Controlling Effects of Irradiance and Heterotrophy on Carbon Translocation in the Temperate Coral Cladocora caespitosa

Temperate symbiotic corals, such as the Mediterranean species Cladocora caespitosa, live in seasonally changing environments, where irradiance can be ten times higher in summer than winter. These corals shift from autotrophy in summer to heterotrophy in winter in response to light limitation of the symbiont's photosynthesis. In this study, we determined the autotrophic carbon budget under different conditions of irradiance (20 and 120 μmol photons m(-2) s(-1)) and feeding (fed three times a week with Artemia salina nauplii, and unfed). Corals were incubated in H(13)CO(3) (-)-enriched seawater, and the fate of (13)C was followed in the symbionts and the host tissue. The total amount of carbon fixed by photosynthesis and translocated was significantly higher at high than low irradiance (ca. 13 versus 2.5-4.5 μg cm(-2) h(-1)), because the rates of photosynthesis and carbon fixation were also higher. However, the percent of carbon translocation was similar under the two irradiances, and reached more than 70% of the total fixed carbon. Host feeding induced a decrease in the percentage of carbon translocated under low irradiance (from 70 to 53%), and also a decrease in the rates of carbon translocation per symbiont cell under both irradiances. The fate of autotrophic and heterotrophic carbon differed according to irradiance. At low irradiance, autotrophic carbon was mostly respired by the host and the symbionts, and heterotrophic feeding led to an increase in host biomass. Under high irradiance, autotrophic carbon was both respired and released as particulate and dissolved organic carbon, and heterotrophic feeding led to an increase in host biomass and symbiont concentration. Overall, the maintenance of high symbiont concentration and high percentage of carbon translocation under low irradiance allow this coral species to optimize its autotrophic carbon acquisition, when irradiance conditions are not favourable to photosynthesis.     

Heterotrophy mitigates the response of the temperate coral Oculina arbuscula to temperature stress

Anthropogenic increases in atmospheric carbon dioxide concentration have caused global average sea surface temperature (SST) to increase by approximately 0.11°C per decade between 1971 and 2010 – a trend that is projected to continue through the 21st century. A multitude of research studies have demonstrated that increased SSTs compromise the coral holobiont (cnidarian host and its symbiotic algae) by reducing both host calcification and symbiont density, among other variables. However, we still do not fully understand the role of heterotrophy in the response of the coral holobiont to elevated temperature, particularly for temperate corals. Here, we conducted a pair of independent experiments to investigate the influence of heterotrophy on the response of the temperate scleractinian coral Oculina arbuscula to thermal stress. Colonies of O. arbuscula from Radio Island, North Carolina, were exposed to four feeding treatments (zero, low, moderate, and high concentrations of newly hatched Artemia sp. nauplii) across two independent temperature experiments (average annual SST (20°C) and average summer temperature (28°C) for the interval 2005–2012) to quantify the effects of heterotrophy on coral skeletal growth and symbiont density. Results suggest that heterotrophy mitigated both reduced skeletal growth and decreased symbiont density observed for unfed corals reared at 28°C. This study highlights the importance of heterotrophy in maintaining coral holobiont fitness under thermal stress and has important implications for the interpretation of coral response to climate change.     

Calcification by juvenile corals under heterotrophy and elevated CO2

Ocean acidification (OA) threatens the existence of coral reefs by slowing the rate of calcium carbonate (CaCO₃) production of framework-building corals thus reducing the amount of CaCO₃ the reef can produce to counteract natural dissolution. Some evidence exists to suggest that elevated levels of dissolved inorganic nutrients can reduce the impact of OA on coral calcification. Here, we investigated the potential for enhanced energetic status of juvenile corals, achieved via heterotrophic feeding, to modulate the negative impact of OA on calcification. Larvae of the common Atlantic golf ball coral, Favia fragum, were collected and reared for 3 weeks under ambient (421 μatm) or significantly elevated (1,311 μatm) CO₂ conditions. The metamorphosed, zooxanthellate spat were either fed brine shrimp (i.e., received nutrition from photosynthesis plus heterotrophy) or not fed (i.e., primarily autotrophic). Regardless of CO₂ condition, the skeletons of fed corals exhibited accelerated development of septal cycles and were larger than those of unfed corals. At each CO₂ level, fed corals accreted more CaCO₃ than unfed corals, and fed corals reared under 1,311 μatm CO₂ accreted as much CaCO₃ as unfed corals reared under ambient CO₂. However, feeding did not alter the sensitivity of calcification to increased CO₂; ? calcification/?Ω was comparable for fed and unfed corals. Our results suggest that calcification rates of nutritionally replete juvenile corals will decline as OA intensifies over the course of this century. Critically, however, such corals could maintain higher rates of skeletal growth and CaCO₃ production under OA than those in nutritionally limited environments.     

