmosphere today, these effects would continue to develop for a few decades more. 350 parts per million CO2 is considered to be a threshold concentration for calcification to be possible but already the atmosphere is at about 415 ppm. Sea levels are rising too as a result, and reefs are degrading and losing their ability to act as breakwaters.

Characterization of a green Stentor with symbiotic algae growing in an extremely oligotrophic environment and storing large amounts of starch granules in its cytoplasm

The genus Stentor is a relatively well-known ciliate owing to its lucid trumpet shape. Stentor pyriformis represents a green, short, and fat Stentor, but it is a little-known species. We investigated 124 ponds and wetlands in Japan and confirmed the presence of S. pyriformis at 23 locations. All these ponds were noticeably oligotrophic. With the improvement of oligotrophic culture conditions, we succeeded in long-term cultivation of three strains of S. pyriformis. The cytoplasm of S. piriformis contains a large number of 1–3 μm refractive granules that turn brown by Lugol’s staining. The granules also show a typical Maltese-cross pattern by polarization microscopy, strongly suggesting that the granules are made of amylopectin-rich starch. By analyzing the algal rDNA, it was found that all S. pyriformis symbionts investigated in this study were Chlorella variabilis. This species is known as the symbiont of Paramecium bursaria and is physiologically specialized for endosymbiosis. Genetic discrepancies between C. variabilis of S. pyriformis and P. bursaria may indicate that algal sharing was an old incident. Having symbiotic algae and storing carbohydrate granules in the cytoplasm is considered a powerful strategy for this ciliate to withstand oligotrophic and cold winter environments in highland bogs.

A review of soft corals (Octocorallia: Alcyonacea) and their symbionts: Distribution of clades and functionality

Even though soft corals are an integral part of the coral reef ecosystem, very little research has focused on theirendosymbiotic association with the zooxanthellae (Symbiodiniaceae). Symbiotic algae appear in soft corals from both the tropical seas as well as temperate regions across the world. The present study provides a list of soft corals along with their associated zooxanthellae using published data. By combining all the data, 52 soft coral species belonging to 21 genera were identified and their corresponding specific Symbiodiniaceae genera were analysed. It was important to initially compare soft corals and distinguish zooxanthellae at clade level to find any host specificity. The vast majority (75 %) of soft corals predominantly hosted the genus Cladocopium (Clade C) and therefore the focus of this study was to evaluate its geographical distribution. The genus Cladocopium consists of many species which are classified as subclades in this study, exhibiting high intra-clade diversity. For 40 of the 52 soft corals reviewed, 18 % had the same clade type C1 (considered as generalists and ancestral) and 13 % contained the clades C15, C64 and C1:3a, respectively. Analysis showed that some clades (C71a, C15b and C64) were confined to specific geographical regions while clade C1 was distributed worldwide. Understanding the distribution of endosymbionts may enable predictions of which symbioses will survive and exist under future climate change scenarios.