Fast detection of nutrient limitation in macroalgae and seagrass with nutrient-induced fluorescence

Photoinhibition and Photoprotective Responses of a Brown Marine Macroalga Acclimated to Different Light and Nutrient Regimes

Plants and brown algae avoid photoinhibition (decline in photosystem II efficiency, Fv/Fm) caused by excess light energy and oxidative stress through several photoprotective mechanisms, such as antioxidant xanthophyll production and heat dissipation. The heat dissipation can be measured as non-photochemical quenching (NPQ) and is strongly driven by de-epoxidation of xanthophyll cycle pigments (XCP). Although NPQ is known to increase under high light acclimation and nutrient-deficient conditions, a few studies have investigated the combined effects of the conditions on both NPQ and associated xanthophyll-to-chlorophyll (Chl) a ratio. The present study investigated the photosynthetic parameters of the brown alga Sargassum fusiforme acclimated to three irradiance levels combined with three nutrient levels. Elevated irradiance decreased Fv/Fm but increased NPQ, XCP/Chl a ratio, and fucoxanthin/Chl a ratio, suggesting the photoprotective role of antioxidant fucoxanthin in brown algae. Reduced nutrient availability increased NPQ but had no effect on the other variables, including XCP/Chl a ratio and its de-epoxidation state. The results indicate that NPQ can be used as a sensitive stress marker for nutrient deficiency, but cannot be used to estimate XCP pool size and state.

Photosynthetic electron transport in pitcher plants (Nepenthes mirabilis)

Pitcher plants (Nepenthes sp.) are insectivorous angiosperm plants with modified leaves known as pitchers best known as acting as traps for insects. Pitcher plants are typically found under boggy conditions under both forest cover and open areas with very poor nutrient status, particularly N-status. The pitchers have low photosynthetic activity. The Chl a content of the pitcher tissue of both Nepenthes mirabilis (green and red) varieties was very low. Chl b/a ratios of the green variety phyllodes (lamina) and pitchers were ≈ 0.24 to 0.29. In the red variety, the mature phyllodes had a Chl b/a ratio ≈ 0.28 but both the pitchers and the young phyllodes had Chl b/a ratios of nearly 0.5. Photosynthetic electron transport (ETR) was measured using PAM technology. Phyllodes of both varieties showed photoinhibition at supra-optimal irradiances [Nepenthes mirabilis (green variety), E_opt ≈ 200-250 µmol photon m^-2 s^-1; red variety, E_opt ≈ 100-150 µmol photon m^-2 s^-1]. Pitchers had low optimum irradiances (E_opt ≈ 40-90 µmol photon m^-2 s^-1). Maximum ETR (ETR_max) of phyllodes of both varieties was low (ETR_max ≈ 50 µmol e^- g^-1 Chl a s^-1); ETR_max was higher for pitchers on a Chl a basis (ETR_max ≈ 80-100 µmol e^- g^-1 Chl a s^-1); a consequence of their low Chl a content on a surface area basis. ETR_max of cut disks of phyllodes did not respond strongly to incubation in NH₄⁺, glutamate or aspartate as N-sources but did respond positively to added urea.

Combination of P-limitation and cadmium in photosynthetic responses of the freshwater microalga Ankistrodesmus densus (Chlorophyceae)

In the environment, microalgae are exposed to a multitude of stressors simultaneously, inducing physiological adjustments. It is well documented that both phosphorus (P) limitation and trace metals exposure affect microalgal physiology. However, investigations regarding the combination of both P limitation and excess trace metals still deserve attention. In the present study, we evaluated the changes in photosynthetic parameters in the green microalga Ankistrodesmus densus acclimated to different P concentrations prior to exposure to Cd. Our results indicate that different concentrations of P in the medium were responsible for significant changes in some parameters, especially those related to photoprotection mechanisms. Cadmium also altered some of these variables in all P scenarios, and greater damage (i.e., synergism) was observed in the combination P-limited and high Cd, with all the evaluated parameters affected under the adverse scenario. Among the parameters analyzed, rapid light curves were the most sensitive to exposure of one or the combination of both stressors (Cd and P limitation). Based on our data, we suggest that P-limited algae activated photoprotective mechanisms as a response to nutrient limitation, especially at the most limited condition. The addition of Cd did not change linearly the parameters related to photoprotection mechanisms under P-limitation, i.e., synergism was observed in the intermediate P-limitation combined with Cd, while in the most P-limited, P seems to be the driving force affecting these mechanisms. Based on our results, we suggest the use of rapid light curves as a tool to complement the assessment of the impacts of stressors, such as metals, in ecotoxicological studies.

