Significant organic carbon acquisition by Prochlorococcus in the oceans

Marine phytoplankton are responsible for about half of the photosynthesis on Earth. Many are mixotrophs, combining photosynthesis with heterotrophic assimilation of organic carbon but the relative contribution of these two carbon sources is not well quantified. Here, single-cell measurements reveal that Prochlorococcus at the base of the photic zone in the Eastern Mediterranean Sea are obtaining only ~20% of carbon required for growth by photosynthesis. Consistently, laboratory-calibrated evaluations of Prochlorococcus photosynthesis indicate that carbon fixation is systematically too low to support published in situ growth rates in the deep photic layer of the Pacific Ocean. Furthermore, agent-based model simulations show that mixotrophic cells maintain realistic growth rates and populations 10s of meters deeper than obligate photo-autotrophs, deepening the nutricline and Deep Chlorophyll Maximum by ~20 m. Time-series of Prochlorococcus ecotype-abundance from the subtropical North Atlantic and North Pacific suggest that up to 30% of the Prochlorococcus cells live where light intensity is not enough to sustain obligate photo-autotrophic populations during warm, stratified periods. Together, these data and models suggest that mixotrophy underpins the ecological success of a large fraction of the global Prochlorococcus population and its collective genetic diversity.

Total Water Column Analysis Shows the Importance of a Single Species in Subsurface Chlorophyll Maximum Thin Layers in Stratified Waters

Marine phytoplankton form the base of marine food webs and are the driving force of the marine carbon cycle, so understanding the dynamics of their blooms is critical. While near-surface marine productivity (<10 m water depths) is extensively documented, that of the subsurface is less well characterised. Increasing evidence of the importance of subsurface chlorophyll maxima (SCM) and climatically driven increases in stratification of surface waters that promote SCM development call for improved sampling of the subsurface. To address this, we targeted the summer stratified waters of the Western English Channel, part of the NW European shelf seas, where SCM are commonly developed. In situ holography was applied to undertake the highest ever resolution, total water column, quantitative analysis of microplankton distribution, and demonstrated the importance of a SCM, co-located with the thermocline, dominated by a single species, the dinoflagellate Ceratium fusus. This species was dominant in the SCM over a wide area of the NW European shelf in the June/July 2015 study period and comprised up to 85% of the SCM biomass. Analysis of similarity and multivariate non-metric multidimensional scaling showed the phytoplankton community of the SCM to be statistically distinct from those of the surface and deep waters. Holography also revealed a fine scale layering of taxa at different levels within the SCM, likely reflecting ecological differences. Some taxa followed the peak abundance of C. fusus, while others reached maximum abundances immediately below or above the C. fusus maximum, suggesting the possible operation of exclusion mechanisms. Additionally, the detection of abundant aggregates located only within and beneath the SCM demonstrates the potential importance of this deep production for the export of carbon to the sea floor. Some predictions of phytoplankton productivity propose a shift to smaller cells in the more stratified oceans of the future resulting in declining production and export. Results presented here, however, contribute to a growing body of evidence that suggests, on the contrary, that key species among the larger celled/colonial, SCM-adapted diatoms and dinoflagellates may instead be selected in stratified conditions, driving increased production and export.

Investigating the effect of nickel concentration on phytoplankton growth to inform the assessment of ocean alkalinity enhancement

Ocean alkalinity enhancement (OAE) is a proposed method for removing carbon dioxide (CO2) from the atmosphere by the accelerated weathering of (ultra-) basic minerals to increase alkalinity – the chemical capacity of seawater to store CO2. During the weathering of OAE-relevant minerals relatively large amounts of trace metals will be released and may perturb pelagic ecosystems. Nickel (Ni) is of particular concern as it is abundant in olivine, one of the most widely considered minerals for OAE. However, so far there is limited knowledge about the impact of Ni on marine biota including phytoplankton. To fill this knowledge gap, this study tested the growth and photo-physiological response of 11 marine phytoplankton species to a wide range of dissolved Ni concentrations (from 0 nmol/L to 50,000 nmol/L). We found that the phytoplankton species were not very sensitive to Ni concentrations under the culturing conditions established in our experiments, but the responses were species-specific. The growth rates of 6 of the 11 tested species showed small but significant responses to changing Ni concentrations. Photosynthetic performance, assessed by measuring the maximum quantum yield (Fv/Fm) and the functional absorption cross-section (σPSII) of photosystem II, was also only mildly sensitive to changing Ni in 3 out of 11 species and 4 out of 11 species, respectively. The limited effect of Ni may be partly due to the provision of nitrate as the nitrogen source for growth, as previous studies suggest higher sensitivities when urea is the nitrogen source. Furthermore, limited influence may be due to the relatively high concentrations of organic ligands in the growth media in our experiments. These ligands reduced bioavailable Ni (i.e., “free Ni2+”) concentrations by binding the majority of the dissolved Ni. Our data suggest that dissolved Ni does not have a strong effect on phytoplankton under our experimental conditions, but we emphasize that a deeper understanding of nitrogen sources, ligand concentrations and phytoplankton composition is needed when assessing the influence of Ni release associated with OAE. We discuss if applications of OAE with Ni-rich minerals may be safer in regions with high organic ligand concentrations and low concentrations of urea as such boundary conditions may lead to less impact of Ni on phytoplankton communities.

