Colloidal organic carbon and phytoplankton speciation during a coastal bloom

During the 1995 spring bloom in Bedford Basin, Nova Scotia,dissolved organic carbon (DOC) in the photic zone was separatedinto low-molecular-weight and colloidal size fractions by cross-flowultrafiltration. DOC, colloidal organic carbon (COC) and low-molecular-weightorganic carbon (LOC) were then analyzed by high-temperaturecatalytic oxidation. COC (associated with the production ofphytoplankton exudates) did not follow the concentration ofbulk chlorophyll a (chi a) or the total number of phytoplanktoncells. Instead, surface-active COC (that adhered to the ultrafiltrationsystem) was produced early during the bloom when the diatomSkeletonema costatum was at a maximum. Later on, as the bloombegan to decline, less surface-active COC (that remained largelyfree of the ultrafiltration surfaces) was produced and was associatedmore with variations in cell number of Chaetoceros socialis,the predominant diatom for most of the bloom. These resultssuggest that chl a or the total number of phytoplankton cellsmay not be reliable indices of the production of COC. On thecontrary, the results suggest that relatively high COC concentrationswere associated with specific diatom species. In addition, themaximum amount of COC was associated during the early stagesof the bloom with a diatom (S.costatum) that was a small fraction(<5.6%) of total phytoplankton cell number. This finding,that relatively large amounts of colloids were associated withexopolymer production during the onset and development of abloom, does not agree with reports suggesting that the productionof exopolymers by diatoms is primarily an end-of-bloom occurrence.     

The seasonal occurrence of chroococcoid cyanobacteria at an Antarctic coastal site

Summary The chroococcoid cyanobacterium Synechococcus was found at 5 m depth at a site 6 km off the Antarctic coast where the water depth was 30 m. These cells reached a peak concentration of 140 cells ml^-1 in August at the time of maximum rate of development of seaice. They were accompanied in the water column by detritus consisting of diatom frustules and sediment particles. A smaller peak of abundance occurred at the time the sea-ice was breaking up. Cyanobacteria were seen only in very low numbers at one, and were absent at the other, of two deep water sampling sites. We propose that the two peaks of cyanobacterial abundance in the water column result from the suspension of a benthic population by ice crystals during the winter and their release from the sea-ice in summer.     

Fortnightly periodicity in the abundance of diatom and dinoflagellate taxa at a coastal study site

From 3 July to 15 September 2000, plankton samples were collected roughly every 2 days at three stations within 1.5 km from shore at Sunset Bay, OR. In these samples we enumerated Coscinodiscus-like diatoms, Protoperidinium spp., and the potentially toxic phytoplankter, Pseudo-nitzschia spp. Using time series analysis, the abundance of these phytoplankters was compared to wind stress, tidal range, and sea surface temperature (SST). SST was significantly cross-correlated with wind stress (lower SST occurred during upwelling favorable winds) and maximum daily tidal range (cold and warm anomalies in SST occurred around the neap and spring tides, respectively). We found no significant cross-correlations between wind stress (upwelling vs. downwelling winds) and the abundance of any of the taxa. Significant cross-correlations were found between phytoplankton abundances and the maximum daily tidal range (peak concentrations occurred between the neap and spring tides) and SST (peak concentrations occurred during periods of warmer SST). We hypothesize that this pattern of abundance may be caused by shoreward transport of offshore phytoplankton populations by internal tidal waves.     

Protist diversity in a permanently ice-covered Antarctic Lake during the polar night transition

The McMurdo Dry Valleys of Antarctica harbor numerous permanently ice-covered lakes, which provide a year-round oasis for microbial life. Microbial eukaryotes in these lakes occupy a variety of trophic levels within the simple aquatic food web ranging from primary producers to tertiary predators. Here, we report the first molecular study to describe the vertical distribution of the eukaryotic community residing in the photic zone of the east lobe (ELB) and west lobe (WLB) of the chemically stratified Lake Bonney. The 18S ribosomal RNA (rRNA) libraries revealed vertically stratified populations dominated by photosynthetic protists, with a cryptophyte dominating shallow populations (ELB–6 m; WLB–10 m), a haptophyte occupying mid-depths (both lobes 13 m) and chlorophytes residing in the deepest layers (ELB–18 and 20 m; WLB–15 and 20 m) of the photic zone. A previously undetected stramenopile occurred throughout the water column of both lobes. Temporal variation in the eukaryotic populations was examined during the transition from Antarctic summer (24-h sunlight) to polar night (complete dark). Protist diversity was similar between the two lobes of Lake Bonney due to exchange between the photic zones of the two basins via a narrow bedrock sill. However, vertical and temporal variation in protist distribution occurred, indicating the influence of the unique water chemistry on the biology of the two dry valley watersheds.     

