Frontal Zone Off Research Articles

Microbial rhodopsins are increasingly favoured over chlorophyll in High Nutrient Low Chlorophyll waters.

Microbial rhodopsins are simple light-harvesting complexes that, unlike chlorophyll photosystems, have no iron requirements for their synthesis and phototrophic functions. Here, we report the environmental concentrations of rhodopsin along the Subtropical Frontal Zone off New Zealand, where Subtropical waters encounter the iron-limited Subantarctic High Nutrient Low Chlorophyll (HNLC) region. Rhodopsin concentrations were highest in HNLC waters where chlorophyll-a concentrations were lowest. Furthermore, while the ratio of rhodopsin to chlorophyll-a photosystems was on average 20 along the transect, this ratio increased to over 60 in HNLC waters. We further show that microbial rhodopsins are abundant in both picoplankton (0.2-3 μm) and in the larger (>3 μm) size fractions of the microbial community containing eukaryotic plankton and/or particle-attached prokaryotes. These findings suggest that rhodopsin phototrophy could be critical for microbial plankton to adapt to resource-limiting environments where photosynthesis and possibly cellular respiration are impaired.

Reducing the arbitrary: fuzzy detection of microbial ecotones and ecosystems \u2013 focus on the pelagic environment

BackgroundOne of the central objectives of microbial ecology is to study the distribution of microbial communities and their association with their environments. Biogeographical studies have partitioned the oceans into provinces and regions, but the identification of their boundaries remains challenging, hindering our ability to study transition zones (i.e. ecotones) and microbial ecosystem heterogeneity. Fuzzy clustering is a promising method to do so, as it creates overlapping sets of clusters. The outputs of these analyses thus appear both structured (into clusters) and gradual (due to the overlaps), which aligns with the inherent continuity of the pelagic environment, and solves the issue of defining ecosystem boundaries.ResultsWe show the suitability of applying fuzzy clustering to address the patchiness of microbial ecosystems, integrating environmental (Sea Surface Temperature, Salinity) and bacterioplankton data (Operational Taxonomic Units (OTUs) based on 16S rRNA gene) collected during six cruises over 1.5 years from the subtropical frontal zone off New Zealand. The technique was able to precisely identify ecological heterogeneity, distinguishing both the patches and the transitions between them. In particular we show that the subtropical front is a distinct, albeit transient, microbial ecosystem. Each water mass harboured a specific microbial community, and the characteristics of their ecotones matched the characteristics of the environmental transitions, highlighting that environmental mixing lead to community mixing. Further explorations into the OTU community compositions revealed that, although only a small proportion of the OTUs explained community variance, their associations with given water mass were consistent through time.ConclusionWe demonstrate recurrent associations between microbial communities and dynamic oceanic features. Fuzzy clusters can be applied to any ecosystem (terrestrial, human, marine, etc) to solve uncertainties regarding the position of microbial ecological boundaries and to refine the relation between the distribution of microorganisms and their environment.

Are oceanic fronts ecotones? Seasonal changes along the subtropical front show fronts as bacterioplankton transition zones but not diversity hotspots.

Ecotones are regarded as diversity hotspots in terrestrial systems, but it is unknown if this 'ecotone effect' occurs in the marine environment. Oceanic fronts are widespread mesoscale features, present in the boundary between different water masses, and are arguably the best potential examples of ecotones in the ocean. Here we performed the first seasonal study along an oceanic front, combining 16S rRNA gene sequencing coupled with a high spatial resolution analysis of the physical properties of the water masses. Using the Subtropical Frontal Zone off New Zealand we demonstrate that fronts delimit shifts in bacterioplankton community composition between water masses, but that the strength of this effect is seasonally dependent. While creating a transition zone where physicochemical parameters and bacterioplankton communities get mixed, this ecotone does not result in increased diversity. Thus unlike terrestrial ecotones, oceanic fronts are boundaries but not hotspots of bacterioplankton diversity in the ocean.