Environmental control of marine phytoplankton stoichiometry in the North Atlantic Ocean

The stoichiometric coupling of carbon to limiting nutrients in marine phytoplankton regulates the magnitude of biological carbon sequestration in the ocean. While clear links between plankton C:N ratios and environmental drivers have been identified, the nature and direction of these links, as well as their underlying physiological and ecological controls, remain uncertain. We show, with a well-constrained mechanistic model of plankton ecophysiology, that while nitrogen availability and temperature emerge as the main drivers of phytoplankton C:N stoichiometry in the North Atlantic, the biological mechanisms involved vary depending on the spatiotemporal scale and region considered. We find that phytoplankton C:N stoichiometry is overall controlled by nitrogen availability below 40° N, predominantly driven by ecoevolutionary shifts in the functional composition of the phytoplankton communities, while phytoplankton stoichiometric plasticity in response to dropping temperatures and increased grazing pressure dominates at higher latitudes. Our findings highlight the potential of “organisms-to-ecosystems” modeling approaches based on mechanistic models of plankton biology accounting for physiology, ecology, and trait evolution to explore and explain complex observational data and ultimately improve the predictions of global ocean models.

BALLAST WATER: A POLYPHASIC APPROACH IN THE EVALUATION OF POTENTIAL PHYTOPLANKTONIC BIOINVASION

The main transport vector for exotic-invasive species around the world is ballast water in merchant ships, crossing natural biogeographic barriers. This is one of the main factors responsible for the reduction and homogenization of the global biota. As phytoplankton is the main group transported by these vessels, this study assesses the risk of bioinvasion in the port complex of Itajaí and Navegantes (southern Brazil), using a polyphasic approach, mixing classical taxonomy with molecular biology. Ballast water collections were carried out to analyze the traditional taxonomy and for clonal cultivation in the laboratory. A successful cultivation of 12 local strains and 10-ballast water was successful. In the latter case, some fresh water, contrary to the statement by the commanders of the exchange of water in the deep ocean (> 200 m). Molecular identification was performed by sequencing the complete ITS region, confirming the presence of Thalassiosira minuscula Krasske, 1941, harmful and of public health interest, previously not mentioned for the region. The species Pleurosigma W. Smith, 1852; Asterionellopsis glacialis (Castracane) Round, 1990; Trieres mobiliensis (Bailey) Ashworth and E.C. Theriot, 2013; Thalassiosira minima Gaarder, 1951; Skeletonema pseudocostatum Medlin, 1991; Pectinodesmus holtmannii E. Hegewald, C. Bock and Krienitz, 2013; Neodesmus Hindák, 1976; and Pseudopediastrum boryanum (Turpin) E. Hegewald, 2005, were identified. The results indicate the possibility of growth of the species found in the ballast environment, which may negatively alter the disposal environment. Keywords: exotic species, molecular identification, marine phytoplankton, port complex of Itajaí and Navegantes.

Thermal plasticity of growth and chain formation of the dinoflagellates Alexandrium affine and Alexandrium pacificum with respect to ocean acidification

The amount of CO2 absorbed by the oceans continues to rise, resulting in further acidification, altering some functional traits of phytoplankton. To understand the effect of elevated partial pressures of CO2 (pCO2) on functional traits of dinoflagellates Alexandrium affine and A. pacificum, the cardinal temperatures and chain formation extent were examined under two pCO2 (400 and 1,000 μatm) over the range of temperature expected to be associated with growth. The growth rate and chain formation extent of A. affine increased with higher pCO2, showing significant changes in cardinal temperatures and a substantial increase in middle chain-length (4‒8 cells) fractionation under elevated pCO2 condition. By contrast, there were no significant differences in specific growth rate and any chain-length fractionation of A. pacificum between ambient and elevated pCO2 conditions. The observed interspecies variation in the functional traits may reflect differences in ability of species to respond to environmental change with plasticity. Moreover, it allows us to understand the shifting biogeography of marine phytoplankton and predict their phenology in the Korea Strait.

Irradiance and nutrient-dependent effects on photosynthetic electron transport in Arctic phytoplankton: A comparison of two chlorophyll fluorescence-based approaches to derive primary photochemistry

We employed Fast Repetition Rate fluorometry for high-resolution mapping of marine phytoplankton photophysiology and primary photochemistry in the Lancaster Sound and Barrow Strait regions of the Canadian Arctic Archipelago in the summer of 2019. Continuous ship-board analysis of chlorophyll a variable fluorescence demonstrated relatively low photochemical efficiency over most of the cruise-track, with the exception of localized regions within Barrow Strait, where there was increased vertical mixing and proximity to land-based nutrient sources. Along the full transect, we observed strong non-photochemical quenching of chlorophyll fluorescence, with relaxation times longer than the 5-minute period used for dark acclimation. Such long-term quenching effects complicate continuous underway acquisition of fluorescence amplitude-based estimates of photosynthetic electron transport rates, which rely on dark acclimation of samples. As an alternative, we employed a new algorithm to derive electron transport rates based on analysis of fluorescence relaxation kinetics, which does not require dark acclimation. Direct comparison of kinetics- and amplitude-based electron transport rate measurements demonstrated that kinetic-based estimates were, on average, 2-fold higher than amplitude-based values. The magnitude of decoupling between the two electron transport rate estimates increased in association with photophysiological diagnostics of nutrient stress. Discrepancies between electron transport rate estimates likely resulted from the use of different photophysiological parameters to derive the kinetics- and amplitude-based algorithms, and choice of numerical model used to fit variable fluorescence curves and analyze fluorescence kinetics under actinic light. Our results highlight environmental and methodological influences on fluorescence-based photochemistry estimates, and prompt discussion of best-practices for future underway fluorescence-based efforts to monitor phytoplankton photosynthesis.