Explore the vast range of titles available.

Abstract Sea ice algal communities are generally dominated by pennate diatoms, which commonly occur at the ice-water interface and in brine channels. They also make a significant contribution to higher trophic levels associated with sea ice habitats. Here, the photosynthetic responses of two sea ice diatom species, Navicula directa and Navicula glaciei, to changes in pCO2 under controlled laboratory conditions were compared. pCO2 (390 ppm and 750 ppm) was manipulated to simulate a shift from present levels (1990) to predicted “IPCC year 2100 worst-case scenario” levels. To investigate these effects, a pulse-amplitude modulation (PAM) fluorometer was used to measure the photosynthetic performance. The ability of the sea ice algae to grow and photosynthesize within physio-chemical gradients in the sea ice suggests that both sea ice species are likely to be well adapted to cope with changes in pCO2 concentrations. Lower pH and higher pCO2 for 7 days resulted in increased biomass, especially for N. directa. However, a decline in photosynthetic capacity (rETRmax) was observed for both species (highest value 11.375 ± 0.163, control; and 8.322 ± 1.282, treatment). Navicula glaciei showed significant effects of elevated pCO2 (p < 0.05) on its photosynthetic response, while N. directa did not. Future changes in CO2 and pH may thus not significantly affect all diatoms but may lead to changes in the photosynthetic activities in some species.

Polar oceans are very susceptible to increased levels of atmospheric CO2 and may act as the world's largest sink for anthropogenic CO2. Simultaneously, as atmospheric CO2 increases, sea surface temperature rises due to global warming. These two factors are important in regulating microalgal ecophysiology, and it has been suggested that future global changes may significantly alter phytoplankton species composition. This study aims to investigate potential consequences of global change in terms of increased temperature and CO2 enrichment on the benthic/sea ice diatom Navicula directa. In a laboratory experiment, the physiological response to elevated temperature and partial pressure of CO2 (pCO2) was investigated in terms of growth, photosynthetic activity and photosynthetic pigment composition. The experiment was performed under manipulated levels of pCO2 (380 and 960 ppm) and temperature (0.5 and 4.5°C) to simulate a change from present levels to predicted levels during a worst-case scenario by the year 2100. After 7 days of treatment, no synergetic effects between temperature and pCO2 were detected. However, elevated temperature promoted effective quantum yield of photosynthesis ([increment]F/ F rm m ′ ) and increased growth rates by approximately 43%. Increased temperature also resulted in an altered pigment composition. In addition, enrichment of CO2 appeared to reduce specific growth rates of N. directa. Even though growth rates were only reduced by approximately 5%, we hereby report that increased pCO2 levels might also have potential negative effects on certain diatom strains.[PUBLICATION ABSTRACT]

The snow cover of Arctic sea ice has recently decreased, and climate models forecast that this will continue and even increase in future. We therefore tested the effect of snow cover on the optical properties of sea ice and the biomass, photobiology, and species composition of sea ice algae at Kangerlussuaq, West Greenland, during March 2011, using a snow-clearance experiment. Sea ice algae in areas cleared of snow was compared with control areas, using imaging variable fluorescence of photosystem II in intact, unthawed ice sections. The study coincided with the onset of spring growth of ice algae, mainly an increase in two pennate diatoms ( Achnanthes taeniata and Navicula directa ), as temperature increased and ice thickness and brine volume stabilized. The increase in biomass was accompanied by an increase in minimum variable fluorescence ( F o ) and the maximum quantum yield of PSII ( F v /F m ) and filling of brine channels with fluorescing cells. In contrast, in the minus snow area, PAR transmittance increased sixfold and there was an exponential decrease in chl- a and no increase in F o , and the area of fluorescing biomass declined to become undetectable. This study suggests that the onset of the spring bloom is predominantly due to temperature effects on brine channel volume, and that the algal decline after snow removal was primarily due to emigration rather than photodamage.