Host pigments: potential facilitators of photosynthesis in coral symbioses

Reef‐building corals occur as a range of colour morphs because of varying types and concentrations of pigments within the host tissues, but little is known about their physiological or ecological significance. Here, we examined whether specific host pigments act as an alternative mechanism for photoacclimation in the coral holobiont. We used the coral Montipora monasteriata (Forskål 1775) as a case study because it occurs in multiple colour morphs (tan, blue, brown, green and red) within varying light‐habitat distributions. We demonstrated that two of the non‐fluorescent host pigments are responsive to changes in external irradiance, with some host pigments up‐regulating in response to elevated irradiance. This appeared to facilitate the retention of antennal chlorophyll by endosymbionts and hence, photosynthetic capacity. Specifically, net P_max Chl a^?1 correlated strongly with the concentration of an orange‐absorbing non‐fluorescent pigment (CP‐580). This had major implications for the energetics of bleached blue‐pigmented (CP‐580) colonies that maintained net P_max cm^?2 by increasing P_max Chl a^?1. The data suggested that blue morphs can bleach, decreasing their symbiont populations by an order of magnitude without compromising symbiont or coral health.     

Coscinaraea marshae corals that have survived prolonged bleaching exhibit signs of increased heterotrophic feeding

Colonies of Coscinaraea marshae corals from Rottnest Island, Western Australia have survived for more than 11 months in various bleached states following a severe heating event in the austral summer of 2011. These colonies are situated in a high-latitude, mesophotic environment, which has made their long-term survival of particular interest as such environments typically suffer from minimal thermal pressures. We have investigated corals that remain unbleached, moderately bleached, or severely bleached to better understand potential survival mechanisms utilised in response to thermal stress. Specifically, Symbiodinium (algal symbiont) density and genotype, chlorophyll-a concentrations, and δ¹³C and δ¹⁵N levels were compared between colonies in the three bleaching categories. Severely bleached colonies housed significantly fewer Symbiodinium cells (p < 0.05) and significantly reduced chlorophyll-a concentrations (p < 0.05), compared with unbleached colonies. Novel Symbiodinium clade associations were observed for this coral in both severely and moderately bleached colonies, with clade C and a mixed clade population detected. In unbleached colonies, only clade B was observed. Levels of δ¹⁵N indicate that severely bleached colonies are utilising heterotrophic feeding mechanisms to aid survival whilst bleached. Collectively, these results suggest that these C. marshae colonies can survive with low symbiont and chlorophyll densities, in response to prolonged thermal stress and extended bleaching, and increase heterotrophic feeding levels sufficiently to meet energy demands, thus enabling some colonies to survive and recover over long time frames. This is significant as it suggests that corals in mesophotic and high-latitude environments may possess considerable plasticity and an ability to tolerate and adapt to large environmental fluctuations, thereby improving their chances of survival as climate change impacts coral ecosystems worldwide.     

First experimental evidence of corals feeding on seagrass matter

We present the first experimental evidence of a coral (Oulastrea crispata) ingesting and assimilating seagrass material. Tropical seagrass meadows export a substantial portion of their productivity and can provide an important source of nutrients to neighbouring systems such as coral reefs; however, little is known about the mechanisms of this link. To investigate whether seagrass nutrient uptake via coral heterotrophy is possible, we conducted a feeding experiment with seagrass particulate and dissolved organic matter. Using gut extractions and stable isotope analyses, we determined that O. crispata ingested ¹⁵N-enriched seagrass particles and assimilated the nitrogen into its tissue at a rate of 0.75 μg N cm^?2 h^?1. Corals took up nitrogen from dissolved matter at a comparable rate of 0.98 μg N cm^?2 h^?1. While other ecological connections between seagrass meadows and reef ecosystems are well known, our results suggest a previously unstudied direct nutritional link between seagrasses and corals.     