Proportion of marine organic carbon present in self-assembled gels along the subtropical front and its increase in response to reduced pH

Dissolved organic matter (DOM), the largest marine reservoir of reduced carbon, which contains a complex mix of polydisperse biopolymers and particularly small molecules in solution. However, we now know that about 10% of these dissolved moieties can spontaneously aggregate forming a huge mass of highly bioreactive microscopic particles by forming self-assembled gels (SAG) with a significant role in global carbon cycling. However, the effect of varying marine environments on DOM self-assembly remains virtually unknown. Here we report variations of the fraction of DOM that self-assemble forming gels (%SAG) measured in seawater samples collected during two different periods — before and during the autumn bloom— along the Subtropical Frontal Zone off New Zealand; an area characterized by strong spatiotemporal physicochemical and biological variability. Results show that %SAG varies in time and space. Measurements of %SAG ranged between 9–28% (14.5±5, mean±SD), and this broad variability could be partly associated to changes in phytoplankton biomass and/or UV radiation. Additional studies on the effect of a pH on the %SAG, reveal that small pH reductions of 0.3units can virtually double the %SAG from 14.5±5 to 27±7% (12.5±4%, mean±SD). The observed increase in the %SAG under reduced pH was observed in all samples, irrespectively of their origin. This outcome suggests that pH might be a critical parameter controlling the formation of gels in marine environments. Decreased pH increases the SW concentration of free ionized Ca ([Ca+2]), the prime DOM crosslinker. Rising [Ca+2] is likely responsible for the observed %SAG increase. The outcome that %SAG can be strongly affected by small changes in seawater properties, suggest that DOM self-assembly is remarkably responsive to subtle variations of environmental conditions. SAG are a leading source of bacterial nutrition and these observations have potentially strong ramifications in the ecology and biogeochemical cycles of the oceans.

Oceanic fronts: transition zones for bacterioplankton community composition.

Oceanic fronts are widespread mesoscale features that exist in the boundary between different water masses. Despite the recognized importance of bacterioplankton (including bacteria and archaea) on the marine biogeochemical cycles and the ubiquitousness of fronts, the effect of frontal zones on the distribution of bacterioplankton community remains unknown. Using 16S rRNA gene sequencing coupled with a high spatial resolution analysis of the physical properties of the water masses, we demonstrate strong shifts in bacterioplankton community composition (BCC) across the subtropical frontal zone off New Zealand. The transition between water masses resulted in a clear modification of the dominant taxa and a significant increase in community dissimilarity. Our results, linking physical oceanography and marine molecular ecology, support the strong role of oceanic frontal zones in delimiting the distribution of bacterioplankton in the ocean.

Bacterioplankton carbon cycling along the Subtropical Frontal Zone off New Zealand

Marine heterotrophic bacterioplankton (Bacteria and Archaea) play a central role in ocean carbon cycling. As such, identifying the factors controlling these microbial populations is crucial to fully understanding carbon fluxes. We studied bacterioplankton activities along a transect crossing three water masses (i.e., Subtropical waters [STW], Sub-Antarctic waters [SAW] and neritic waters [NW]) with contrasting nutrient regimes across the Subtropical Frontal Zone. In contrast to bacterioplankton production and community respiration, bacterioplankton respiration increased in the offshore SAW, causing a seaward increase in the contribution of bacteria to community respiration (from 7% to 100%). Cell-specific bacterioplankton respiration also increased in SAW, but cell-specific production did not, suggesting that prokaryotic cells in SAW were investing more energy towards respiration than growth. This was reflected in a 5-fold decline in bacterioplankton growth efficiency (BGE) towards SAW. One way to explain this decrease in BGE could be due to the observed reduction in phytoplankton biomass (and presumably organic matter concentration) towards SAW. However, this would not explain why bacterioplankton respiration was highest in SAW, where phytoplankton biomass was lowest. Another factor affecting BGE could be the iron limitation characteristic of high-nutrient low-chlorophyll (HNLC) regions like SAW. Our field-study based evidences would agree with previous laboratory experiments in which iron stress provoked a decrease in BGE of marine bacterial isolates. Our results suggest that there is a strong gradient in bacterioplankton carbon cycling rates along the Subtropical Frontal Zone, mainly due to the HNLC conditions of SAW. We suggest that Fe-induced reduction of BGE in HNLC regions like SAW could be relevant in marine carbon cycling, inducing bacterioplankton to act as a link or a sink of organic carbon by impacting on the quantity of organic carbon they incorporate into biomass or respire as CO2.

Distribution and modification of diatom assemblages in and around a warm core ring in the western North Pacific Frontal Zone east of Hokkaido

Spatial distribution relationships between diatom assemblages and water types were inves- tigated in and around a 2-year-old warm core ring in the western North Pacific Polar Frontal Zone off Hokkaido, Japan. From the oceanographic and diatom data, principal component analysis and a clustering technique identified four water types and four diatom assemblages. The Background Assemblage had a low standing stock and was distributed over the entire study area, including the core water of the ring. This assemblage was associated with water conditions that were not highly suitable for diatom growth. A Cold Assemblage, probably linked to submerging Oyashio Water, was found in sub-surface waters. Its standing crop was rather high. The two other assemblages also had relatively high standing stocks: one, the Stratified Assemblage, was found around the nutricline in the sub-surface layer and the other, the Warm Streamer Assemblage, in a warm streamer flowing along the outer edge of the warm core ring. We argue here that the Stratified Assemblage emerged from the Background Assemblage because of favorable local conditions (nutrient level), and, conversely, that in the downstream of the warm streamer, the Warm Streamer Assemblage reversed to the Back- ground Assemblage because the nutrient supply was depleted.

