Filamentous epiphyte outbreaks, a serious problem in the commercial cultivation of Kappaphycus/Eucheuma, are often triggered by large changes in abiotic factors. However, the physiological characteristics of epiphytes are poorly understood. In this study, the main pigment contents of Neosiphonia savatieri and its healthy host strains, including green thalli of Kappaphycus striatum (G-KS), brown and green thalli of Kappaphycus alvarezii (B-KA, G-KA), were quantified. The light absorption capacities of lipid- and water-soluble pigments of these seaweeds were also measured. Photosynthetic parameters, including pigment content and fast chlorophyll a fluorescence kinetics, were further studied in N. savatieri exposed to different salinities (16–34 psu) for 10 h in an outdoor experiment. The phycobiliprotein (PBP), Chl-a, and carotenoid contents of N. savatieri were comparatively higher than the hosts, with the values of 2.50, 0.29, and 0.12 mg g−1 (fw), respectively. The mean extinction coefficients of lipid- and water-soluble pigments in N. savatieri were both higher than G-KS, B-KA, and G-KA in the 400–700 nm bands. Salinity of 28 psu increased the pigment contents of the epiphyte, and 16–22 psu did not have a negative impact. Polyphasic fluorescence transients revealed that salinities at 22–28 psu were beneficial to photosynthetic electron transport, mainly attributed to the enhancement of energy absorption per active PSII reaction centre. Given these observations, N. savatieri has a higher light-harvesting capacity and better tolerance to low salinities. These two physiological properties made N. savatieri well suited to the low light and hypo-saline conditions that are frequently triggered by fluctuations in abiotic factors.

Photorespiration inChlorella protothecoidesplays an important role in photoprotection of photosystem (PS) II in the late phase of H2photoproduction, allowing PSII to supply more electrons to hydrogenase.

Arthrospira (Spirulina) is widely used as human health food and animal feed. In cultures grown outdoors in open ponds, Arthrospira cells are subjected to various environmental stresses, such as high temperature. A better understanding of the effects of high temperature on photosynthesis may help optimize the productivity of Arthrospira cultures. In this study, the effects of heat stress on photosynthetic rate, chlorophyll a fluorescence transients, and photosystem (PS) II, PSI activities in a marine cyanobacterium Arthrospira sp. were examined. Arthrospira cells grown at 25 °C were treated for 30 min at 25 (control), 30, 34, 37, or 40 °C in the dark. Heat stress (30–37 °C) enhanced net photosynthetic O2 evolution rate. Heat stress caused over-reduction PSII acceptor side, damage of donor side of PSII, decrease in the energetic connectivity of PSII units, and decrease in the performance of PSII. When the temperature changed from 25 to 37 °C, PSII activity decreased, while PSI activity increased, the enhancement of photosynthetic O2 evolution was synchronized with the increase in PSI activity. When temperature was further increased to 40 °C, it induced a decrease in photosynthetic O2 evolution rate and a more severe decrease in PSII activity, but an increase in PSI activity. These results suggest that PSI activity was the decisive factor determining the change of photosynthetic O2 evolution when Arthrospira was exposed to a temperature from 25 to 37 °C, but then, PSII activity became the decisive factor adjusting the change of photosynthetic O2 evolution when the temperature was increased to 40 °C.

•The hydrogenase of Chlorella pyrenoidosa could tolerate a low concentration of O2.•Nitrogen (N) deprivation lowers the photosynthetic activities of C. pyrenoidosa.•N deprivation significantly enhances H2 photoproduction of C. pyrenoidosa.•Pre-culture in N-deprived medium severely inhibits the photosynthetic activities.•Pre-culture in N-deprived medium increases the H2 yield of C. pyrenoidosa further.Previously, we found that marine Chlorella pyrenoidosa strain IOAC707S could generate hydrogen (H2) under nitrogen deprivation in seawater medium, without dark incubation, N2 flushing or addition of protonophore. In this work, the H2 yield of C. pyrenoidosa was enhanced, and the mechanism of H2 photoproduction was investigated. It was found that C. pyrenoidosa could produce H2 under hypoxic conditions, which indicated the hydrogenase of C.pyrenoidosa could tolerate a low concentration of O2. Nitrogen deprivation could lower the efficiency of photosyntem (PS) II photochemical activity and PSII oxygenic activities, which were favorable for rapid establishment of anoxia and efficient H2 photoproduction, and thus significantly enhance H2 photoproduction of C. pyrenoidosa. Pre-culture in nitrogen-deprived medium could further increase the H2 yield of C. pyrenoidosa, for its more severe inhibition of the oxygen-evolving complex, electron transport and photochemical efficiency of PSII, and PSII oxygenic activities, which facilitated the establishment of anaerobiosis and activation of hydrogenase.

