Temperate Diatom Research Articles

Freezing stress tolerance of benthic freshwater diatoms from the genus Pinnularia: Comparison of strains from polar, alpine, and temperate habitats.

Diatoms are among the most important primary producers in alpine and polar freshwaters. Although temperate diatoms are sensitive to freezing, polar diatoms often exhibit more resistance. This is particularly true for members of the (predominantly terrestrial) Pinnularia borealis species complex. However, it remains unclear to what extent this resistance applies to other representatives of the genus. Here, we compare the freezing-stress tolerance of 11 freshwater, benthic strains representing different species of Pinnularia (including Caloneis) from polar, alpine, and temperate habitats. As vegetative cells, strains were exposed to freezing temperatures of -4, -10, -20, -40, -80, and -196°C. Survival was assessed by light microscopy and photosynthetic measurements. We observed vegetative cells to be sensitive to low freezing temperatures; only "mild" freezing was survived by all tested strains, and most tested strains did not survive treatments ≤-10°C. However, individual strain sensitivities appeared related to their original habitats. For example, polar and alpine strains better withstood "mild" and "moderate" freezing (-4 and -10°C, respectively); although temperate strains were significantly affected by the "mild" freezing treatment, polar and alpine strains were not. The -10°C treatment was survived exclusively by polar strains, and only P. catenaborealis survived all treatments. Interestingly, this species exhibited the lowest survival in the -10°C treatment, potentially implying some metabolic activity even at freezing temperatures. Thus, despite more extensive sampling throughout the genus and finer temperature scaling compared to previous studies, the remarkable freezing-stress tolerance of the P. borealis species complex remains unique within the genus.

Short-term responses of temperate and subarctic marine diatoms to Irgarol 1051 and UV radiation: Insights into temperature interactions.

Phytoplankton face numerous pressures resulting from chemical and physical stressors, primarily induced by human activities. This study focuses on investigating the interactive effects of widely used antifouling agent Irgarol 1051 and UV radiation on the photo-physiology of marine diatoms from diverse latitudes, within the context of global warming. Our findings clearly shown that both Irgarol and UV radiation have a significant inhibitory impact on the photochemical performance of the three diatoms examined, with Irgarol treatment exhibiting more pronounced effects. In the case of the two temperate zone diatoms, we observed a decrease in the inhibition induced by Irgarol 1051 and UVR as the temperature increased up to 25°C. Similarly, for the subarctic species, an increase in temperature resulted in a reduction in the inhibition caused by Irgarol and UVR. These results suggest that elevated temperatures can mitigate the short-term inhibitory effects of both Irgarol and UVR on diatoms. Furthermore, our data indicate that increased temperature could significantly interact with UVR or Irgarol for temperate diatoms, while this was not the case for cold water diatoms, indicating temperate and subarctic diatoms may respond differentially under global warming.

Lipidome Plasticity Enables Unusual Photosynthetic Flexibility in Arctic vs. Temperate Diatoms.

The diatom lipidome actively regulates photosynthesis and displays a high degree of plasticity in response to a light environment, either directly as structural modifications of thylakoid membranes and protein-pigment complexes, or indirectly via photoprotection mechanisms that dissipate excess light energy. This acclimation is crucial to maintaining primary production in marine systems, particularly in polar environments, due to the large temporal variations in both the intensity and wavelength distributions of downwelling solar irradiance. This study investigated the hypothesis that Arctic marine diatoms uniquely modify their lipidome, including their concentration and type of pigments, in response to wavelength-specific light quality in their environment. We postulate that Arctic-adapted diatoms can adapt to regulate their lipidome to maintain growth in response to the extreme variability in photosynthetically active radiation. This was tested by comparing the untargeted lipidomic profiles, pigmentation, specific growth rates and carbon assimilation of the Arctic diatom Porosira glacialis vs. the temperate species Coscinodiscus radiatus during exponential growth under red, blue and white light. Here, we found that the chromatic wavelength influenced lipidome remodeling and growth in each strain, with P. glacialis showing effective utilization of red light coupled with increased inclusion of primary light-harvesting pigments and polar lipid classes. These results indicate a unique photoadaptation strategy that enables Arctic diatoms like P. glacialis to capitalize on a wide chromatic growth range and demonstrates the importance of active lipid regulation in the Arctic light environment.