Microbial Community Structure and Associations During a Marine Dinoflagellate Bloom

Interactions between microorganisms and algae during bloom events significantly impacts their physiology, alters ambient chemistry, and shapes ecosystem diversity. The potential role these interactions have in bloom development and decline are also of particular interest given the ecosystem impacts of algal blooms. We hypothesized that microbial community structure and succession is linked to specific bloom stages, and reflects complex interactions among taxa comprising the phycosphere environment. This investigation used pyrosequencing and correlation approaches to assess patterns and associations among bacteria, archaea, and microeukaryotes during a spring bloom of the dinoflagellate Alexandrium catenella. Within the bacterial community, Gammaproteobacteria and Bacteroidetes were predominant during the initial bloom stage, while Alphaproteobacteria, Cyanobacteria, and Actinobacteria were the most abundant taxa present during bloom onset and termination. In the archaea biosphere, methanogenic members were present during the early bloom period while the majority of species identified in the late bloom stage were ammonia-oxidizing archaea and Halobacteriales. Dinoflagellates were the major eukaryotic group present during most stages of the bloom, whereas a mixed assemblage comprising diatoms, green-algae, rotifera, and other microzooplankton were present during bloom termination. Temperature and salinity were key environmental factors associated with changes in bacterial and archaeal community structure, respectively, whereas inorganic nitrogen and inorganic phosphate were associated with eukaryotic variation. The relative contribution of environmental parameters measured during the bloom to variability among samples was 35.3%. Interaction analysis showed that Maxillopoda, Spirotrichea, Dinoflagellata, and Halobacteria were keystone taxa within the positive-correlation network, while Halobacteria, Dictyochophyceae, Mamiellophyceae, and Gammaproteobacteria were the main contributors to the negative-correlation network. The positive and negative relationships were the primary drivers of mutualist and competitive interactions that impacted algal bloom fate, respectively. Functional predictions showed that blooms enhance microbial carbohydrate and energy metabolism, and alter the sulfur cycle. Our results suggest that microbial community structure is strongly linked to bloom progression, although specific drivers of community interactions and responses are not well understood. The importance of considering biotic interactions (e.g., competition, symbiosis, and predation) when investigating the link between microbial ecological behavior and an algal bloom's trajectory is also highlighted.

Nitrogen and phosphorus uptake rates of different species from a coral reef community after a nutrient pulse

Terrestrial runoff after heavy rainfall can increase nutrient concentrations in waters overlying coral reefs that otherwise experience low nutrient levels. Field measurements during a runoff event showed a sharp increase in nitrate (75-fold), phosphate (31-fold) and ammonium concentrations (3-fold) in waters overlying a fringing reef at the island of Curaçao (Southern Caribbean). To understand how benthic reef organisms make use of such nutrient pulses, we determined ammonium, nitrate and phosphate uptake rates for one abundant coral species, turf algae, six macroalgal and two benthic cyanobacterial species in a series of laboratory experiments. Nutrient uptake rates differed among benthic functional groups. The filamentous macroalga Cladophora spp., turf algae and the benthic cyanobacterium Lyngbya majuscula had the highest uptake rates per unit biomass, whereas the coral Madracis mirabilis had the lowest. Combining nutrient uptake rates with the standing biomass of each functional group on the reef, we estimated that the ammonium and phosphate delivered during runoff events is mostly taken up by turf algae and the two macroalgae Lobophora variegata and Dictyota pulchella. Our results support the often proposed, but rarely tested, assumption that turf algae and opportunistic macroalgae primarily benefit from episodic inputs of nutrients to coral reefs.