Seasonal variation in abundance and species composition of choanoflagellates (Acanthoecideae) at Antarctic coastal sites

Summary The abundance of loricate choanoflagellates at three shallow inshore sites (< 30 m) near Davis, Antarctica from March 1982 to February 1983 showed marked variation which followed the seasonal cycle of chlorophyll a concentration. In summer the concentration of choanoflagellates ranged from 10⁴–10⁶ cells 1^–1 while in winter they were in the order of 10² cells 1^–1. Species seasonal succession was observed with Bicosta spinifera, Crinolina aperta and Parvicorbicula socialis, organisms which dominated the summer population, being absent in winter whereas Cosmoeca ventricosa and Saepicula leadbeateri were found only in winter. Acanthocorbis unguiculata, Calliacantha simplex and Diaphanoeca grandis were present throughout the year.     

Seasonal changes in the abundance and composition of marine heterotrophic bacterial communities in an Antarctic coastal area

During a 1-year period, systematic observations of the Antarctic coastal marine bacterioplankton were recorded. Three field stations were sampled weekly in 1989 in Terre Adélie area. The survey included physicochemical (temperature and particulate organic matter) and bacteriological (total and heterotrophic counts, estimation of bacterial production) measurements. The bacterial community structure was investigated by carrying out 27 morphological and biochemical tests on 254 strains isolated during each season. Gram-negative non-fermentative rods were always dominant. However, an obvious difference exists between the communities inhabiting ice-free and ice-covered seawater. The potential metabolic abilities which were relatively significant in the summer community were severely reduced in the winter community. A general increase in bacterial biomass and production was observed in surface water after sea ice formation. The results suggest a close coupling between heterotrophic bacterioplankton and the input of allochthonous organic carbon, for example from the overlying sea-ice communities or from nearby penguin rookeries.     

Effect of Temperature and Prey Type on Nutrient Regeneration by an Antarctic Bacterivorous Protist

Experimental studies were carried out on an Antarctic isolate of the heterotrophic nanoflagellate Paraphysomonas imperforata to examine the efficiency of incorporation and remineralization of nitrogen and phosphorus from bacterial prey. Experiments were carried out over a temperature range from ambient Antarctic temperature (0 degrees C) to 10 degrees C. Temperature had a marked effect on the maximal growth rate of the phagotrophic nanoflagellate. Growth rate in the presence of high prey abundance ranged from 0.6 day(-1) at 0 degrees C to 2.6 day(-1) at 10 degrees C. In contrast, temperature had no discernable effect on the efficiencies of incorporation and remineralization of major nutrients by P. imperforata. The efficiencies of phosphorus and nitrogen incorporation from prey biomass averaged over the temperature range examined were 58 and 39%, respectively, for the two elements. Ammonium and phosphate were the dominant forms of dissolved nitrogen and phosphorus appearing in the culture medium during the grazing phase of the experiments. Overall, dissolved organic nitrogen and phosphorus constituted minor components of these elements released by the grazing activities of the protist. The results of this study indicated that incorporation/remineralization of nitrogen and phosphorus contained in prey was relatively unaffected by culture temperature in this heterotrophic nanoflagellate, although low temperature significantly depressed its growth rate. This finding has important implications for energy utilization and elemental cycling in perennially cold ecosystems and is at odds with conclusions that have been reached in some previous studies regarding the growth efficiency of phagotrophic Antarctic protists.     

Understanding the blob bloom: Warming increases toxicity and abundance of the harmful bloom diatom Pseudo-nitzschia in California coastal waters

The toxic diatom genus Pseudo-nitzschia produces environmentally damaging harmful algal blooms (HABs) along the U.S. west coast and elsewhere, and a recent ocean warming event coincided with toxic blooms of record extent. This study examined the effects of temperature on growth, domoic acid toxin production, and competitive dominance of two Pseudo-nitzschia species from Southern California. Growth rates of cultured P. australis were maximal at 23 °C (∼0.8 d⁻¹), similar to the maximum temperature recorded during the 2014–2015 warming anomaly, and decreased to ∼0.1 d⁻¹ by 30 °C. In contrast, cellular domoic acid concentrations only became detectable at 23 °C, and increased to maximum levels at 30 °C. In two incubation experiments using natural Southern California phytoplankton communities, warming also increased the relative abundance of another potentially toxic local species, P. delicatissima. These results suggest that both the toxicity and the competitive success of particular Pseudo-nitzschia spp. can be positively correlated with temperature, and therefore there is a need to determine whether harmful blooms of this diatom genus may be increasingly prevalent in a warmer future coastal ocean.     

