Transcriptional remodelling upon light removal in a model cnidarian: Losses and gains in gene expression.

Organismal responses to light:dark cycles can result from two general processes: (a) direct response to light or (b) a free-running rhythm (i.e., a circadian clock). Previous research in cnidarians has shown that candidate circadian clock genes have rhythmic expression in the presence of diel lighting, but these oscillations appear to be lost quickly after removal of the light cue. Here, we measure whole-organism gene expression changes in 136 transcriptomes of the sea anemone Nematostella vectensis, entrained to a light:dark environment and immediately following light cue removal to distinguish two broadly defined responses in cnidarians: light entrainment and circadian regulation. Direct light exposure resulted in significant differences in expression for hundreds of genes, including more than 200 genes with rhythmic, 24-hr periodicity. Removal of the lighting cue resulted in the loss of significant expression for 80% of these genes after 1 day, including most of the hypothesized cnidarian circadian genes. Further, 70% of these candidate genes were phase-shifted. Most surprisingly, thousands of genes, some of which are involved in oxidative stress, DNA damage response and chromatin modification, had significant differences in expression in the 24 hr following light removal, suggesting that loss of the entraining cue may induce a cellular stress response. Together, our findings suggest that a majority of genes with significant differences in expression for anemones cultured under diel lighting are largely driven by the primary photoresponse rather than a circadian clock when measured at the whole animal level. These results provide context for the evolution of cnidarian circadian biology and help to disassociate two commonly confounded factors driving oscillating phenotypes.

Enhanced photoprotection pathways in symbiotic dinoflagellates of shallow-water corals and other cnidarians.

Photoinhibition, exacerbated by elevated temperatures, underlies coral bleaching, but sensitivity to photosynthetic loss differs among various phylotypes of Symbiodinium, their dinoflagellate symbionts. Symbiodinium is a common symbiont in many cnidarian species including corals, jellyfish, anemones, and giant clams. Here, we provide evidence that most members of clade A Symbiodinium, but not clades B-D or F, exhibit enhanced capabilities for alternative photosynthetic electron-transport pathways including cyclic electron transport (CET). Unlike other clades, clade A Symbiodinium also undergo pronounced light-induced dissociation of antenna complexes from photosystem II (PSII) reaction centers. We propose these attributes promote survival of most cnidarians with clade A symbionts at high light intensities and confer resistance to bleaching conditions that conspicuously impact deeper dwelling corals that harbor non-clade A Symbiodinium.

Effects of freezing, drying, ultraviolet irradiation, chlorine, and quaternary ammonium treatments on the infectivity of myxospores of Myxobolus cerebralis for Tubifex tubifex.

The effects of freezing, drying, ultraviolet irradiation (UV), chlorine, and a quaternary ammonium compound on the infectivity of the myxospore stage of Myxobolus cerebralis (the causative agent of whirling disease) for Tubifex tubifex were examined in a series of laboratory trials. Freezing at either -20 degrees C or -80 degrees C for a period of 7 d or 2 months eliminated infectivity as assessed by the absence of production of the actinospore stage (triactinomyxons [TAMs]) from T. tubifex cultures inoculated with treated myxospores over a 4-5-month period. Myxospores retained infectivity when held in well water at 5 degrees C or 22 degrees C for 7 d and when held at 4 degrees C or 10 degrees C d for 2 months. In contrast, no TAMs were produced from T. tubifex cultures inoculated with myxospores held at 20 degrees C for 2 months. Drying of myxospores eliminated any evidence of infectivity for T. tubifex. Doses of UV from 40 to 480 mJ/cm2 were all effective for inactivating myxospores of M. cerebralis, although a few TAMs were detected in one replicate T. tubifex culture at 240 mJ/cm2 and in one replicate culture at 480 mJ/cm2. Treatments of myxospores with chlorine bleach at active concentrations of at least 500 mg/L for 15 min largely inactivated myxospore infectivity for T. tubifex. Likewise, there was no evidence of TAMs produced by T. tubifex inoculated with myxospores treated with alkyl dimethyl benzyl ammonium chloride (ADBAC) at 1,500 mg/L for 10 min. Treatments of myxospores with 1,000-mg/L ADBAC for 10 min reduced TAM production in T. tubifex cultures sevenfold relative to that in cultures inoculated with an equal number of untreated myxospores. These results indicate that myxospores of M. cerebralis demonstrate a selective rather than broad resistance to selected physical and chemical treatments, and this selective resistance is consistent with conditions that myxospores are likely to experience in nature.

Cellular mechanisms of Cnidarian bleaching: stress causes the collapse of symbiosis.

Cnidarian bleaching is a breakdown in the mutualistic symbiosis between host Cnidarians, such as reef building corals, and their unicellular photosynthetic dinoflagellate symbionts. Bleaching is caused by a variety of environmental stressors, most notably elevated temperatures associated with global climate change in conjunction with high solar radiation, and it is a major contributor to coral death and reef degradation. This review examines the underlying cellular events that lead to symbiosis dysfunction and cause bleaching, emphasizing that, to date, we have only some pieces of a complex cellular jigsaw puzzle. Reactive oxygen species (ROS), generated by damage to both photosynthetic and mitochondrial membranes, is shown to play a central role in both injury to the partners and to inter-partner communication of a stress response. Evidence is presented that suggests that bleaching is a host innate immune response to a compromised symbiont, much like innate immune responses in other host-microbe interactions. Finally, the elimination or exit of the symbiont from host tissues is described through a variety of mechanisms including exocytosis, host cell detachment and host cell apoptosis.

