Connectivity among Photosystem II centers in phytoplankters: Patterns and responses

The mechanism of nanoparticle toxicity to cyanobacteria

The demand for nanoparticles is increasing tremendously, and so is the risk of their foreseeable discharge into the environment. Nanoparticles contain a variety of features, including anti-microbial properties, and have been shown to have toxic effects on aquatic organisms previously. However, the causes of nanoparticle toxicity under environmental conditions are still unknown. Exposure to nanoparticles in the environment is unavoidable as nanomaterials are used more prevalent in our daily lives, and as a result, nanotoxicity research is gaining traction. To understand the impact of nanoparticle toxicity on aquatic biota, cyanobacteria (blue-green algae) are an ideal model system. The cyanobacteria play an important role in ecological balance, nutrient cycling, energy flow, biological nitrogen fixation, and environmental remediation, and their susceptibility to nanoparticles can help in making a wise strategy for the mitigation of possible nano-pollution. This article presents an analysis of recent research findings on the toxicological influences of nanoparticles on the growth rate, biochemical changes, ultra-structural changes as well as the nanoparticle toxicity mechanisms in cyanobacteria. The finding suggests that the shading effect, generation of reactive oxygen species, membrane damage and disintegration of pigments are the main reasons for nanoparticle toxicity to the cyanobacteria.

Different functional traits among closely related algal symbionts dictate stress endurance for vital Indo-Pacific reef-building corals

Reef-building corals in the genus Porites are one of the most important constituents of Indo-Pacific reefs. Many species within this genus tolerate abnormally warm water and exhibit high specificity for particular kinds of endosymbiotic dinoflagellates that cope with thermal stress better than those living in other corals. Still, during extreme ocean heating, some Porites exhibit differences in their stress tolerance. While corals have different physiological qualities, it remains unknown whether the stability and performance of these mutualisms is influenced by the physiology and genetic relatedness of their symbionts. We investigated two ubiquitous Pacific reef corals, Porites rus and Porites cylindrica, from warmer inshore and cooler offshore reef systems in Palau. While these corals harbored a similar kind of symbiont in the genus Cladocopium (within the ITS2 C15 subclade), rapidly evolving genetic markers revealed evolutionarily diverged lineages corresponding to each Porites species living in each reef habitat. Furthermore, these closely related Cladocopium lineages were differentiated by their densities in host tissues, cell volume, chlorophyll concentration, gross photosynthesis, and photoprotective pathways. When assessed using several physiological proxies, these previously undifferentiated symbionts contrasted in their tolerance to thermal stress. Symbionts within P. cylindrica were relatively unaffected by exposure to 32℃ for 14 days, whereas P. rus colonies lost substantial numbers of photochemically compromised symbionts. Heating reduced the ability of the offshore symbiont associated with P. rus to translocate carbon to the coral. By contrast, high temperatures enhanced symbiont carbon assimilation and delivery to the coral skeleton of inshore P. cylindrica. This study indicates that large physiological differences exist even among closely related symbionts, with significant implications for thermal susceptibility among reef-building Porites.

Toxic and protective mechanisms of cyanobacterium Synechocystis sp. in response to titanium dioxide nanoparticles

An increasing production and use of titanium dioxide nanoparticles (TiO₂ NPs) pose a huge threat to phytoplankton since they are largely released into aquatic environments, which represent a sink for TiO₂ NPs. However, toxicity and protective mechanisms of cyanobacteria in response to TiO₂ NPs remain elusive. Here we investigated toxic effects of two sizes of TiO₂ NPs (50 and 10 nm) and one bulk TiO₂ (200 nm) on a cyanobacterium, Synechocystis sp. and their possible protective mechanisms. We found that 10 nm TiO₂ NPs caused significant growth and photosynthesis inhibition in Synechocystis sp. cells, largely reflected in decreased growth rate (38%), operational PSII quantum yields (40%), phycocyanin (51%) and allophycocyanin (63%), and increased reactive oxygen species content (245%), superoxide dismutase activity (46%). Also, transcriptomic analysis of Synechocystis sp. exposure to 10 nm TiO₂ NPs showed the up-regulation of D1 and D2 protein genes (psbA and psbD), ferredoxin gene (petF) and F-type ATPase genes (e.g., atpB), and the down-regulation of psbM and psb28-2 in PS II. We further proposed a conceptual model to explore possible toxic and protective mechanisms for Synechocystis sp. under TiO₂ nanoparticle exposure. This study provides mechanistic insights into our understanding of Synechocystis sp. responses to TiO₂ NPs. This is essential for more accurate environmental risk assessment approaches of nanoparticles in aquatic ecosystems by governmental environmental agencies worldwide.