Aboveground,belowground biomass and nutrients pool in <Emphasis Type="Italic">Salicornia brachiata</Emphasis> at coastal area of India: interactive effects of soil characteristics

The present study determined the plant biomass (aboveground and belowground) of Salicornia brachiata from six different salt marshes distributed in Indian coastal area over one growing season (September 2014–May 2015). The nutrients concentration and their pools were estimated in plant as well as soil. Belowground biomass in S. brachiata was usually lower than the aboveground biomass. Averaged over different locations, highest biomass was observed in the month of March (2.1 t ha^?1) followed by May (1.64 t ha^?1), February (1.60 t ha^?1), November (0.82 t ha^?1) and September (0.05 t ha^?1). The averaged aboveground to belowground ratio was 12.0. Aboveground and belowground biomass were negatively correlated with pH of soil, while positively with soil electrical conductivity. Further, there were positive relationships between organic carbon and belowground biomass; and available sodium and aboveground biomass. The nutrient pools in aboveground were always higher than to belowground biomass. Aboveground pools of carbon (543 kg ha^?1), nitrogen (48 kg ha^?1), phosphorus (4 kg ha^?1), sodium (334 kg ha^?1) and potassium (37 kg ha^?1) were maximum in the month of March 2015. Bioaccumulation and translocation factors for sodium of S. brachiata were more than one showing tolerance to salinity and capability of phytoremediation for the saline soil.     

Productivity links morphology,symbiont specificity and bleaching in the evolution of Caribbean octocoral symbioses

Many cnidarians host endosymbiotic dinoflagellates from the genus Symbiodinium. It is generally assumed that the symbiosis is mutualistic, where the host benefits from symbiont photosynthesis while providing protection and photosynthetic substrates. Diverse assemblages of symbiotic gorgonian octocorals can be found in hard bottom communities throughout the Caribbean. While current research has focused on the phylo- and population genetics of gorgonian symbiont types and their photo-physiology, relatively less work has focused on biogeochemical benefits conferred to the host and how these benefits vary across host species. Here we examine this symbiosis among 11 gorgonian species collected in Bocas del Toro, Panama. By coupling light and dark bottle incubations (P/R) with ¹³C-bicarbonate tracers, we quantified the link between holobiont oxygen metabolism with carbon assimilation and translocation from symbiont to host. Our data show that P/R varied among species, and was correlated with colony morphology and polyp size. Sea fans and sea plumes were net autotrophs (P/R>1.5), while nine species of sea rods were net heterotrophs with most below compensation (P/R<1.0). ¹³C assimilation corroborated the P/R results, and maximum δ¹³C_host values were strongly correlated with polyp size, indicating higher productivity by colonies with high polyp SA:V. A survey of gorgonian-Symbiodinium associations revealed that productive species maintain specialized, obligate symbioses and are more resistant to coral bleaching, whereas generalist and facultative associations are common among sea rods that have higher bleaching sensitivities. Overall, productivity and polyp size had strong phylogenetic signals with carbon fixation and polyp size showing evidence of trait covariance.     

Feeding increases the number of offspring but decreases parental investment of Red Sea coral Stylophora pistillata

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Chronic low-level nutrient enrichment benefits coral thermal performance in a fore reef habitat