Phytoplankton Chl a, primary production and nutrient distributions across a coastal frontal zone off Georgia, U.S.A.

Using results obtained by ship and aircraft surveys of near-shore waters off Georgia, U.S.A. we describe the gradients in major plant nutrients, near-surface cholorphyll (Chl a) and near-surface primary production in relation to near-surface density gradients of a coastal frontal zone. The results show that a relatively narrow band (∼10 km wide) of high phytoplankton biomass (Chl a) is a ubiquitous feature of the shoreward side of the coastal frontal zone. Chl a decreases across the frontal zone as salinity increases and near surface Chl a concentrations are approximately 10-fold greater in the high Chl a band than on the seaward side of the frontal zone. Near-surface phosphate and silicate concentrations are distributed similarly to Chl a. In contrast to silicate and phosphate, we did not detect significant cross-shelf gradients of inorganic nitrogen (N) concentrations. The distributions imply high rates of primary production in the coastal band throughout the year, total consumption of available inorganic N brought in by rivers or other sources and high rates of N recycling.

Subsurface energetics of the Gulf Stream cyclonic frontal zone off Onslow Bay, North Carolina

It has been shown with the use of 4‐month‐long time series of velocity, temperature, and conductivity that fluctuating kinetic and potential energy was converted into kinetic and potential energy of the mean flow following a fluid particle in the subsurface Gulf Stream cyclonic frontal zone off Onslow Bay, North Carolina, during early 1979. This result agrees well with earlier measurements made in the surface layer off Onslow Bay. These flux calculations represent an important step in verifying the direction of the net cross‐stream energy flux within the stream off Onslow Bay. According to an hypothesis presented for the growth and decay of Gulf Stream meanders along the continental margin of the southeastern United States, Onslow Bay is an area of decreasing meander amplitude. The direction of the energy conversion from meanders to the mean flow, determined from our calculations, is consistent with this hypothesis. Relatively low velocity covariances were found to be associated with relatively small transfers of kinetic energy during a period of low meander activity. This finding supports the notion that meanders play a significant role in the energy transformation processes. The presence of such ‘quiet’ periods may indicate a low‐frequency modulation of Gulf Stream meander activity.

Icthyoplankton of Slope and Gulf Stream Waters off Nova Scotia in late Autumn 1974

Ichthyoplankton, collected in the Gulf Stream frontal zone off Nova Scotia in late autumn 1974, was represented by 22 families. The larvae of some species of these families have not been previously found in the area. Myctophid larvae were the most abundant and were represented in many samples. Zoogeographical structure of the ichthyoplankton community and the larval distribution of some groups are briefly discussed.

Observation and interpretation of a high-frequency internal wave packet and surface slick pattern

A packet of high-frequency (period ∼8 min) internal gravity waves was observed on a precision-depth recorder (PDR) chart while an anchor station was being conducted during the Cue 1 experiment off Oregon. The anchor station was located over the continental shelf and in the frontal zone associated with coastal upwelling. The packet appeared as a set of three oscillations (maximum peak-to-peak amplitude of 11 m) of a scattering layer located at a depth of about 20 m, which was immediately below the center of the permanent pycnocline (25.5–26.0 sigma t) at that time and location (water depth 80 m, 10-km offshore). The scattering layer occurred at the depth of a chlorophyll A maximum; strong temperature, salinity, and sound speed gradients; and strong vertical shear of the horizontal velocity. Simultaneously, a series of surface slicks, oriented parallel to the isobaths and propagating onshore, was observed under conditions of light winds. From the spacing of the slicks a horizontal scale of 100 m was estimated; this scale corresponded most nearly to the wavelength of the first internal wave mode for the observed wave period. From time series of nearby moored current and temperature sensors, bursts of high-frequency oscillations were noted to occur on semidiurnal tidal cycles. Such bursts had occurred a few hours before the internal wave packet was observed on the PDR. It is concluded that the semidiurnal baroclinic tide breaks down in the frontal zone off Oregon, forming large-amplitude high-frequency internal gravity waves. These waves can induce detectable surface slicks under light wind conditions and major perturbations of a shallow scattering layer when it is present.