Dynamic Photophysiological Stress Response of a Model Diatom to Ten Environmental Stresses

Stressful environmental conditions can induce many different acclimation mechanisms in marine phytoplankton, resulting in a range of changes in their photophysiology. Here we characterize the common photophysiological stress response of the model diatom Thalassiosira pseudonana to ten environmental stressors and identify diagnostic responses to particular stressors. We quantify the magnitude and temporal trajectory of physiological parameters including the functional absorption cross-section of PSII (σ_PSII ), quantum efficiency of PSII, non-photochemical quenching (NPQ), cell volume, Chl a, and carotenoid (Car) content in response to nutrient starvation (nitrogen (N), phosphorus (P), silicon (Si), and iron (Fe)), changes in temperature, irradiance, pH, and reactive oxygen species (ROS) over 5 time points (0, 2, 6, 24, 72 h). We find changes in conditions: temperature, irradiance, and ROS, often result in the most rapid changes in photophysiological parameters (<2 h), and in some cases are followed by recovery. In contrast, nutrient starvation (N, P, Si, Fe) often has slower (6-72 h) but ultimately larger magnitude effects on many photophysiological parameters. Diagnostic changes include large increases in cell volume under Si-starvation, very large increases in NPQ under P-starvation, and large decreases in the σ_PSII under high light. The ultimate goal of this analysis is to facilitate and enhance the interpretation of fluorescence data and our understanding of phytoplankton photophysiology from laboratory and field studies.

Lhcx proteins provide photoprotection via thermal dissipation of absorbed light in the diatom Phaeodactylum tricornutum

Diatoms possess an impressive capacity for rapidly inducible thermal dissipation of excess absorbed energy (qE), provided by the xanthophyll diatoxanthin and Lhcx proteins. By knocking out the Lhcx1 and Lhcx2 genes individually in Phaeodactylum tricornutum strain 4 and complementing the knockout lines with different Lhcx proteins, multiple mutants with varying qE capacities are obtained, ranging from zero to high values. We demonstrate that qE is entirely dependent on the concerted action of diatoxanthin and Lhcx proteins, with Lhcx1, Lhcx2 and Lhcx3 having similar functions. Moreover, we establish a clear link between Lhcx1/2/3 mediated inducible thermal energy dissipation and a reduction in the functional absorption cross-section of photosystem II. This regulation of the functional absorption cross-section can be tuned by altered Lhcx protein expression in response to environmental conditions. Our results provide a holistic understanding of the rapidly inducible thermal energy dissipation process and its mechanistic implications in diatoms.

Photosynthetic organisms can dissipate excess absorbed light energy as heat to avoid photodamage. Here the authors show that induced thermal dissipation in the diatom Phaeodactylum tricornutum Pt4 is Lhcx protein-dependent and correlates with a reduced functional absorption cross-section of photosystem II.

Time-dependent upregulation of electron transport with concomitant induction of regulated excitation dissipation in Haslea diatoms

Photoacclimation by strains of Haslea "blue" diatom species H. ostrearia and H. silbo sp. nov. ined. was investigated with rapid light curves and induction-recovery curves using fast repetition rate fluorescence. Cultures were grown to exponential phase under 50 µmol m^-2 s^-1 photosynthetic available radiation (PAR) and then exposed to non-sequential rapid light curves where, once electron transport rate (ETR) had reached saturation, light intensity was decreased and then further increased prior to returning to near growth light intensity. The non-sequential rapid light curve revealed that ETR was not proportional to the instantaneously applied light intensity, due to rapid photoacclimation. Changes in the effective absorption cross sections for open PSII reaction centres (σ_PSII') or reaction centre connectivity (ρ) did not account for the observed increases in ETR under extended high light. σ_PSII' in fact decreased as a function of a time-dependent induction of regulated excitation dissipation Y(NPQ), once cells were at or above a PAR coinciding with saturation of ETR. Instead, the observed increases in ETR under extended high light were explained by an increase in the rate of PSII reopening, i.e. Q_A^- oxidation. This acceleration of electron transport was strictly light dependent and relaxed within seconds after a return to low light or darkness. The time-dependent nature of ETR upregulation and regulated NPQ induction was verified using induction-recovery curves. Our findings show a time-dependent induction of excitation dissipation, in parallel with very rapid photoacclimation of electron transport, which combine to make ETR independent of short-term changes in PAR. This supports a selective advantage for these diatoms when exposed to fluctuating light in their environment.