The AOX pathway inC.protothecoidesplays an important role in the photoprotection of PSII by alleviating the inhibition of the repair of the photodamaged PSII during H2 photoproduction.We had demonstrated that nitrogen limitation (LN) substantially enhanced H2 photoproduction in Chlorella protothecoides. In the present study, the mitochondrial alternative oxidase (AOX) pathway capacity was found to increase significantly during H2 photoproduction under LN or under LN simultaneously with sulfur deprivation (LNS) conditions. The purpose of this study was to clarify the role of the AOX pathway during H2 photoproduction in C. protothecoides. The AOX pathway can affect H2 photoproduction in the following ways: (1) consuming O2, which is favorable for the establishment of anaerobiosis; (2) consuming NADPH and competing with hydrogenase for photosynthetic electrons, which would decrease the H2 photoproduction; (3) protecting photosystem (PS) II, which is a direct electron source for H2 photoproduction, from photoinhibition. In LN and LNS cultures, the inhibition of the AOX pathway reduced the H2 photoproduction significantly, and did not increase the amount of O2. But, the inhibition of the AOX pathway decreased the maximal photochemical efficiency of PSII (Fv/Fm) and the actual photochemical efficiency of PSII (ΦPSII) significantly, leading to photoinhibition, which would decrease the photosynthetic electrons transferred to hydrogenase. And, the inhibition of the AOX pathway did not change the level of photoinhibition in the presence of D1 protein synthesis inhibitor chloramphenicol, indicating that the inhibition of the AOX pathway did not accelerate the photodamage to PSII directly but inhibited the repair of the photodamaged PSII. Therefore, the mitochondrial AOX pathway in C. protothecoides plays an important role in the photoprotection of PSII by alleviating the inhibition of the repair of the photodamaged PSII during H2 photoproduction, which is thus able to supply more electrons to hydrogenase under LN and LNS conditions.

•Nitrogen limitation induced photoproduction of H2 efficiently in Chlorella protothecoides.•N limitation rather than S deprivation was favorable for the H2 photoproduction.•Sulfur deprivation enhanced H2 photoproduction in N-limited C. protothecoides.•N limitation and S deprivation enhanced H2 production due to enhanced electron sources.The aim of this study was to understand the enhancement mechanism of H2 photoproduction in Chlorella protothecoides under simultaneous nitrogen limitation and sulfur deprivation (LNS). Nitrogen limitation (LN) rather than sulfur deprivation significantly inhibited relative variable fluorescence at K-step (WK) and J-step (VJ), photochemical efficiency of PSII (photosystem II), Fv/Fm, during the process of incubation in the light. Under such conditions, photosynthetic O2 evolution decreased and the anaerobiosis was established after 12 h of incubation. The algae generated large amounts of H2 under nitrogen limitation but generated only trace amounts under sulfur deprivation. Obviously, nitrogen limitation rather than sulfur deprivation was the decisive factor that induced H2 photoproduction in C. protothecoides under LNS. The LNS culture generated much more H2 than the LN culture in the presence of DCMU during incubation, suggesting that a PSII-independent electron source contributed many more electrons for transfer to hydrogenase in the LNS culture. PSII electron transport includes linear electron flow (LEF) and cyclic electron flow (CEF) of PSII in C. protothecoides. In the PSII-dependent electron source for H2 photoproduction, PSII supplies electrons to hydrogenase through the LEF. The LNS culture showed much higher LEF and lower CEF than the LN culture during the H2 photoproduction phase, as indicated by the large lower quantum yield of PSII electron transport (ΦPSII) in the LNS culture in the presence of DCMU. Therefore, compared with nitrogen limitation, simultaneous nitrogen limitation and sulfur deprivation enhanced H2 photoproduction in C. protothecoides mainly due to enhanced PSII-dependent and -independent electron sources.