Global- and local-scale anthropogenic stressors have been the main drivers of coral reef decline, causing shifts in coral reef community composition and ecosystem functioning. Excess nutrient enrichment can make corals more vulnerable to ocean warming by suppressing calcification and reducing photosynthetic performance. However, in some environments, corals can exhibit higher growth rates and thermal performance in response to nutrient enrichment. In this study, we measured how chronic nutrient enrichment at low concentrations affected coral physiology, including endosymbiont and coral host response variables, and holobiont metabolic responses of Pocillopora spp. colonies in Mo'orea, French Polynesia. We experimentally enriched corals with dissolved inorganic nitrogen and phosphate for 15 months on an oligotrophic fore reef in Mo'orea. We first characterized symbiont and coral physiological traits due to enrichment and then used thermal performance curves to quantify the relationship between metabolic rates and temperature for experimentally enriched and control coral colonies. We found that endosymbiont densities and total tissue biomass were 54% and 22% higher in nutrient-enriched corals, respectively, relative to controls. Algal endosymbiont nitrogen content cell⁻¹ was 44% lower in enriched corals relative to the control colonies. In addition, thermal performance metrics indicated that the maximal rate of performance for gross photosynthesis was 29% higher and the rate of oxygen evolution at a reference temperature (26.8 °C) for gross photosynthesis was 33% higher in enriched colonies compared to the control colonies. These differences were not attributed to symbiont community composition between corals in different treatments, as C42, a symbiont type in the Cladocopium genus, was the dominant endosymbiont type found in all corals. Together, our results show that in an oligotrophic fore reef environment, nutrient enrichment can cause changes in coral endosymbiont physiology that increase the performance of the coral holobiont.

     

Effects of temperature, light and heterotrophy on the growth rate and budding of the temperate coral Cladocora caespitosa

Recent investigations have shown the temperate scleractinian coral Cladocora caespitosa to be a new potential climate archive for the Mediterranean Sea. Whilst earlier studies have demonstrated a seasonal variation in growth rates, they were unable to distinguish which environmental parameter (light, temperature, or food) was influencing growth. In this study, the effect of these three factors on the coral physiology and calcification rate was characterized to aid the correct interpretation of skeletal trace element variations. Two temperatures (13 and 23°C), irradiances (50 and 120 μmol m⁻² s⁻¹), and feeding regimes (unfed and fed with nauplii of Artemia salina) were tested under controlled laboratory conditions on the growth, zooxanthellae density, chlorophyll (chl) content, and asexual reproduction (budding) of C. caespitosa during a 7-week factorial experiment. Unlike irradiance, which had no effect, high temperature and food supply increased the growth rates of C. caespitosa. The effect of feeding was however higher for corals maintained at low temperature, suggesting that heterotrophy is especially important during the cold season, and that temperature is the predominant factor affecting the coral’s growth. At low temperature, fed samples had higher zooxanthellae density and chl content, possibly for maximizing photosynthetic efficiency. Sexual reproduction investment of C. caespitosa was higher during favourable conditions characterised by high temperatures and zooplankton availability.     

Light Respiratory Processes and Gross Photosynthesis in Two Scleractinian Corals

The light dependency of respiratory activity of two scleractinian corals was examined using O₂ microsensors and CO₂ exchange measurements. Light respiration increased strongly but asymptotically with elevated irradiance in both species. Light respiration in Pocillopora damicornis was higher than in Pavona decussata under low irradiance, indicating species-specific differences in light-dependent metabolic processes. Overall, the coral P. decussata exhibited higher CO₂ uptake rates than P. damicornis over the experimental irradiance range. P. decussata also harboured twice as many algal symbionts and higher total protein biomass compared to P. damicornis, possibly resulting in self-shading of the symbionts and/or changes in host tissue specific light distribution. Differences in light respiration and CO₂ availability could be due to host-specific characteristics that modulate the symbiont microenvironment, its photosynthesis, and hence the overall performance of the coral holobiont.     

A quantitative study of eicosapentaenoic acid (EPA) production by Nannochloropsis gaditana for aquaculture as a function of dilution rate,temperature and average irradiance

Different pilot-scale outdoor photobioreactors using medium recycling were operated in a greenhouse under different environmental conditions and the growth rates (0.1 to 0.5 day^?1) obtained evaluated in order to compare them with traditional systems used in aquaculture. The annualized volumetric growth rate for Nannochloropsis gaditana was 0.26 g l^?1 day^?1 (peak 0.4 g l^?1 day^?1) at 0.4 day^?1 in a 5-cm wide flat-panel bioreactor (FP-PBR). The biomass productivity achieved in this reactor was 10-fold higher than in traditional reactors, reaching values of 28 % and 45 % dry weight (d.w.) of lipids and proteins, respectively, with a 4.3 % (d.w.) content of eicosapentaenoic acid (EPA). A model for predicting EPA productivity from N. gaditana cultures that takes into account the existence of photolimitation and photoinhibition of growth under outdoor conditions is presented. The effect of temperature and average irradiance on EPA content is also studied. The maximum EPA productivity attained is 30 mg l^?1 day^?1.     