Microbiome and photoperiod interactively determine thermal sensitivity of polar and temperate diatoms.

The effect of temperature on ectothermic organisms in the context of climate change has long been considered in isolation (i.e. as a single driver). This is challenged by observations demonstrating that temperature-dependent growth is correlated to further factors. However, little is known how the chronobiological history of an organism reflected in its adaptation to re-occurring cyclic patterns in its environment (e.g. annual range of photoperiods in its habitat) and biotic interactions with its microbiome, contribute to shaping its realized niche. To address this, we conducted a full-factorial microcosm multi-stressor experiment with the marine diatoms Thalassiosira gravida (polar) and Thalassiosira rotula (temperate) across multiple levels of temperature (4°C; 9°C; 13.5°C) and photoperiod (4 h; 16 h; 24 h), both in the presence or absence of their microbiomes. While temperature-dependent growth of the temperate diatom was constrained by short and long photoperiods, the polar diatom coped with a 24 h photoperiod up to its thermal optimum (9°C). The algal microbiomes particularly supported host growth at the margins of their respective fundamental niches except for the combination of the warmest temperature tested at 24 h photoperiod. Overall, this study demonstrates that temperature tolerances may have evolved interactively and that the mutualistic effect of the microbiome can only be determined once the multifactorial abiotic niche is defined.

Warming exacerbates the impacts of ultraviolet radiation in temperate diatoms but alleviates the effect on polar species.

Under global change scenarios, the sea surface temperature is increasing steadily along with other changes to oceanic environments. Consequently, marine diatoms are influenced by multiple ocean global change drivers. We hypothesized that temperature rise mediates the responses of polar and temperate diatoms to UV radiation (UVR) to different extents, and exposed the temperate centric diatoms, Thalassiosira weissflogii and Skeletonema costatum, and a polar pennate diatom Entomoneis sp., to warming (+5°C) for 10 days, then performed short-term incubations under different radiation treatments with or without UVR. The effective quantum yields of the three diatoms were stable during exposure to PAR, but decreased when exposed to PAR + UVR, leading to significant UV-induced inhibition, which was 3% and 9%, respectively, for T. weissflogii and S. costatum under ambient temperature but increased to 12% and 17%, respectively, in the cells acclimated to the warming treatment. In contrast, UVR induced much higher inhibition, by about 45%, in the polar diatom Entomoneis sp. at ambient temperature, and the warming treatment alleviated the UV-induced inhibition, which dropped to 36%. The growth rates were significantly inhibited by UVR in S. costatum under the warming treatment and in Entomoneis sp. under ambient temperature, while there was no significant effect for T. weissflogii. Our results indicate that the polar diatom was more sensitive to UVR though warming could alleviate its impact, whereas the temperate diatoms were less sensitive to UVR but warming exacerbated its impacts, implying that diatoms living in different regions may exhibit differential responses to global changes.

Interrelated influence of iron, light, and CO2 on carbon fixation in a Southern Ocean diatom

AbstractThe productivity of diatoms in the Southern Ocean plays a key role in the transfer of carbon (C) from the atmosphere to the ocean's interior, which impacts climate. However, diatom growth in the Southern Ocean is limited by several environmental factors including iron (Fe) and light. Ongoing increases in ocean CO2 concentrations may increase diatom carbon fixation, but it is uncertain how this will interact with extant Fe and light limitation. Here we grew the Southern Ocean diatom Fragilariopsis cylindrus under a matrix of growth sufficient and limiting Fe and light levels and current and elevated CO2 concentrations, and found that a decrease in Fe concentration at high light, or in light intensity at high Fe, caused a similar 28–35% decrease in growth rate. Combined low Fe and low light caused a much larger (71–75%) decrease in growth rate than occurred with low Fe or low light alone, indicating Fe and light co‐limitation. At a given concentration of bioavailable dissolved inorganic Fe (Fe′), increasing pCO2 from 400 to 750 μatm had no significant effect on growth or C‐fixation rates under any Fe and light conditions. These results suggest that unlike previous measurements in Fe‐ and light‐limited temperate diatoms, increased CO2 should have little effect on C‐fixation rates in Southern Ocean diatoms. The different physiological responses of cold‐water and temperate diatoms to the changing environment warrant further investigation for understanding and predicting changes in the efficiency of the biological carbon pump and the associated potential feedback to the climate change.