Dinoflagellate cyst production at a coastal Mediterranean site

To assess the diversity and seasonality of dinoflagellate cystproduction, surface sediment and trap samples were studied inthe Gulf of Naples (Mediterranean Sea). A total of 59 differentcyst morphotypes were recorded. At the stations within the 70m isobath, sediment assemblages were dominated by calcareousPeridiniales (66–79%), while at the deepest stations non-calcareousPeri-diniales attained the highest percentages (40–49%).The sediment trap sampling, carried out fortnightly over twoannual cycles, revealed high production rates (up to 1.7 x 10⁶cysts m^–2 day^–1) from spring to late autumn of bothyears, with a distinct seasonal production pattern. Althoughrather similar in species composition, the total cyst flux differedmarkedly between the 2 years (1.26 and 0.55 x 108 cysts m^–2year^–1, respectively). Species-specific production patternswere observed: some species formed cysts over several months,others in restricted periods of the year. Cyst-forming speciesconstituted a small part of the planktonic dinoflagellate populationsrecorded in the area. A coupling between the trap material andsurface water plankton was observed for calcareous Peridiniales.This sampling approach allowed the detection of some speciesnever recorded before in the gulf, including two potentiallytoxic species: Alexandrium andersoni and Gymnodinium catenatum-likespecies.     

Feeding ecology of benthic filter-feeders at Potter Cove,an Antarctic coastal ecosystem

A rich benthic filter-feeding community is present at Potter Cove, South Shetland, in spite of some usually unfavorable conditions affecting their feeding ecology, such as low phytoplankton production in summer and high sedimentation rates. However, organic material that could fuel the benthic system, such as macroalgal fragments, was detected in the water-bottom interface year-round. Mean assimilation efficiency of several sizes of macroalgal particles under different inorganic particulate percentages varied between 26–51% in a clam, and 26–72% in two ascidian species. Estimated particle flux (feces) produced by ascidians varied according to species and abundance. The C:N ratio of feces was relatively high. Plasticity and profit from the available food sources contribute to the success of the filter-feeding community in Potter Cove.     

Drivers of interannual variability in virioplankton abundance at the coastal western Antarctic peninsula and the potential effects of climate change

An 8‐year time‐series in the Western Antarctic Peninsula (WAP) with an approximately weekly sampling frequency was used to elucidate changes in virioplankton abundance and their drivers in this climatically sensitive region. Virioplankton abundances at the coastal WAP show a pronounced seasonal cycle with interannual variability in the timing and magnitude of the summer maxima. Bacterioplankton abundance is the most influential driving factor of the virioplankton, and exhibit closely coupled dynamics. Sea ice cover and duration predetermine levels of phytoplankton stock and thus, influence virioplankton by dictating the substrates available to the bacterioplankton. However, variations in the composition of the phytoplankton community and particularly the prominence of Diatoms inferred from silicate drawdown, drive interannual differences in the magnitude of the virioplankton bloom; likely again mediated through changes in the bacterioplankton. Their findings suggest that future warming within the WAP will cause changes in sea ice that will influence viruses and their microbial hosts through changes in the timing, magnitude and composition of the phytoplankton bloom. Thus, the flow of matter and energy through the viral shunt may be decreased with consequences for the Antarctic food web and element cycling.     

Protist community composition during spring in an Arctic flaw lead polynya

The overwintering deployment of an icebreaker during the Canadian Flaw Lead study provided an opportunity to evaluate how protist communities (phytoplankton and other single-celled eukaryotes) respond to changing spring irradiance conditions in flaw lead polynyas, where open water persists between the central pack ice and land fast ice. We combined microscopic analysis of the protist communities (all cell sizes) with clone libraries of 18S rRNA genes and 18S rRNA (from RNA converted to cDNA) of size-fractionated seawater (0.2–3.0 μm) from 10 to 12 m depth in the surface mixed layer. The rRNA gene analysis provided information on the presence of organisms, while the rRNA analysis provided information on the most active members of the community. There was little overlap between the two types of clone libraries, and there were large community shifts over time. Heterotrophic dinoflagellates and ciliates were the most common sequences recovered. The relative proportion of photosynthetic protist sequences increased in March and April, and there was greater representation of Bacillariophyta, Prasinophyta, Haptophyta, and Cryptophyta in the rRNA compared to rRNA gene libraries. Microscopy indicated that large-celled diatoms dominated the community in May, when chlorophyll concentrations were greatest. However, the RNA sequencing showed that heterotrophic and putative parasitic protists were proportionately more active, and the concomitant decrease in nutrients suggested that the spring phytoplankton bloom had begun to decline by this time. These observations provide evidence of substantial changes in protist community structure and function during the spring transition.     