Inorganic carbon and pH effect on growth and lipid productivity of Tetraselmis suecica and Chlorella sp (Chlorophyta) grown outdoors in bag photobioreactors

There has been considerable interest in cultivation of green microalgae (Chlorophyta) as a source of lipid that can alternatively be converted to biodiesel. However, almost all mass cultures of algae are carbon-limited. Therefore, to reach a high biomass and oil productivities, the ideal selected microalgae will most likely need a source of inorganic carbon. Here, growth and lipid productivities of Tetraselmis suecica CS-187 and Chlorella sp were tested under various ranges of pH and different sources of inorganic carbon (untreated flue gas from coal-fired power plant, pure industrial CO₂, pH-adjusted using HCl and sodium bicarbonate). Biomass and lipid productivities were highest at pH 7.5 (320?±?29.9 mg biomass L^?1 day^?1and 92?±?13.1 mg lipid L^?1 day^?1) and pH 7 (407?±?5.5 mg biomass L^?1 day^?1 and 99?±?17.2 mg lipid L^?1 day^?1) for T. suecica CS-187 and Chlorella sp, respectively. In general, biomass and lipid productivities were pH 7.5?>?pH 7?>?pH 8?>?pH 6.5 and pH 7?>?pH 7.5?=?pH 8?>?pH 6.5?>?pH 6?>?pH 5.5 for T. suecica CS-187 and Chlorella sp, respectively. The effect of various inorganic carbon on growth and productivities of T. suecica (regulated at pH?=?7.5) and Chlorella sp (regulated at pH?=?7) grown in bag photobioreactors was also examined outdoor at the International Power Hazelwood, Gippsland, Victoria, Australia. The highest biomass and lipid productivities of T. suecica (51.45?±?2.67 mg biomass L^?1 day^?1 and 14.8?±?2.46 mg lipid L^?1 day^?1) and Chlorella sp (60.00?±?2.4 mg biomass L^?1 day^?1 and 13.70?±?1.35 mg lipid L^?1 day^?1) were achieved when grown using CO₂ as inorganic carbon source. No significant differences were found between CO₂ and flue gas biomass and lipid productivities. While grown using CO₂ and flue gas, biomass productivities were 10, 13 and 18 %, and 7, 14 and 19 % higher than NaHCO₃, HCl and unregulated pH for T. suecica and Chlorella sp, respectively. Addition of inorganic carbon increased specific growth rate and lipid content but reduced biomass yield and cell weight of T. suecica. Addition of inorganic carbon increased yield but did not change specific growth rate, cell weight or content of the cell weight of Chlorella sp. Both strains showed significantly higher maximum quantum yield (F_v/F_m) when grown under optimum pH.     

Dissolved inorganic carbon and total alkalinity of a Hawaiian fringing reef: chemical techniques for monitoring the effects of ocean acidification on coral reefs

There is an interest in developing approaches to “ecosystem-based” management for coral reefs. One aspect of ecosystem performance is to monitor carbon metabolism of whole communities. In an effort to explore robust techniques to monitor the metabolism of fringing reefs, especially considering the possible effects of ocean acidification, a yearlong study of the carbonate chemistry of a nearshore fringing reef in Hawaii was conducted. Diurnal changes in seawater carbonate chemistry were measured once a week in an algal-dominated and a coral-dominated reef flat on the Waimanalo fringing reef, Hawaii, from April of 2010 until May of 2011. Calculated rates of gross primary production (GPP) and net community calcification (G) were similar to previous estimates of community metabolism for other coral reefs (GPP 971 mmol C m^?2 d^?1; G 186 mmol CaCO₃ m^?2 d^?1) and indicated that this reef was balanced in terms of organic metabolism, exhibited net calcification, and was a net source of CO₂ to the atmosphere. Average slopes of total alkalinity versus dissolved inorganic carbon (TA–DIC slope) for the coral-dominated reef flat exhibited a greater calcification-to-net photosynthesis ratio than for the algal-dominated reef flat (coral slope vs. algal slope). Over the course of the time series, TA–DIC slopes remained significantly different between sites and were not correlated with diurnal averages in reef-water residence time or solar irradiance. These characteristic slopes for each reef flat reflect the relationship between carbon and carbonate community metabolism and can be used as a tool to monitor ecosystem function in response to ocean acidification.     

