Explore the vast range of titles available.

Key Points Phytoplankton are the most abundant primary producers in the oceans, and phytoplankton blooms are recognizable signs of the annual productivity cycle in aquatic systems. Phytoplankton blooms contain dense and diverse heterotrophic bacterial populations that determine the fate of much of the carbon that is fixed by these primary producers. This is achieved by the transformation of phytoplankton-derived organic matter, which returns carbon to the atmosphere as CO 2 and converts carbon to bacterial biomass, which enters the marine food web or renders it resistant to microbial degradation, such that it contributes to a vast pool of recalcitrant carbon in the ocean. Although blooms vary in terms of phytoplankton composition and environmental conditions, a limited number of bacterial taxa dominate bloom-associated microbial communities. The most frequently observed bacteria belong to the Flavobacteriia and Proteobacteria. Cultivated representatives of both flavobacteria and roseobacters are currently the main models that are used to study phytoplankton–bacteria interactions. These two lineages show substantial metabolic versatility, which seems to fuel these interactions. Culture-based studies of roseobacters suggest that they form more intimate associations with specific phytoplankton than flavobacteria. Specific physiological processes that have been identified in cultured representatives and are supported by metagenomic data from natural populations have been proposed to facilitate these interactions. These include the production of secondary metabolites, catabolism of various phytoplankton-derived low molecular weight compounds and cell surface structures that facilitate cellular adhesion. Genomic, metatranscriptomic and metaproteomic data suggest that flavobacteria are particularly well equipped to use the high molecular weight components of phytoplankton-derived material. Other flavobacterial physiologies, including cell adhesion and motility, may be important in facilitating interactions between flavobacteria and phytoplankton. Marine phytoplankton blooms are annual spring events that are accompanied by a surge in heterotrophic bacteria, primarily roseobacters, flavobacteria and members of the Gammaproteobacteria, which recycle most of the carbon that is fixed by the primary producers. In this Review, Buchan et al . describe the emerging physiological features and functions of these bacterial communities and their interactions with phytoplankton. Marine phytoplankton blooms are annual spring events that sustain active and diverse bloom-associated bacterial populations. Blooms vary considerably in terms of eukaryotic species composition and environmental conditions, but a limited number of heterotrophic bacterial lineages — primarily members of the Flavobacteriia, Alphaproteobacteria and Gammaproteobacteria — dominate these communities. In this Review, we discuss the central role that these bacteria have in transforming phytoplankton-derived organic matter and thus in biogeochemical nutrient cycling. On the basis of selected field and laboratory-based studies of flavobacteria and roseobacters, distinct metabolic strategies are emerging for these archetypal phytoplankton-associated taxa, which provide insights into the underlying mechanisms that dictate their behaviours during blooms.

In the nutrient-rich region surrounding marine phytoplankton cells, heterotrophic bacterioplankton transform a major fraction of recently fixed carbon through the uptake and catabolism of phytoplankton metabolites. We sought to understand the rules by which marine bacterial communities assemble in these nutrient-enhanced phycospheres, specifically addressing the role of host resources in driving community coalescence. Synthetic systems with varying combinations of known exometabolites of marine phytoplankton were inoculated with seawater bacterial assemblages, and communities were transferred daily to mimic the average duration of natural phycospheres. We found that bacterial community assembly was predictable from linear combinations of the taxa maintained on each individual metabolite in the mixture, weighted for the growth each supported. Deviations from this simple additive resource model were observed but also attributed to resource-based factors via enhanced bacterial growth when host metabolites were available concurrently. The ability of photosynthetic hosts to shape bacterial associates through excreted metabolites represents a mechanism by which microbiomes with beneficial effects on host growth could be recruited. In the surface ocean, resource-based assembly of host-associated communities may underpin the evolution and maintenance of microbial interactions and determine the fate of a substantial portion of Earth’s primary production.