Effect of light and nutrient availability on the release of dissolved organic carbon (DOC) by Caribbean turf algae

Turf algae increasingly dominate benthic communities on coral reefs. Given their abundance and high dissolved organic carbon (DOC) release rates, turf algae are considered important contributors to the DOC pool on modern reefs. The release of photosynthetically fixed carbon as DOC generally, but not always, increases with increased light availability. Nutrient availability was proposed as an additional factor to explain these conflicting observations. To address this proposed but untested hypothesis, we documented the interactive contributions of light and nutrient availability on the release of DOC by turf algae. DOC release rates and oxygen production were quantified in incubation experiments at two light levels (full and reduced light) and two nutrient treatments (natural seawater and enriched seawater). In natural seawater, DOC release at full light was four times higher than at reduced light. When nutrients were added, DOC release rates at both light levels were similar to the natural seawater treatment at full light. Our results therefore show that low light in combination with low nutrient availability reduces the release of DOC by turf algae and that light and nutrient availability interactively determine DOC release rates by this important component of Caribbean reef communities.

Rapid Accumulation of Total Lipid in Rhizoclonium africanum Kutzing as Biodiesel Feedstock under Nutrient Limitations and the Associated Changes at Cellular Level

Increase of total lipid and the proportion of the favorable fatty acids in marine green filamentous macroalga Rhizoclonium africanum (Chlorophyceae) was studied under nitrate and phosphate limitations. These stresses were given by both eliminating and doubling the required amounts of nitrate and phosphate salts in the growth media. A significant twofold increase in total lipid (193.03 mg/g) was achieved in cells in absence of nitrate in the culture medium, followed by phosphate limitation (142.65 mg/g). The intracellular accumulation of neutral lipids was observed by fluorescence microscopy. The scanning electron microscopic study showed the major structural changes under nutrient starvation. Fourier transform infrared spectroscopy (FTIR) revealed the presence of ester (C-O-C stretching), ketone (C-C stretching), carboxylic acid (O-H bending), phosphine (P-H stretching), aromatic (C-H stretching and bending), and alcohol (O-H stretching and bending) groups in the treated cells indicating the high accumulation of lipid hydrocarbons in the treated cells. Elevated levels of fatty acids favorable for biodiesel production, that is, C16:0, C16:1, C18:1, and C20:1, were identified under nitrate- and phosphate-deficient conditions. This study shows that the manipulation of cultural conditions could affect the biosynthetic pathways leading to increased lipid production while increasing the proportion of fatty acids suitable for biodiesel production.

Coral mucus fuels the sponge loop in warm- and cold-water coral reef ecosystems

Shallow warm-water and deep-sea cold-water corals engineer the coral reef framework and fertilize reef communities by releasing coral mucus, a source of reef dissolved organic matter (DOM). By transforming DOM into particulate detritus, sponges play a key role in transferring the energy and nutrients in DOM to higher trophic levels on Caribbean reefs via the so-called sponge loop. Coral mucus may be a major DOM source for the sponge loop, but mucus uptake by sponges has not been demonstrated. Here we used laboratory stable isotope tracer experiments to show the transfer of coral mucus into the bulk tissue and phospholipid fatty acids of the warm-water sponge Mycale fistulifera and cold-water sponge Hymedesmia coriacea, demonstrating a direct trophic link between corals and reef sponges. Furthermore, 21–40% of the mucus carbon and 32–39% of the nitrogen assimilated by the sponges was subsequently released as detritus, confirming a sponge loop on Red Sea warm-water and north Atlantic cold-water coral reefs. The presence of a sponge loop in two vastly different reef environments suggests it is a ubiquitous feature of reef ecosystems contributing to the high biogeochemical cycling that may enable coral reefs to thrive in nutrient-limited (warm-water) and energy-limited (cold-water) environments.