H2 photoproduction, hydrogenase activities and PSII photochemical activities in Chlorella protothecoides under sulfur (S–) or nitrogen (N–) deprivation or simultaneous N-limitation and S-deprivation were studied. C. protothecoides pre-cultured in full nutrient TAP medium containing 7 mM NH4Cl was found to produce a detectable but low level of H2, once the cells were inoculated either in S-free or N-free medium. However, cells pre-grown in a low concentration of NH4Cl (0.35 and 0.7 mM) generated a large amount of H2 after transfer to N-limited and S-free medium. The maximal H2 outputs of ∼233.7 and ∼129.1 ml/l were obtained within 100 h in the cultures exposed to S-deprived medium containing 0.35 mM and 0.7 mM NH4Cl, with the average H2 production rates being ∼2.19 and ∼1.37 ml/l/h, respectively. Our studies further indicated that N-limitation resulted in considerable starch accumulation, chlorophyll synthesis reduction, photosynthetic electron transfer block and oxygen evolving complex (OEC) injury, as well as attenuation in PSII oxygenic activity. Significant starch degradation was not observed during the H2 photoevolution process. Attenuation of PSII O2 evolution favored a rapid establishment of anaerobiosis for hydrogenase induction. Meanwhile, a constant high level of hydrogenase activities in C. protothecoides exposed to simultaneous N-limitation and S-deprivation were measured. Based on the above results, a possible mechanism of high H2 photoproduction in C. protothecoides exposed to N-limitation and S-deprivation was discussed. Low net photosynthetic oxygenic rates, together with high hydrogenase activities were thought to contribute to the enhancement of H2 photoproduction by C. protothecoides.Highlights▶ H2 production in Chlorella protothecoides was enhanced under N-limitation and S-deprivation. ▶ The optimal nitrogen concentration to attain maximal H2 yield was 0.35 mM NH4Cl. ▶ N-limitation leads to reduced PSII oxygenic activity favored anoxia establishment.

Water samples from various marine, freshwater and terrestrial environments in China have been collected and screened in our laboratory for wild clones of microalgae since 2008. Hundreds of microalgal clones were purified through plate streaking, of which 52 strains have been screened for their ability to produce H2 under anaerobic conditions. Twenty eight strains including 13 freshwater green algae, 12 marine green algae and 3 cyanobacteria were found to evolve H2 under sulfur or nitrogen deprivation. Parietochloris incisa, Chlorella protothecoides, Chlorella capsulata, Nannochloropsis sp., Tetraselmis helgolandica, Pyramimonas sp., Chlorella autotrophica, Dunaliella apiculata, Tetraselmis striata and Tetraselmis tetrathele were first reported as new H2 producing species. Among those H2 producing strains, freshwater C. protothecoides showed the highest H2 producing capacity (2.93 H2 ml/l/h) and accumulated up to 123.6 ml H2 per liter culture under sulfur deprivation. Marine C. autotrophica and T. striata were two notable green marine species that produced H2 phototrophically, with 3.73 ml/l and 20.11 ml/l of H2 yield respectively. Our results also demonstrated that both the likelihood of a strain producing H2, and the average H2 output, in freshwater strains were much higher than that in marine strains. In addition, the time for inducing H2 production in freshwater strains was short compared to the marine ones. All three Arthrospira strains tested were able to generate H2 under sulfur (S-) deprivation though the cultures would quickly lose their vitality in S-deprived medium. Interestingly, Arthrospira produced H2 under nitrogen deprivation, and the H2 yield was even higher than under S-depletion. However, neither tested chrysophyte nor diatom produced any H2 under anaerobiosis caused by S-deprivation.Highlights► Ten species of microalgae as novel H2 production species were first reported. ► Chlorella protothecoides is a high H2 yield freshwater strain in sulfur starvation. ► Chlorella autotrophica and Tetraselmis striata are marine strains photoproduce H2. ► Arthrospira generated more H2 under nitrogen deprivation than sulfur depletion. ► No tested chrysophyta and diatom produced H2 under sulfur-deprived condition.

Neosiphonia savatieri, a filamentous red alga, had spread and caused a massive death of its host Kappaphycus alvarezii since March 2009 in China. With an aim to found a specific method to eliminate the N. savatieri efficiently from carrageenan producing K. alvarezii, the effects of glyphosate on the photosynthetic behaviors of K. alvarezii and N. savatieri were comparatively studied by using fast chlorophyll a (Chl a) fluorescence kinetics. A dose- and time-dependent changes of fast Chl a fluorescence kinetics were obtained in N. savatieri treated by glyphosate, meanwhile no significant change was detected in the K. alvarezii under the same treatment conditions. Moreover, the maximum PSII photochemical efficiency for dark-adapted tissues (FV/Fm) of N. savatieri decreased significantly when the N. savatieri was treated with glyphosate. Above results were further supported by transitory offshore glyphosate soak experiment. The brownish-red N. savatieri turned to be olivine then drew off within 5 days after soaking in >1 g L−1 of glyphosate for more than 1 min, meanwhile, no visible harmful effects were detected on K. alvarezii. Based on above results, glyphosate is suggested to be an effective chemical to eliminate N. savatieri from K. alvarezii.