Phytoplankton blooms in coastal oceans can be beneficial to coastal fisheries production and ecosystem function, but can also cause major environmental problems 1 , 2 —yet detailed characterizations of bloom incidence and distribution are not available worldwide. Here we map daily marine coastal algal blooms between 2003 and 2020 using global satellite observations at 1-km spatial resolution. We found that algal blooms occurred in 126 out of the 153 coastal countries examined. Globally, the spatial extent (+13.2%) and frequency (+59.2%) of blooms increased significantly ( P  < 0.05) over the study period, whereas blooms weakened in tropical and subtropical areas of the Northern Hemisphere. We documented the relationship between the bloom trends and ocean circulation, and identified the stimulatory effects of recent increases in sea surface temperature. Our compilation of daily mapped coastal phytoplankton blooms provides the basis for global assessments of bloom risks and benefits, and for the formulation or evaluation of management or policy actions. Satellite observations reveal global increases in the extent and frequency of phytoplankton blooms between 2003 and 2020 and provide insights into the relationship between blooms, ocean circulation and sea surface temperature.

Essential fatty acids (EFA), which are primarily generated by phytoplankton, limit growth and reproduction in diverse heterotrophs. The biochemical composition of phytoplankton is well-known to be governed both by phylogeny and environmental conditions. Nutrients, light, salinity, and temperature all affect both phytoplankton growth and fatty acid composition. However, the relative importance of taxonomy and environment on algal fatty acid content has yet to be comparatively quantified, thus inhibiting predictions of changes to phytoplankton food quality in response to global environmental change. We compiled 1145 published marine and freshwater phytoplankton fatty acid profiles, consisting of 208 species from six major taxonomic groups, cultured in a wide range of environmental conditions, and used a multivariate distance-based linear model to quantify the total variation explained by each variable. Our results show that taxonomic group accounts for 3-4 times more variation in phytoplankton fatty acids than the most important growth condition variables. The results underscore that environmental conditions clearly affect phytoplankton fatty acid profiles, but also show that conditions account for relatively low variation compared to phylogeny. This suggests that the underlying mechanism determining basal food quality in aquatic habitats is primarily phytoplankton community composition, and allows for prediction of environmental-scale EFA dynamics based on phytoplankton community data. We used the compiled dataset to calculate seasonal dynamics of long-chain EFA (LCEFA; ≥C20 ɷ-3 and ɷ-6 polyunsaturated fatty acid) concentrations and ɷ-3:ɷ-6 EFA ratios in Lake Washington using a multi-decadal phytoplankton community time series. These analyses quantify temporal dynamics of algal-derived LCEFA and food quality in a freshwater ecosystem that has undergone large community changes as a result of shifting resource management practices, highlighting diatoms, cryptophytes and dinoflagellates as key sources of LCEFA. Moreover, the analyses indicate that future shifts towards cyanobacteria-dominated communities will result in lower LCEFA content in aquatic ecosystems.

Phytoplankton growth ( μ ) and grazing loss ( g ) rates were measured monthly by the Landry-Hassett dilution method over a 2-year period at both estuarine (Klang) and coastal water (Port Dickson) systems along the Straits of Malacca. Chlorophyll a (Chl a ) concentration ranged from 0.20 to 4.47 μg L −1 at Klang except on two occasions when Chl a spiked above 10 μg L −1 . In contrast, Chl a concentrations were relatively stable at Port Dickson (0.14 to 2.76 μg L −1 ). From the rate measurements, μ was higher ( t  = 2.01, df  = 43, p  < 0.05) at Klang (0.30 to 2.26 day −1 ) than at Port Dickson (0.18 to 1.66 day −1 ), but g was not significantly different ( p  > 0.80). g ranged from 0.30 to 1.50 and 0.21 to 1.51 day −1 at Klang and Port Dickson, respectively. In this study, grazing loss was coupled to phytoplankton growth, and the ratio of g / μ or grazing pressure which estimates the proportion of primary production grazed was 50 % at Klang and lower than at Port Dickson (68 %; t  = 2.213, df  = 36, p  < 0.05). We found that the higher growth rates in a eutrophic system, i.e., Klang, were not matched by higher grazing loss, and this may have implications for the biogeochemical cycling in coastal waters.

