Enhancement of diatom growth and phytoplankton productivity with reduced O2 availability is moderated by rising CO2

Many marine organisms are exposed to decreasing O2 levels due to warming-induced expansion of hypoxic zones and ocean deoxygenation (DeO2). Nevertheless, effects of DeO2 on phytoplankton have been neglected due to technical bottlenecks on examining O2 effects on O2-producing organisms. Here we show that lowered O2 levels increased primary productivity of a coastal phytoplankton assemblage, and enhanced photosynthesis and growth in the coastal diatom Thalassiosira weissflogii. Mechanistically, reduced O2 suppressed mitochondrial respiration and photorespiration of T. weissflogii, but increased the efficiency of their CO2 concentrating mechanisms (CCMs), effective quantum yield and improved light use efficiency, which was apparent under both ambient and elevated CO2 concentrations leading to ocean acidification (OA). While the elevated CO2 treatment partially counteracted the effect of low O2 in terms of CCMs activity, reduced levels of O2 still strongly enhanced phytoplankton primary productivity. This implies that decreased availability of O2 with progressive DeO2 could boost re-oxygenation by diatom-dominated phytoplankton communities, especially in hypoxic areas, with potentially profound consequences for marine ecosystem services in coastal and pelagic oceans.

Domoic Acid and Pseudo-nitzschia spp. Connected to Coastal Upwelling along Coastal Inhambane Province, Mozambique: A New Area of Concern

Harmful algal blooms (HABs) are increasing globally in frequency, persistence, and geographic extent, posing a threat to ecosystem and human health. To date, no occurrences of marine phycotoxins have been recorded in Mozambique, which may be due to absence of a monitoring program and general awareness of potential threats. This study is the first documentation of neurotoxin, domoic acid (DA), produced by the diatom Pseudo-nitzschia along the east coast of Africa. Coastal Inhambane Province is a biodiversity hotspot where year-round Rhincodon typus (whale shark) sightings are among the highest globally and support an emerging ecotourism industry. Links between primary productivity and biodiversity in this area have not previously been considered or reported. During a pilot study, from January 2017 to April 2018, DA was identified year-round, peaking during Austral winter. During an intense study between May and August 2018, our research focused on identifying environmental factors influencing coastal productivity and DA concentration. Phytoplankton assemblage was diatom-dominated, with high abundances of Pseudo-nitzschia spp. Data suggest the system was influenced by nutrient pulses resulting from coastal upwelling. Continued and comprehensive monitoring along southern Mozambique would provide critical information to assess ecosystem and human health threats from marine toxins under challenges posed by global change.

Recovery Responses of Freshwater Phytoplankton Assemblage After the Disappearance of Metal Toxicity in Semi-Continuous Cultures

The present study demonstrates the recovery of phytoplankton assemblage from metal stress. Phytoplankton assemblage consisting of different freshwater algal species isolated from a tropical pond was exposed to sublethal concentrations of Cu and Zn for 25 days in semi-continuous culture (Toxicity phase). Subsequently, algal assemblage grown in the toxicity phase were transferred to the fresh culture medium without elevated levels of the test metals for 25 days in semi-continuous culture (Recovery phase). We monitored the total biovolume of each algal species during the toxicity and recovery phases. The members of Cyanophyta were most sensitive against metal toxicity, followed by the members of Bascillariophyta. However, the members of Chlorophyta showed relatively lesser sensitivity against test metals. Among chlorophytes, Scendesmus opolinensis and Cosmarium bioculatum were tolerant to both the test metals. Metal-stressed algal species showed recovery after transferring to the basal medium depending on the concentration of metals during the toxicity phase. The members of Cyanophyta were unable to recover from metal stress. However, members of Chlorophyta showed faster recovery than others from Zn stress, and the members of Bascillariophyta showed quicker recovery from Cu stress. The differential abilities of various algal species to recover from metal stress perhaps depend on their ability to counterbalance metal toxicity. Further, research is warranted to characterize the differential ability of various algal groups to recover from metal stress. The present findings would help understand the efficiency of different algal groups to restore their position within the freshwater algal community after the disappearance for metal stress.

Recovery Responses of Freshwater Phytoplankton Assemblage After the Disappearance of Metal Toxicity in Semi-Continuous Cultures

The present study demonstrates the recovery of phytoplankton assemblage from metal stress. Phytoplankton assemblage consisting of different freshwater algal species isolated from a tropical pond was exposed to sublethal concentrations of Cu and Zn for 25 days in semi-continuous culture (Toxicity phase). Subsequently, algal assemblage grown in the toxicity phase were transferred to the fresh culture medium without elevated levels of the test metals for 25 days in semi-continuous culture (Recovery phase). We monitored the total biovolume of each algal species during the toxicity and recovery phases. The members of Cyanophyta were most sensitive against metal toxicity, followed by the members of Bascillariophyta. However, the members of Chlorophyta showed relatively lesser sensitivity against test metals. Among chlorophytes, Scendesmus opolinensis and Cosmarium bioculatum were tolerant to both the test metals. Metal-stressed algal species showed recovery after transferring to the basal medium depending on the concentration of metals during the toxicity phase. The members of Cyanophyta were unable to recover from metal stress. However, members of Chlorophyta showed faster recovery than others from Zn stress, and the members of Bascillariophyta showed quicker recovery from Cu stress. The differential abilities of various algal species to recover from metal stress perhaps depend on their ability to counterbalance metal toxicity. Further, research is warranted to characterize the differential ability of various algal groups to recover from metal stress. The present findings would help understand the efficiency of different algal groups to restore their position within the freshwater algal community after the disappearance for metal stress.