Winners and Losers of Atlantification: The Degree of Ocean Warming Affects the Structure of Arctic Microbial Communities.

Arctic microbial communities (i.e., protists and bacteria) are increasingly subjected to an intrusion of new species via Atlantification and an uncertain degree of ocean warming. As species differ in adaptive traits, these oceanic conditions may lead to compositional changes with functional implications for the ecosystem. In June 2021, we incubated water from the western Fram Strait at three temperatures (2 °C, 6 °C, and 9 °C), mimicking the current and potential future properties of the Arctic Ocean. Our results show that increasing the temperature to 6 °C only minorly affects the community, while an increase to 9 °C significantly lowers the diversity and shifts the composition. A higher relative abundance of large hetero- and mixotrophic protists was observed at 2 °C and 6 °C compared to a higher abundance of intermediate-sized temperate diatoms at 9 °C. The compositional differences at 9 °C led to a higher chlorophyll a:POC ratio, but the C:N ratio remained similar. Our results contradict the common assumption that smaller organisms and heterotrophs are favored under warming and strongly indicate a thermal limit between 6 °C and 9 °C for many Arctic species. Consequently, the magnitude of temperature increase is a crucial factor for microbial community reorganization and the ensuing ecological consequences in the future Arctic Ocean.

Large Diversity in Nitrogen- and Sulfur-Containing Compatible Solute Profiles in Polar and Temperate Diatoms.

Intense bottom-ice algal blooms, often dominated by diatoms, are an important source of food for grazers, organic matter for export during sea ice melt, and dissolved organic carbon. Sea-ice diatoms have a number of adaptations, including accumulation of compatible solutes, that allows them to inhabit this highly variable environment characterized by extremes in temperature, salinity, and light. In addition to protecting them from extreme conditions, these compounds present a labile, nutrient-rich source of organic matter, and include precursors to climate active compounds (e.g., dimethyl sulfide [DMS]), which are likely regulated with environmental change. Here, intracellular concentrations of 45 metabolites were quantified in three sea-ice diatom species and were compared to two temperate diatom species, with a focus on compatible solutes and free amino acid pools. There was a large diversity of metabolite concentrations between diatoms with no clear pattern identifiable for sea-ice species. Concentrations of some compatible solutes (isethionic acid, homarine) approached 1 M in the sea-ice diatoms, Fragilariopsis cylindrus and Navicula cf. perminuta, but not in the larger sea-ice diatom, Nitzschia lecointei or in the temperate diatom species. The differential use of compatible solutes in sea-ice diatoms suggests different adaptive strategies and highlights which small organic compounds may be important in polar biogeochemical cycles.

Growth and Elemental Stoichiometry of the Ecologically-Relevant Arctic Diatom Chaetoceros gelidus: A Mix of Polar and Temperate

In the wake of modest surface blooms that occur at the onset of the growth season in the nitrogen-poor surface waters of the Beaufort Sea, subsurface chlorophyll maxima (SCM) develop and persist within the nutrient-rich halocline. Algal communities of these SCM can realize a major portion of annual net primary production and are often dominated by the widespread diatom Chaetoceros gelidus in coastal waters. In order to assess how changing growth conditions at the SCM may affect the biological carbon pump, grazer nutrition and dissolved nutrient inventories across the transpolar Pacific-Atlantic conduit, the elemental stoichiometry of a C. gelidus strain (RCC2046) isolated from the Beaufort Sea and its response to the availability of light and different forms of nitrogen (N) were examined in the laboratory. The cells were grown in semi-continuous batch cultures at 0 °C under sub-saturating (LL ; 5.5 µmol photons m-2 s-1) or saturating irradiance (HL ; 200 5.5 µmol photons m-2 s-1) and with ammonium (NH4+), nitrate (NO3-) or urea as sole N form in nutrient-replete conditions. The growth rate, cell size, maximum quantum efficiency of photosystem II (Fv/Fm) and the concentrations of chlorophyll a (Chl a), biogenic silica (Si) and particulate N, phosphorus (P) and organic carbon (C) were measured during the exponential growth phase. Despite a clear response of volumetric quotas to N form, the growth rates and elemental ratios of the cells were unaffected by N form in the two irradiance treatments. Elemental ratios were affected by light and the responses were remarkably similar to those observed for temperate diatoms. Overall, this study shows that the growth and elemental composition of C. gelidus in the Arctic Ocean are highly resistant to changes in nitrogen form at near-freezing temperatures and suggests that this diatom possesses the ability to remain competitive despite ongoing environmental changes.