Irradiance optimization of outdoor microalgal cultures using solar tracked photobioreactors

Photosynthetic activity and temperature regulation of microalgal cultures (Chlorella vulgaris and Scenedesmus obliquus) under different irradiances controlled by a solar tracker and different cell densities were studied in outdoor flat panel photobioreactors. An automated process control unit regulated light and temperature as well as pH value and nutrient concentration in the culture medium. CO₂ was supplied using flue gas from an attached combined block heat and power station. Photosynthetic activity was determined by pulse amplitude modulation fluorometry. Compared to the horizontal irradiance of 55 mol photons m^?2 d^?1 on a clear day, the solar tracked photobioreactors enabled a decrease and increase in the overall light absorption from 19 mol photons m^?2 d^?1 (by rotation out of direct irradiance) to 79 mol photons m^?2 d^?1 (following the position of the sun). At biomass concentrations below 1.1 g cell dry weight (CDW) L^?1, photoinhibition of about 35 % occurred at irradiances of ≥1,000 μmol photons m^?2 s^?1 photosynthetic active radiation (PAR). Using solar tracked photobioreactors, photoinhibition can be reduced and at optimum biomass concentration (≥2.3 g CDW L^?1), the culture was irradiated up to 2,000 μmol photons m^?2 s^?1 to overcome light limitation with biomass yields of 0.7 g CDW mol photons^?1 and high photosynthetic activities indicated by an effective quantum yield of 0.68 and a maximum quantum yield of 0.80 (F _v/F _m). Overheating due to high irradiance was avoided by turning the PBR out of the sun or using a cooling system, which maintained the temperature close to the species-specific temperature optima.     

Coral overgrowth by an encrusting red alga (Ramicrusta sp.): a threat to Caribbean reefs?

An encrusting red alga (Ramicrusta sp., Peyssonneliaceae) present in Lac Bay, Bonaire, overgrows and kills corals and other sessile organisms. Living coral tissue comprises 7.2 % of the benthic composition of the shallow reef, while Ramicrusta sp. covers 18.7 % of the substratum. Of 1374 coral colonies surveyed, 45.8 % were partially overgrown by Ramicrusta sp., with P. porites, P. astreoides and M. complanata being the most susceptible to overgrowth. Mean Ramicrusta sp. maximum overgrowth rates ± SD were 0.08 ± 0.05 mm d^?1, 0.07 ± 0.03 mm d^?1 and 0.06 ± 0.02 mm d^?1 for M. complanata, P. porites and P. astreoides, respectively. None of the 71 coral recruits surveyed were growing on Ramicrusta sp. Ramicrusta sp. is an immediate threat to corals, reduces the area of suitable substratum for coral settlement and may have the ability to influence coral species composition.     

Relative growth rate,biomass partitioning and nutrient allocation in seedlings of two threatened trees grown under different light conditions

The present study investigates the variation in the relative growth, biomass and nutrient allocation in two threatened tree species viz. Magnolia punduana Hook.f. & Th. and Elaeocarpus prunifolius Wall. ex Müll. Berol. grown under three different levels of irradiance. The irradiance ranged between 1 and 12 mol m^?2 d^?1. Results showed that the highest relative growth rate (RGR) was achieved under the intermediate light treatment for both the species (mean: 0.005 mg mg^?1 d^?1). The growth response coefficient (GRC) model revealed that net assimilation rate (NAR) was the factor driving the RGR in both species. A significant positive correlation was found between NAR and RGR (R² = 0.33, p = 0.000) whereas specific leaf area (SLA) and leaf mass fraction (LMF) was negatively correlated to RGR. Overall, multiple regression of the studied species based on the independent variables viz. NAR, SLA, and LMF showed a significant relation with RGR (F(3,50,53 = 13.001, p = 0.000, R² = 0.43). The biomass distribution in the studied species is in agreement with the “balanced-growth hypothesis” where high irradiance increased allocation to below ground biomass fraction and decreased irradiance increased allocation to the above ground fraction. The highest nitrogen concentration in leaves was observed under the intermediate light treatment. Overall seedlings growth under intermediate light had a higher mean RGR indicating the species' preference for partial light conditions. Long-term experiments under varied light conditions as in the present study would provide useful insight into plant growth strategies in varied environmental conditions.     