The distribution variation in chromophoric dissolved organic matter (CDOM) content in mid-latitude subtropical drinking water source reservoirs (MDWSRs) has great significance in the security of aquatic environments and human health. CDOM distribution is heavily influenced by biogeochemical processes and anthropogenic activity. However, little is known regarding the impact of component variation and phytoplankton growth on CDOM distribution variation in MDWSR. Therefore, samples were collected from a representative MDWSR (the Shanzai Reservoir) for analysis. CDOM absorption and fluorescence coupling with parallel factor analysis were measured and calculated. The results indicated that only two CDOM components were found in the surface water of Shanzai Reservoir, fulvic acid, and high-excitation tryptophan, originating from terrestrial and autochthonous sources, respectively. The types of components did not change with the season. The average molecular weight of CDOM increased in proportion to its fulvic acid content. The distribution variation in CDOM content mainly resulted from the variation in two CDOM components in summer and from high-excitation tryptophan in winter. Phytoplankton growth strongly influenced the distribution variation of CDOM content in summer; the metabolic processes of Cyanobacteria and Bacillariophyta consumed fulvic acid, while that of Cryptophyta produced high-excitation tryptophan.

The organic carbon produced in the ocean’s surface by phytoplankton is either passed through the food web or exported to the ocean interior as marine snow. The rate and efficiency of such vertical export strongly depend on the size, structure and shape of individual particles, but apart from size, other morphological properties are still not quantitatively monitored. With the growing number of in situ imaging technologies, there is now a great possibility to analyze the morphology of individual marine snow. Thus, automated methods for their classification are urgently needed. Consequently, here we present a simple, objective categorization method of marine snow into a few ecologically meaningful functional morphotypes using field data from successive phases of the Arctic phytoplankton bloom. The proposed approach is a promising tool for future studies aiming to integrate the diversity, composition and morphology of marine snow into our understanding of the biological carbon pump. Marine snow is a major route through which photosynthetically fixed carbon is transported to the deep ocean, but the factors affecting flux are largely unknown. Here the authors use high frequency imaging of marine snow particles collected during phytoplankton blooms to categorize and quantify transport.

From microbes to large predators, there is increasing evidence that marine life is shaped by short-lived submesoscales currents that are difficult to observe, model, and explain theoretically. Whether and how these intense three-dimensional currents structure the productivity and diversity of marine ecosystems is a subject of active debate. Our synthesis of observations and models suggests that the shallow penetration of submesoscale vertical currents might limit their impact on productivity, though ecological interactions at the submesoscale may be important in structuring oceanic biodiversity. Short-lived three-dimensional submesoscale currents, responsible for swirling ocean color chlorophyll filaments, have long been thought to affect productivity. Current research suggests they may not be effective in enhancing phytoplankton growth, but may have important contributions to biodiversity.

Phytoplankton blooms characterize temperate ocean margin zones in spring. We investigated the bacterioplankton response to a diatom bloom in the North Sea and observed a dynamic succession of populations at genus-level resolution. Taxonomically distinct expressions of carbohydrate-active enzymes (transporters; in particular, TonB-dependent transporters) and phosphate acquisition strategies were found, indicating that distinct populations of Bacteroidetes, Gammaproteobacteria, and Alphaproteobacteria are specialized for successive decomposition of algal-derived organic matter. Our results suggest that algal substrate availability provided a series of ecological niches in which specialized populations could bloom. This reveals how planktonic species, despite their seemingly homogeneous habitat, can evade extinction by direct competition.

Residual macronutrients in the surface Southern Ocean result from restricted biological utilization, caused by low wintertime irradiance, cold temperatures, and insufficient micronutrients. Variability in utilization alters oceanic CO 2 sequestration at glacial-interglacial timescales. The role for insufficient iron has been examined in detail, but manganese also has an essential function in photosynthesis and dissolved concentrations in the Southern Ocean can be strongly depleted. However, clear evidence for or against manganese limitation in this system is lacking. Here we present results from ten experiments distributed across Drake Passage. We found manganese (co-)limited phytoplankton growth and macronutrient consumption in central Drake Passage, whilst iron limitation was widespread nearer the South American and Antarctic continental shelves. Spatial patterns were reconciled with the different rates and timescales for removal of each element from seawater. Our results suggest an important role for manganese in modelling Southern Ocean productivity and understanding major nutrient drawdown in glacial periods. Southern Ocean productivity is a crucial component of the carbon cycle, but phytoplankton there are thought to be limited by iron. Here the authors conduct trace metal incubation experiments across the Drake Passage, finding that manganese can play an unexpected role in restricting phytoplankton growth.