Effects of light on nitrogen and carbon uptake during a Prorocentrum minimum bloom

During a 4-week period in late spring 1998 an extensive Prorocentrum minimum (Pavillard) Schiller bloom developed in several tributaries of the Chesapeake Bay. Experiments were carried out in one of these tributaries using ¹³C and ¹⁵N isotopic techniques to characterize C and N uptake as a function of irradiance during the course of this bloom. Uptake rates of N substrates (NO₃⁻, NH₄⁺, urea, and an amino acid mixture) and C substrates (bicarbonate and urea) were measured. For each N substrate, short-term uptake rates (0.5 h) were not substantially different over the irradiance range measured, suggesting that N uptake of this dinoflagellate was not strongly light-dependent over this time scale. Dark uptake rates of all N substrates ranged between 35 and 113% of light uptake rates. Over the duration of the P. minimum bloom, however, total ambient N uptake rates increased with increasing natural irradiance. Uptake of bicarbonate showed typical light-dependent photosynthetic characteristics and the measured photosynthetic parameters suggested that at least on the short time scale (0.5 h), P. minimum cells were adapted to high light. Rates of C uptake from the substrate urea were minimal, <1% of total C uptake from photosynthesis, but doubled over the course of the bloom, and like N uptake, were not strongly light-dependent on the short time scale (0.5 h). Significant N dark uptake by P. minimum was likely to have been important by providing N sources over the daily scale to sustain the bloom.     

Virus succession observed during an Emiliania huxleyi bloom

Denaturing gradient gel electrophoresis was used as a molecular tool to determine the diversity and to monitor population dynamics of viruses that infect the globally important coccolithophorid Emiliania huxleyi. We exploited variations in the major capsid protein gene from E. huxleyi-specific viruses to monitor their genetic diversity during an E. huxleyi bloom in a mesocosm experiment off western Norway. We reveal that, despite the presence of several virus genotypes at the start of an E. huxleyi bloom, only a few virus genotypes eventually go on to kill the bloom.     

Predation impact of ciliated and flagellated protozoa during a summer bloom of brown sulfur bacteria in a meromictic coastal lake

Anaerobic phagotrophic protozoa may play an important role in the carbon flux of chemically stratified environments, especially when phototrophic sulfur bacteria account for a high proportion of the primary production. To test this assumption, we investigated the vertical and temporal distribution of microbial heterotrophs and of autotrophic picoplankton throughout the water column of the meromictic coastal lake Faro (Sicily, Italy), in the summer of 2004, coinciding with a bloom of brown-colored green sulfur bacteria. We also assessed the grazing impact of ciliated and flagellated protozoa within the sulfur bacteria plate using a modification of the fluorescently labeled bacteria uptake approach, attempting to minimize the biases intrinsic to the technique and to preserve the in situ anoxic conditions. Significant correlations were observed between ciliate biomass and bacteriochlorophyll e concentration, and between heterotrophic nanoflagellate biomass and chlorophyll a concentration in the water column. The major predators of anaerobic picoplankton were pleuronematine ciliates and cryptomonad flagellates, with clearances of 26.6 and 9.5 nL per cell h⁻¹, respectively, and a cumulative impact on the picoplankton gross growth rate ranging between 36% and 72%. We concluded that protozoan grazing channels a large proportion of anaerobic picoplankton production to higher trophic levels without restraining photosynthetic bacteria productivity.     

Seasonal variation in marine phytoplankton and ice algae at a shallow antarctic coastal site

The phytoplankton population near Davis, Vestfold Hills, Antarctica was monitored throughout 1982. Chlorophyll-a determinations and counts of living cells in both the water column and sea ice demonstrated a marked seasonality in phytoplankton abundane and species composition. From April to October nanoplanktonic organisms contributed most of the chlorophyll-a in both the sea ice and water column. Blooms of diatoms occurred in May, November and December in the bottom of the sea-ice and in January and February in the water column. Phaeocystis pouchetii was dominant during December in the water column. Large numbers of dead diatoms were found in winter. The concentrations of nitrate, dissolved inorganic phosphate and dissolved silicate increased throughout the year until December, when the concentrations of nitrate and silicate fell sharply, followed a month later by a reduction in phosphate concentration. The diversity of phytoplankton was greatest during the summer months.