Understanding Diatoms’ Past and Future Biogeochemical Role in High‐Latitude Seas

AbstractBecause cold‐water diatoms’ baseline elemental density (BED) is substantially higher than temperate diatoms, previous polar studies may have underestimated diatoms’ contribution to elemental standing stocks, contribution to particulate organic carbon (POC) export and incorrectly modeled their susceptibility to future warming. We apply cold‐water diatom allometry to Arctic field samples and derive diatom growth rates ranging from 0.01–0.68 day−1, versus unrealistically high rates estimated using temperate diatom allometry. Reanalysis of published Southern Ocean data (Antarctic Environment and Southern Ocean Process Study and European Iron Fertilization Experiment) shows that diatom POC was significantly underestimated and diatoms could have accounted for a majority of POC export. However, during some field programs (Kerguelen Plateau), temperate allometry properly accounted for diatom biomass. We also predict that warming sea surface temperature may alter high‐latitude diatom BED, suggesting that even if abundances do not change with warming, the reduced diatom BED will likely lower the trophic‐transfer efficiency and their total carbon flow to consumers.

Tolerance of pennate diatoms (Bacillariophyceae) to experimental freezing: comparison of polar and temperate strains

ABSTRACTAlthough polar regions are characterised by extreme environment conditions with overall low temperatures and often pronounced diurnal to seasonal temperature fluctuations, diatoms (Bacillariophyceae) thrive in a wide range of nonmarine polar habitats, suggesting that they are well adapted to these harsh conditions. In this study, tolerance to freezing of vegetative and resting cells of 17 polar and nine temperate diatom strains, belonging to different morphospecies and originating from both freshwater and terrestrial habitats, was experimentally tested. Most strains were newly isolated from sites in Maritime Antarctica (James Ross Island and Vega Island), the High Arctic (Spitsbergen) and Europe. Cultures were exposed to five freezing treatments differing in temperature (−4, −20, −40 and −180 °C), freezing rate (gradual, abrupt), time (1 h, 12 h) and thawing rate (slow, fast). Results indicated that diatoms were sensitive to experimental freezing. Freezing temperatures had a significant effect on strain survival: all strains survived −4 °C; most survived −20 °C; five survived −40 °C, and four of these (all belonging to the Pinnularia borealis complex) survived freezing in liquid nitrogen (−180 °C). The cooling and thawing rate had a significant impact on survival: abrupt cooling and slow thawing resulted in much lower survival rates than gradual cooling and fast thawing. Resting cells showed better growth than vegetative cells after freezing treatment but only in the −4 °C treatment. Surprisingly, no striking differences in growth recovery were observed between polar and temperate strains.

Effects of iron limitation on silicon uptake kinetics and elemental stoichiometry in two Southern Ocean diatoms, Eucampia antarctica and Proboscia inermis, and the temperate diatom Thalassiosira pseudonana