•Thirty-seven microalgal strains are identified and compared based on lipid content, growth rate and biomass production.•The average lipid content of genus Isochrysis is the highest among the tested genera.•A taxonomic pattern of lipid accumulation is found in microalgae.•No significant correlation is found between lipid content and growth rate or biomass production.•Nannochloropsis maritime and Isochrysis galbana are promising candidates for biodiesel feedstock.Microalgae are considered to be a potential alternative source for the production of biofuels, and isolation of microalgae capable of high lipid concentration and high biomass production is the foundation for microalgal biofuel development. To date, over 1200 microalgal clones have been isolated in our laboratory. Among them, 37 strains were identified and further compared based on total lipid content, growth rate and biomass production. From the ITS1-5.8S-ITS2 sequences and morphological characteristics, these strains were identified as belonging to the genera Scenedesmus, Chlorella, Stichococcus, Nannochloropsis, Tetraselmis, Isochrysis, Phaeodactylum and Cylindrotheca. The lipid content of these strains varied from 6% dry weight (dw) to 42% dw. All three Isochrysis galbana strains could accumulate lipid at more than 35% dw, and their average was the highest among the tested genera (P < 0.05), followed by genera Nannochloropsis and Chlorella. Based on the phylogenetic data, a taxonomic pattern of lipid accumulation was found in microalgae, which suggested that a genera strategy should be considered in screening for lipid-rich microalgal strains. A comprehensive consideration of growth rate, biomass production and lipid content indicated that marine species Nannochloropsis maritima strain IOAC710S, and I. galbana strains IOAC683S and IOAC724S could be promising candidates for biodiesel feedstock.

•Inhibition of the photorespiratory pathway did not change dry weight accumulation, chlorophyll content or OJIP transients.•The inhibition of the photorespiratory pathway decreased the photochemical activity of photosystem II (PSII) significantly.•The inhibition of photorespiration decreased total photosynthetic O2 evolution capacity significantly.•The suppression of photorespiration correlated with the decrease of astaxanthin content.•The photorespiratory pathway plays an important role in astaxanthin accumulation in Haematococcus pluvialis.Most previous studies on Haematococcus pluvialis have been focused on growth and astaxanthin accumulation. However, the relationships between photorespiration and astaxanthin accumulation have not been clarified. The purpose of this study was to examine the role of photorespiration during the process of astaxanthin accumulation in H. pluvialis. During astaxanthin accumulation, the astaxanthin content was reduced significantly when photorespiration was inhibited by its specific inhibitor, carboxymethoxylamine. The inhibition of photorespiration did not change the dry weight, chlorophyll content and OJIP transients during the incubation; however, the inhibition of photorespiration significantly decreased the photochemistry of photosystem II and total photosynthetic O2 evolution capacity. Moreover, the restriction in photorespiration was synchronized with a decrease of astaxanthin accumulation. These results suggest that the photorespiratory pathway in H. pluvialis can accelerate astaxanthin accumulation. We speculate that photorespiration can enhance astaxanthin accumulation in the following ways: (i) photorespiration directly affects the glycerate-3-phosphate (PGA) level, which is intrinsically related to the accumulation of astaxanthin in H. pluvialis; (ii) the photorespiratory pathway indirectly affects the PGA level by effecting the dark reactions of photosynthesis, which then results in the enhancement of astaxanthin accumulation in H. pluvialis.

•Sublethal concentrations of toosendanin shrank body size of survivors in rotifers.•Toosendanin had no effect on egg size but decreased number of eggs per rotifer.•Toosendanin showed antifeedant effect by reducing rotifer algal consumption.•Toosendanin directly inhibited activities of digestive proteases in rotifers.•These results herald the beginning of toxic mechanism of toosendanin on rotifers.Toosendanin has been suggested as a potential agent for controlling zooplankton contamination in microalgal cultivation. Growth inhibitory and antifeedant influences of toosendanin on the rotifer Brachionus plicatilis were investigated. Sublethal concentrations of toosendanin (1.76–2.59 mg m−3) reduced population abundance of rotifers, decreased the lorica length and width, and shrank the body size of survivors notably. No changes were observed in the amictic egg sizes, although the egg production per female rotifer was decreased by toosendanin. Exposure to toosendanin for 4 h significantly decreased the rotifer chlorophyll consumption per individual, indicating the antifeedant effect on rotifers. Analysis on the digestive enzymes showed that exposure to toosendanin had no effect on cellulose activity, but reduced the activities of diastase, pepsase and tryptase in rotifer homogenate. In addition, the half-saturation constant (Km) of tryptase was increased and the maximum velocity (Vmax) of pepsase was decreased by toosendanin, suggesting the direct inhibition of these two proteases by toosendanin in B. plicatilis. This may contribute to the feeding deterrent property of toosendanin on rotifers. The present study heralds the very beginning of studies regarding the toxic mechanism of toosendanin on rotifers, and will accelerate its potential uses for zooplankton extermination in microalgal cultivation.