Elemental Stoichiometry and Photophysiology Regulation of Synechococcus sp. PCC7002 Under Increasing Severity of Chronic Iron Limitation

Iron (Fe) is an essential cofactor for many metabolic enzymes of photoautotrophs. Although Fe limits phytoplankton productivity in broad areas of the ocean, phytoplankton have adapted their metabolism and growth to survive in these conditions. Using the euryhaline cyanobacterium Synechococcus sp. PCC7002, we investigated the physiological responses to long-term acclimation to four levels of Fe availability representative of the contemporary ocean (36.7, 3.83, 0.47 and 0.047 pM Fe'). With increasing severity of Fe limitation, Synechococcus sp. cells gradually decreased their volume and growth while increasing their energy allocation into organic carbon and nitrogen cellular pools. Furthermore, the total cellular content of pigments decreased. Additionally, with increasing severity of Fe limitation, intertwined responses of PSII functional cross-section (σPSII), re-oxidation time of the plastoquinone primary acceptor QA (τ) and non-photochemical quenching revealed a shift in the photophysiological response between mild to strong Fe limitation compared with severe limitation. Under mild and strong Fe limitation, there was a decrease in linear electron transport accompanied by progressive loss of state transitions. Under severe Fe limitation, state transitions seemed to be largely supplanted by alternative electron pathways. In addition, mechanisms to dissipate energy excess and minimize oxidative stress associated with high irradiances increased with increasing severity of Fe limitation. Overall, our results establish the sequence of physiological strategies adopted by the cells under increasing severity of chronic Fe limitation, within a range of Fe concentrations relevant to modern ocean biogeochemistry.

Nitrogen starvation induces distinct photosynthetic responses and recovery dynamics in diatoms and prasinophytes

Nitrogen stress is an important control on the growth of phytoplankton and varying responses to this common condition among taxa may affect their relative success within phytoplankton communities. We analyzed photosynthetic responses to nitrogen (N) stress in two classes of phytoplankton that often dominate their respective size ranges, diatoms and prasinophytes, selecting species of distinct niches within each class. Changes in photosynthetic structures appeared similar within each class during N stress, but photophysiological and growth responses were more species- or niche-specific. In the coastal diatom Thalassiosira pseudonana and the oceanic diatom T. weissflogii, N starvation induced large declines in photosynthetic pigments and Photosystem II (PSII) quantity and activity as well as increases in the effective absorption cross-section of PSII photochemistry (σ'PSII). These diatoms also increased photoprotection through energy-dependent non-photochemical quenching (NPQ) during N starvation. Resupply of N in diatoms caused rapid recovery of growth and relaxation of NPQ, while recovery of PSII photochemistry was slower. In contrast, the prasinophytes Micromonas sp., an Arctic Ocean species, and Ostreococcus tauri, a temperate coastal eutrophile, showed little change in photosynthetic pigments and structures and a decline or no change, respectively, in σ'PSII with N starvation. Growth and PSII function recovered quickly in Micromonas sp. after resupply of N while O. tauri failed to recover N-replete levels of electron transfer from PSII and growth, possibly due to their distinct photoprotective strategies. O. tauri induced energy-dependent NPQ for photoprotection that may suit its variable and nutrient-rich habitat. Micromonas sp. relies upon both energy-dependent NPQ and a sustained, energy-independent NPQ mechanism. A strategy in Micromonas sp. that permits photoprotection with little change in photosynthetic structures is consistent with its Arctic niche, where low temperatures and thus low biosynthetic rates create higher opportunity costs to rebuild photosynthetic structures.