AbstractWe investigated the effects of iron (Fe) limitation on the elemental stoichiometry, silicic acid (Si(OH)4) uptake kinetics and cell morphology in two Southern Ocean diatoms Eucampia antarctica and Proboscia inermis and the temperate diatom Thalassiosira pseudonana. An increase in Fe‐stress resulted in reductions in specific growth rate and decreases in cellular nitrogen (N) and carbon (C) content relative to cellular biogenic silica (BSi) in both Southern Ocean diatoms and a reduction in growth rate only for T. pseudonana. Both E. antarctica and P. inermis exhibited an increase in cell volume in response to Fe‐limitation resulting in a decrease in the cell surface to volume ratio, while normalization of BSi content to cell surface area suggests these diatoms do not become more heavily silicified under Fe limitation. Kinetic Si(OH)4 uptake experiments performed on all three diatom species show that Si(OH)4 uptake is reduced under Fe‐limited conditions. For Southern Ocean diatoms, this was manifested through a decrease in the maximum specific uptake rate of Si(OH)4 (VSi‐max), along with a decrease in the half‐saturation constant for Si(OH)4 uptake (KSi) under Fe stress for E. antarctica. Our data also show that when normalized to cell surface area, VSi‐max of the three diatoms species exhibited a linear relationship with cellular growth rate, and was independent of cell morphological variations. Our results suggest that the morphological adaptations of Southern Ocean diatoms in response to Fe‐stress have the potential to affect phytoplankton community dynamics and Si(OH)4 : NO3 uptake and export ratios in Southern Ocean waters.

Nutrient limitation in two marine diatoms inhibits release of bromoform

Bromoform released from phytoplankton and kelp in the ocean is the largest known carrier of bromine to the atmosphere. The photoproducts of atmospheric bromoform catalyse ozone depletion. Laboratory investigations were conducted into the link between nutrient limitation and bromoform production using axenic cultures of two warm-water diatoms (Chaetoceros neogracile and Phaeodactylum tricornutum). During exponential growth the bromoform production was 2 000–3 000 nmol bromoform (g Chl a)−1 h−1, i.e. 10–100 times higher than earlier values for temperate and cold-water diatoms. Bromoform production decreased down to zero under CO2 and nitrate limitation for both species. These results suggest that the bromoform production could be directly related to bromoperoxidase activity (and irradiance) only during exponential growth, whereas compounds other than bromoform might be formed under nutrient limitation.

Chemical Diversity as a Function of Temperature in Six Northern Diatom Species

In this study, we investigate how metabolic fingerprints are related to temperature. Six common northern temperate diatoms (Attheya longicornis, Chaetoceros socialis, Chaetoceros furcellatus, Porosira glacialis, Skeletonema marinoi, and Thalassiosira gravida) were cultivated at two different temperatures, 0.5 and 8.5 °C. To exclude metabolic variations due to differences in growth rates, the growth rates were kept similar by performing the experiments under light limited conditions but in exponential growth phase. Growth rates and maximum quantum yield of photosynthesis were measured and interpreted as physiological variables, and metabolic fingerprints were acquired by high-resolution mass spectrometry. The chemical diversity varied substantially between the two temperatures for the tested species, ranging from 31% similarity for C. furcellatus and P. glacialis to 81% similarity for A. longicornis. The chemical diversity was generally highest at the lowest temperature.

Iron‐light interactions differ in Southern Ocean phytoplankton

In laboratory experiments we examined the interplay of light and iron availability on the intracellular iron concentrations, specific growth rates, and photosynthetic physiology of Southern (S.) Ocean diatoms (Eucampia antarctica and Proboscia inermis) and the haptophyte Phaeocystis antarctica. Intracellular iron concentrations and iron : carbon (Fe : C) molar ratios increased with decreasing irradiance in temperate coastal (Thalassiosira weissflogii) and oceanic (Thalassiosira oceanica) diatoms, in support of the well‐established antagonistic iron‐light relationship. In contrast, S. Ocean species required lower cellular iron concentrations and Fe : C ratios than temperate diatoms to grow at comparable rates, and their iron requirements decreased or remained relatively constant with decreasing light. These results suggest that the current paradigm that low light increases algal cellular iron requirements (supplied through “biodilution”) is not applicable to S. Ocean phytoplankton. Although iron use efficiencies decreased at sub‐saturating light in all species, these reductions were due primarily to lower growth rates, but not higher intracellular Fe : C ratios, in S. Ocean species. We propose that S. Ocean species have overcome the antagonistic iron‐light relationship by increasing the size, rather than the number, of photosynthetic units under low irradiances, resulting in an acclimation strategy that does not increase their cellular iron requirements.