Phytoplankton (>15 µm) was investigated in three shallow coastal areas at Admiralty Bay (AB) between the summers of 2002–03 and 2008–09. Phytoplankton abundance was low (103 cells l-1) and, over time, the prevailing cell size decreased due to a shift in phytoplankton dominant species from diatoms to dinoflagellates. In situ and remote sensing data showed that oscillations in sea surface temperature, precipitation, ice formation/melting, irradiance (cloud cover) and bottom circulation (indexed by the Antarctic Oscillation Index; AAO) were shown to govern the structure of the phytoplankton. Under negative AAO, diatoms prevailed, with the dominance of large (>80 µm) benthic diatoms (e.g. Corethron pennatum and Navicula directa) in periods of low production (102 cells l-1 in 2002–03), and medium-sized (31–80 µm) centrics (e.g. Thalassiosira spp. and Stellarima microtrias) when the abundance was higher (104 cells l-1 in 2003–04). Conversely, positive AAO led to the co-dominance of dinoflagellates and planktonic diatoms (e.g. Pseudo-nitzschia spp.) in the summers of 2007–08 and 2008–09. These results suggest that the AAO can be a good predictor of phytoplankton in coastal areas around the western Antarctic Peninsula, and may help our understanding of changes in other trophic levels of the food web.

Changes in the phytoplankton biomass (chlorophyll a), production rate, and species composition were studied over two seasons using the time series measurements in the northern limb of the Cochin estuary in relation to the prevailing hydrological conditions. The present study showed the significant seasonal variation in water temperature (F = 69.4, P < 0.01), salinity (F = 341.93, P < 0.01), dissolved inorganic phosphorous (F = 17.71, P < 0.01), and silica (F = 898.1, P < 0.01) compared to nitrogen (F = 1.646, P > 0.05). The uneven input of ammonia (3.4-224.8 μM) from upstream (Periyar River) leads to the inconsistency in the N/P ratio (range 6.8-262). A distinct seasonality was observed in Si/N (F = 382.9, P < 0.01) and Si/P (F = 290.3, P < 0.01) ratios compared to the N/P ratio (F = 1.646, P > 0.05). The substantial increase in chlorophyll a (average, 34.8 ± 10 mg m ⁻ ³) and primary production (average, 1,304 ± 694 mg C m ⁻ ³ day ⁻ ¹) indicated the mesotrophic condition of the study area during the premonsoon (PRM) and it was attributed to the large increase in the population of nanoplankton (size < 20 μ ) such as Skeletonema costatum, Thalassiosira subtilis, Nitzschia closterium, and Navicula directa. In contrast, during the post monsoon (PM), low chlorophyll a concentration (average, 9.3 ± 9.2 mg m ⁻ ³) and primary production (average, 124 ± 219 mg C m ⁻ ³ day ⁻ ¹) showed heterotrophic condition. It can be stated that favorable environmental conditions (optimum nutrients and light intensity) prevailing during the PRM have enhanced the abundance of the nanoplankton community in the estuary, whereas during the PM, the light limitation due to high turbidity can reduce the nanoplankton growth and abundance, even though high nutrient level exists.

Diatoms are unicellular algae that synthesize cell wall with silica that has highly ornate features on the nano to microscale. The porous silica nanoparticulate structure of three marine centric and one pennate diatoms namely, Coscinodiscus concinnus, Coscinodiscus sp., Odontella mobiliensis and Navicula directa were investigated by Field Emission Scanning Electron Microscopy (FESEM). Important morphological features like porous pattern, topography, pore size and shape were studied. The external layer (cribellum) of C. concinnus was found to be consisting of a characteristic pentagonal array of pores which were star in shape and irregular in size, with a diameter of 224.7 nm and a pore-to-pore distance of 160.6 nm. The second diatom species investigated, Coscinodiscus sp. showed frustule with radially-oriented pattern of alternating grid-like arrangements of pores with honeycomb topography with pore diameter of 132.1 and distance between arrays were 61.01 nm. The O . mobiliensis images showed well organisation of holes (foramen) showed hexagonal organisation and all the pores are circular with same size and pores of 328.6 nm diameter with pore to pore distance was 252.8 nm. The girdle view of N. directa was about 5 μm in diameter with values showing striae are parallel in whole and porous were observed in N. directa in the range of 278.3 nm, the gaps between regularly arranged pores were 145.6 nm was clearly observed. The internal and external structures of all the diatom frustules were different in pore arrangements. The present study showed that high-resolution FESEM results revealed the silica nanostructure with nanoporous material exhibited interesting application in antireflection, drug delivery and heavy metal adsorbing studies, which should be investigated further research will be a subject of future proposals by a flat form of present investigation.