Multiple driving factors explain spatial and temporal variability in coral calcification rates on the Bermuda platform

Experimental studies have shown that coral calcification rates are dependent on light, nutrients, food availability, temperature, and seawater aragonite saturation (Ω _arag), but the relative importance of each parameter in natural settings remains uncertain. In this study, we applied Calcein fluorescent dyes as time indicators within the skeleton of coral colonies (n = 3) of Porites astreoides and Diploria strigosa at three study sites distributed across the northern Bermuda coral reef platform. We evaluated the correlation between seasonal average growth rates based on coral density and extension rates with average temperature, light, and seawater Ω _arag in an effort to decipher the relative importance of each parameter. The results show significant seasonal differences among coral calcification rates ranging from summer maximums of 243 ± 58 and 274 ± 57 mmol CaCO₃ m^?2 d^?1 to winter minimums of 135 ± 39 and 101 ± 34 mmol CaCO₃ m^?2 d^?1 for P. astreoides and D. strigosa, respectively. We also placed small coral colonies (n = 10) in transparent chambers and measured the instantaneous rate of calcification under light and dark treatments at the same study sites. The results showed that the skeletal growth of D. strigosa and P. astreoides, whether hourly or seasonal, was highly sensitive to Ω _arag. We believe this high sensitivity, however, is misleading, due to covariance between light and Ω _arag, with the former being the strongest driver of calcification variability. For the seasonal data, we assessed the impact that the observed seasonal differences in temperature (4.0 °C), light (5.1 mol photons m^?2 d^?1), and Ω _arag (0.16 units) would have on coral growth rates based on established relationships derived from laboratory studies and found that they could account for approximately 44, 52, and 5 %, respectively, of the observed seasonal change of 81 ± 14 mmol CaCO₃ m^?2 d^?1. Using short-term light and dark incubations, we show how the covariance of light and Ω _arag can lead to the false conclusion that calcification is more sensitive to Ω _arag than it really is.     

Utilization of cane molasses towards cost-saving astaxanthin production by a Chlorella zofingiensis mutant

The aim of the present study was to survey the growth and astaxanthin production of E17, an astaxanthin-rich mutant of Chlorella zofingiensis, through feeding the low-cost carbon source cane molasses. In heterotrophic batch cultivation, E17 fed with pretreated molasses achieved biomass (1.79 g L^?1 day^?1) and astaxanthin (1.99 mg L^?1 day^?1) productivities comparable to those with glucose, which were about 2- and 2.8-fold of those fed with untreated molasses, respectively. Molasses-induced astaxanthin accumulation may be attributed to the elicited expression of carotenogenic genes, in particular the genes specifically responsible for the ketolation and hydroxylation of β-carotene to form astaxanthin. A two-stage fed-batch strategy was employed to grow E17 and induce astaxathin accumulation, resulting in 45.6 g L^?1 biomass and 56.1 mg L^?1 astaxanthin, the highest volumetric astaxanthin yield ever reported for this alga. In addition, the astaxanthin production by E17 was tested with a semi-continuous culture method, where the directly diluted raw molasses (giving 5 g L^?1 sugar) was used as the carbon source. Little growth inhibition of E17 was observed in the semi-continuous culture with a biomass productivity of 1.33 g L^?1 day^?1 and an astaxanthin productivity of 0.83 mg L^?1 day^?1. The mixotrophic semi-continuous cultures enhanced the biomass and astaxanthin productivities by 29.3 % and 42.2 %, respectively. This study highlights the potential of using the industrially cheap cane molasses towards large-scale cost-saving production of the high-value ketocarotenoid astaxanthin.