Effects of Grazing and Nutrients on Phytoplankton Blooms and Microplankton Assemblage Structure in Four Temperate Lakes Spanning a Eutrophication Gradient

Phytoplankton assemblage dynamics are sensitive to biotic and abiotic factors, as well as anthropogenic stressors such as eutrophication, and thus are likely to vary between lakes of differing trophic state. We selected four lakes in Washington State, USA, ranging from oligo- to hypereutrophic, to study the separate and interactive effects of enhanced nutrient availability and zooplankton grazing on phytoplankton net growth rates and overall microplankton (phytoplankton and microzooplankton) assemblage structure. We collected water quality and plankton samples monthly in each lake from May to October 2014, and also conducted laboratory incubation experiments using ambient plankton assemblages from each lake with amendments of zooplankton grazers (5× ambient densities) and nutrients (Nitrogen + Phosphorus) in June, August, and October. In each set of monthly experiments, nested two-way ANOVAs were used to test the effects of enhanced grazers and nutrients on net chlorophyll a-based phytoplankton growth rates. Nested PERMANOVAs were used to test the effects of each factor on microplankton assemblage structure. Enhanced grazing reduced phytoplankton net growth in oligotrophic Cle Elum Lake and oligo-mesotrophic Lake Merwin in August (p < 0.001) and Merwin again in October (p < 0.05), while nutrient enhancement increased phytoplankton net growth in Lake Merwin in June (p < 0.01). Changes in microplankton assemblage composition were not detected as a result of either factor, but they were significantly different between sites (p < 0.001) during each month, and varied by month within each lake. Significant effects of both enhanced grazers and nutrients were detected in systems of low, but not high, trophic state, although this varied by season. We suggest that it is critical to consider trophic state when predicting the response of phytoplankton to bottom-up and top-down factors in lakes.

The Combined Effects of Increased pCO2 and Warming on a Coastal Phytoplankton Assemblage: From Species Composition to Sinking Rate

In addition to ocean acidification, a significant recent warming trend in Chinese coastal waters has received much attention. However, studies of the combined effects of warming and acidification on natural coastal phytoplankton assemblages here are scarce. We conducted a continuous incubation experiment with a natural spring phytoplankton assemblage collected from the Bohai Sea near Tianjin. Experimental treatments used a full factorial combination of temperature (7 and 11°C) and pCO2 (400 and 800 ppm) treatments. Results suggest that changes in pCO2 and temperature had both individual and interactive effects on phytoplankton species composition and elemental stoichiometry. Warming mainly favored the accumulation of picoplankton and dinoflagellate biomass. Increased pCO2 significantly increased particulate organic carbon to particulate organic phosphorus (C:P) and particulate organic carbon to biogenic silica (C:BSi) ratios, and decreased total diatom abundance; in the meanwhile, higher pCO2 significantly increased the ratio of centric to pennate diatom abundance. Warming and increased pCO2 both greatly decreased the proportion of diatoms to dinoflagellates. The highest chlorophyll a biomass was observed in the high pCO2, high temperature phytoplankton assemblage, which also had the slowest sinking rate of all treatments. Overall, there were significant interactive effects of increased pCO2 and warming on dinoflagellate abundance, pennate diatom abundance, diatom vs. dinoflagellates ratio and the centric vs. pennate ratio. These findings suggest that future ocean acidification and warming trends may individually and cumulatively affect coastal biogeochemistry and carbon fluxes through shifts in phytoplankton species composition and sinking rates.

Temporal variation revealed the composition change of phytoplankton assemblage responds to varied environmental indicators within an artificial freshwater engineered ecosystem

Artificial freshwater engineering ecosystems (AFwEEs) have attracted more and more attention. Phytoplankton is critical to the fluctuation of water quality in the AFwEEs. However, there is still a major knowledge gap regarding the ecology, composition, and temporal dynamics of phytoplankton assemblage composition in AFwEEs. Hence, an AFwEEs designed base on the submerged macrophytes (i.e., Vallisneria natans, Najas marina, and Potamogeton crispus) and fishes (i.e., Hypophthalmichthys molitrix, Hypophthalmichthys nobilis, and Ctenopharyngodon idellus) was established. The initial purpose for this survey was to conduct almost entire year (April/2019 – December/2019) sampling investigation to explain the interactions and relationship between multifarious environmental indicators and phytoplankton, also for the different phenomenon explanation of composition change in phytoplankton assemblage between AFwEEs and natural water environments. Consequences exhibited the assemblage dynamics of the phytoplankton were significant varied with seasonal succession, in which Cyanophyta dominated the phytoplankton assemblage while the mean relative abundance accounted for 66.4%, and then was Chlorophyta, for 18.8%. Principal component analysis (PCA) demonstrated temporal variation was significant impacted on phytoplankton assemblages. The dominant species, different assemblages, and distribution of phytoplankton were closely related to the dynamic changes of seasonal succession, which might change the composition of phytoplankton assemblage in the FwAEEs. Results of redundancy analysis (RDA) revealed the water temperature, nutrient concentration, DO concentration, and pH value were the main abiotic environmental indicators affecting phytoplankton assemblage. The trend of the four diversity indices indicated the phytoplankton assemblage became more stable in winter while the change extent of that was more significant in summer.