The effect of gravity on coral morphology.

Coral morphological variability reflects either genetic differences or environmentally induced phenotypic plasticity. We present two coral species that sense gravity and accordingly alter their morphology, as characterized by their slenderness (height to diameter) ratio (SR). We experimentally altered the direction (and intensity) of the gravitational resultant force acting along or perpendicular to the main body axis of coral polyps. We also manipulated light direction, in order to uncouple gravity and light effects on coral development. In the experiments, vertically growing polyps had significantly higher SR than their horizontal siblings even when grown in a centrifuge (experiencing different resultant gravitational forces in proximal and distal positions). Lowest SR was in horizontal side-illuminated polyps, and highest in vertical top-illuminated polyps. Adult colonies in situ showed the same pattern. Gravitational intensity also affected polyp growth form. However, polyp volume, dry skeleton weight and density in the various centrifuge positions, and in aquaria experiments, did not differ significantly. This reflects the coral's ability to sense altered gravity direction and intensity, and to react by changing the development pattern of their body morphology, but not the amount of skeleton deposited.

Communication arising. Is coral bleaching really adaptive?

From an experiment in which corals are transplanted between two depths on a Panamanian coral reef, Baker infers that bleaching may sometimes help reef corals to survive environmental change. Although Baker's results hint at further mechanisms by which reef-building corals may acclimatize to changing light conditions, we do not consider that the evidence supports his inference.

Mycosporine-like amino acids and related Gadusols: biosynthesis, acumulation, and UV-protective functions in aquatic organisms.

Organisms living in clear, shallow water are exposed to the damaging wavelengths of solar ultraviolet radiation (UVR) coincident with the longer wavelengths of photosynthetically available radiation (PAR) also necessary for vision. With the general exception of bacteria, taxonomically diverse marine and freshwater organisms have evolved the capacity to synthesize or accumulate UV-absorbing mycosporine-like amino acids (MAAs), presumably for protection against environmental UVR. This review highlights the evidence for this UV-protective role while also considering other attributed functions, including reproductive and osmotic regulation and vision. Probing the regulation and biosynthesis of MAAs provides insight to the physiological evolution and utility of UV protection and of biochemically associated antioxidant defenses.

An analytical model for the SO2- centre, ESR signal at g = 2.0057 in carbonates.

Detailed experiments were conducted to test the behaviour of the ESR signal at the g-value of 2.0057 in corals after irradiation and heating. On the basis of the results an analytical model for this signal was developed. We assume the existence of a precursor to the SO2- radical. On irradiation traps are produced, some in the precursor state and some in the radical state. Heating then causes transfer of electrons into the precursor state, from the precursor state into the radical state and out of the radical state into a base state. On the base of this model, we suggest that the signal at g = 2.0057 can be applied for dating. Our first dating attempts on corals delivered promising results for the suggested procedure.

Fluorescent pigments in corals are photoprotective.

All reef-forming corals depend on the photosynthesis performed by their algal symbiont, and such corals are therefore restricted to the photic zone. The intensity of light in this zone declines over several orders of magnitude--from high and damaging levels at the surface to extreme shade conditions at the lower limit. The ability of corals to tolerate this range implies effective mechanisms for light acclimation and adaptation. Here we show that the fluorescent pigments (FPs) of corals provide a photobiological system for regulating the light environment of coral host tissue. Previous studies have suggested that under low light, FPs may enhance light availability. We now report that in excessive sunlight FPs are photoprotective; they achieve this by dissipating excess energy at wavelengths of low photosynthetic activity, as well as by reflecting of visible and infrared light by FP-containing chromatophores. We also show that FPs enhance the resistance to mass bleaching of corals during periods of heat stress, which has implications for the effect of environmental stress on the diversity of reef-building corals, such as enhanced survival of a broad range of corals allowing maintenance of habitat diversity.

Photobiology of natural populations of zooxanthellae from the sea anemone Aiptasia pallida: assessment of the host's role in protection against ultraviolet radiation.

Natural populations of the sea anemone Aiptasia pallida containing endosymbiotic dinoflagellates were acclimated to different irradiance regimes, with and without ultraviolet radiation (UV). They showed a compensatory response in the amount of chlorophyll and the activities of enzymes responsible for detoxifying active species of oxygen that are produced by the interaction between visible or ultraviolet radiation and photosynthetically produced oxygen. Protection from active species of oxygen is essential to prevent the photooxidation of chlorophyll and the concomitant loss of productivity. Bulk analyses of chlorophyll showed differences between the populations exposed to varying irradiance regimes, but revealed no significant independent effect of UV. However, analysis by flow cytometry of the individual cells from treated populations did detect statistically significant differences in cell size and the amount of chlorophyll fluorescence per cell, which could be attributed to treatment with ultraviolet radiation. With flow cytometry we are able to detect the population variability that is undetectable by bulk measurements which is important in assessing the effects of environmental parameters in both symbiotic and free-living microalgae. Research using simultaneous measurements by flow cytometry could add considerable insight into the population dynamics of both zooxanthellae and host cells.