Short-term antioxidative responses of 15 microalgae exposed to excessive irradiance including ultraviolet radiation

Short-term photosensitivity and oxidative stress responses were compared for three groups of marine microalgae: Antarctic microalgae, temperate diatoms and temperate flagellates. In total, 15 low-light-acclimated species were exposed to simulated surface irradiance including ultraviolet radiation (SSI). Photosensitivity was assessed as the rate of recovery of Fv/Fm in the hours following SSI treatment. Before, during and after the SSI treatment, oxidative stress responses were assessed by following xanthophyll content and cycling, and activities of superoxide dismutase, ascorbate peroxidase and glutathione reductase, and glutathione redox status. When acclimated to low irradiance, antioxidant levels were not group specific. Superoxide dismutase activity was positively correlated with cell size, whereas in general, ascorbate peroxidase activity appeared to be lower and glutathione redox status appeared to be higher in the Antarctic than in the temperate species. After SSI exposure, the strong inhibition of PSII was followed by variable rates of recovery, although four species remained photosynthetically inactive. SSI tolerance appeared unrelated to geographic or taxonomic background, or to cell size. PSII recovery was enhanced in species with decreasing superoxide dismutase activity, glutathione redox status and increased xanthophyll cycle activity. We conclude that antioxidant responses are highly species specific and not related to the geographic or taxonomic background. Furthermore, xanthophyll cycling seems more important than antioxidants. Finally, it can be hypothesized that glutathione could function as a stress sensor and response regulator.

Release of reactive bromine and iodine from diatoms and its possible role in halogen transfer in polar and tropical oceans

An in situ incubation assay measuring the bromination and iodination of phenol red was developed to detect the release of reactive bromine and iodine (primarily hypobromous acid [HOBr] and hypoiodous acid [HOI], respectively) from a putative extracellular bromoperoxidase of marine diatoms. Six of 11 species showed significant release compared to controls. Polar species were particularly active, releasing 0.6–180 fmol HOBr cell−1 h−1 (0.04–1.8 µmol HOBr [mg total chlorophyll]−1 h−1; at the seawater bromide concentration, 840 µmol L−1) and 1.9–271 fmol HOI cell−1 h−1 (0.02–2.7 µmol HOI [mg total chlorophyll]21 h−1, at 100 µmol L−1 iodide). Porosira glacialis consistently showed the highest rates of release. Several temperate diatoms, including Achnanthes cf longipes, known to have a bromide‐sensitive peroxidase involved in stalk formation, and warmwater species also showed the ability to release reactive bromine and iodine. This release was influenced by light, temperature, bromide (and iodide) concentration, H2O2 concentration, and pH. The rate of HOBr release by polar diatoms was much greater than bromoform emissions measured by others from laboratory cultures and seaice algae in the field. This indicates that most of the HOBr released may react with dissolved organic matter (DOM) to form nonvolatile bromine organics. Some fraction of diatom‐produced HOBr and HOI may also form volatile Br2 and I2, which could transfer to the polar troposphere. The reaction of diatom‐released reactive bromine and iodine with seawater DOM may represent the major mechanism in the formation of oceanic polybromo‐ and polyiodo‐methanes.

Vitamin B12 and cobalt cycling among diatoms and bacteria in Antarctic sea ice microbial communities

Within McMurdo SoundÂs annual sea ice, assimilation and concentrations of vitamin B12 (cobalamin), microbial community productivity, and biomass were examined among three 100‐m2 quadrats where light penetration was manipulated by snow cover during austral summer. From late October through December, B12 concentrations (6−32 pmol L−1) and assimilation rates (17−780 pmol m−2 d−1) in congelation ice covaried with primary productivity (0.0001−250 mmol C m−2 d−1) and chlorophyll a (0.6−36 mg m−2). Within ice core samples, incorporation of 57Co‐B12 into the >1.0‐mm size fraction (mostly diatoms) was almost 100 times faster than into 0.2−1.0−mm particles (mostly heterotrophic bacteria) and was dependent on light and active transport across cell membranes. Microalgal B12:C cell quotas in field communities varied widely (2.6‐150,000 nmol B12 (mol C)−1; x̄x = 500) and generally exceeded those of cultured temperate diatoms (0.4‐55 nmol B12 (mol C)−1; x̄x = 4.1). Comparisons of dissolved B12 pools in the ice and their turnover (0.02‐0.6 d‐1) with underlying seawater suggest that this vitamin is produced in situ rather than delivered from waters below. Production and uptake of B12 and uptake of cobalt, required for B12 synthesis, were then examined among bacteria isolated from these communities. Only 23% of 78 bacterial isolates were incapable of B12 uptake, but these clones assimilated dissolved cobalt. Intracellular B12 production was evident in 9 of the 11 isolates screened and their cell quotas varied widely, 0.6‐6,800 nmol B12 (mol C)‐1. Mass balance analyses and published kinetics data independently suggest that microalgal growth in sea ice was not limited by vitamin B12 in most of our field observations and that in situ bacterial B12 production could potentially meet microalgal demands. Similar analyses, however, suggest that cobalt supply from underlying waters may have limited community growth and B12 production.

TAXONOMY AND DISTRIBUTION OF DIATOMS IN THE GENUS GOMPHONEMA FROM THE FLORIDA EVERGLADES, U.S.A.

Located at a subtropical latitude, the expansive Florida Everglades contains a mixture of tropical and temperate diatom taxa, as well as a unique flora adapted to the calcareous, often excessively hot, seasonally flooded wetland conditions. This flora has been poorly documented taxonomically, although diatoms are recognized as important indicators of environmental change in this threatened ecosystem. Gomphonema is a dominant genus in the freshwater marsh, and is represented by highly variable species complexes, including Gomphonema gracile Ehrenberg, Gomphonema intricatum var. vibrio Ehrenberg sensu Fricke, Gomphonema vibrioides Reichardt & Lange-Bertalot and Gomphonema parvulum (Kützing) Grunow. These taxa have been shown to exhibit wide morphological variation in other regions, resulting in considerable nomenclatural confusion. We collected Gomphonema from 237 sites distributed throughout the freshwater Everglades and used qualitative and quantitative morphological data to identify 20 distinguishable populations. Taxonomie assignments were based on descriptions and/or observations of type material of relevant taxa when possible, but deviations from original morphological range descriptions were common. We then compared morphological variation in Everglades Gomphonema taxa to that reported for the same taxa in other regions and suggest revisions of taxonomie concepts when necessary.

The lipid composition of Euphausia superba Dana in relation to the nutritional value of Phaeocystis pouchetii (Hariot) Lagerheim

The fatty acid profiles of Euphausia superba, the Antarctic prymnesiophyte, Phaeocystis pouchetii, and a temperate diatom, Phaeodactylum tricornutum were analysed and compared. The lipid content, lipid class, fatty acid and sterol composition of E. superba fed on each cultured phytoplankton and a mixed diet of both species, were determined. No significant difference was found between total lipid levels of E. superba reared on each of these different diets. Phaeocystis pouchetii, although deficient in a number of the essential fatty acids, is apparently an adequate food source for E. superba. The proportion of polyenoic fatty acids varied within lipid classes although there was no significant difference between levels of the long chain polyunsaturate 20:5(n-3) in the total lipid of E. superba fed on these diets. This acid was found to be less than 1% of the total lipid in Phaeocystis pouchetii compared to 37% in Phaeodactylum tricornutum. This suggests that krill may possess the ability to convert exogenous shorter chain fatty acids to 20:5(n-3) and 22:6(n-3). Significant differences were detected in the isomeric ratio of 16:1(n-7c)/16:0 between krill fed the diatom compared to the prymnesiophyte diet. Significant differences were also detected in several shorter chain fatty acids and between fatty acids within their lipid classes. Such differences may have the potential to be used as biochemical signatures to provide information on food sources and possible feeding grounds of E. superba. Phaeocystis pouchetii in a very late stationary phase, although not used in this feeding study, was found to contain 11% of 22:6(n-3) for which there are few substantive sources in natural algal populations.