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ABSTRACT Cluster 5 Synechococcus species are widely acknowledged for their broad distribution and biogeochemical importance. In particular, subcluster 5.2 strains inhabit freshwater, estuarine, and marine environments but are understudied, compared to other subclusters. Here, we present the genome for Synechococcus sp. strain LA31, a strain that was recently isolated from Narragansett Bay, Rhode Island, USA. 

Marine microbial communities in coastal environments are subject to both seasonal fluctuations and anthropogenic alterations of environmental conditions. The separate influences of temperature and resource‐dependency on phytoplankton growth, community, and ecosystem metabolism are relatively well understood. However, winners and losers in the ocean are determined based on the interplay among often rapidly changing biological, chemical and physical drivers. The direct, indirect, and interactive effects of these conditions on planktonic food web structure and function are poorly constrained. Here, we investigated how simultaneous manipulation of temperature and nutrient availability affects trophic transfer from phytoplankton to herbivorous protists, and their resulting implications at the ecosystem level. Temperature directly affected herbivorous protist composition; ciliates dominated (66%) in colder treatment and dinoflagellates (60%) at warmer temperatures. Throughout the experiments, grazing rates were < 0.1 d⁻¹, with higher rates at subzero temperatures. Overall, the nutrient–temperature interplay affected trophic transfer rates antagonistically when nutrients were amended, and synergistically, when nutrients were not added. This interaction resulted in higher percentages of primary production consumed under nutrient unamended compared to nutrient amended conditions. At the ecosystem level, these changes may determine the fate of primary production, with most of the production likely exported out of the pelagic zone in high‐temperature and nutrient conditions, while high‐temperature and low‐nutrient availability strengthened food web coupling and enhanced trophic transfer. These results imply that in warming oceans, management of coastal nutrient loading will be a critical determinant of the degree of primary production removal by microzooplankton and dependent ecosystem production.

 

A complex interplay of environmental variables impacts phytoplankton community composition and physiology. Temperature and nutrient availability are two principal factors driving phytoplankton growth and composition, but are often investigated independently and on individual species in the laboratory. To assess the individual and interactive effects of temperature and nutrient concentration on phytoplankton community composition and physiology, we altered both the thermal and nutrient conditions of a cold‐adapted spring phytoplankton community in Narragansett Bay, Rhode Island, when surface temperature was 2.6°C and chlorophyll > 9 μg L⁻¹. Water was incubated in triplicate at −0.5°C, 2.6°C, and 6°C for 10 d. At each temperature, treatments included both nutrient amendments (N, P, Si addition) and controls (no macronutrients added). The interactive effects of temperature and resource availability altered phytoplankton growth and community structure. Nutrient amendments resulted in species sorting and communities dominated by larger species. Under replete nutrients, warming tripled phytoplankton growth rates, but under in situ nutrient conditions, increased temperature acted antagonistically, reducing growth rates by as much as 33%, suggesting communities became nutrient limited. The temperature–nutrient interplay shifted the relative proportions of each species within the phytoplankton community, resulting in more silica rich cells at decreasing temperatures, irrespective of nutrients, and C : N that varied based on resource availability, with nutrient limitation inducing a 47% increase in C : N at increasing temperatures. Our results illustrate how the temperature–nutrient interplay can alter phytoplankton community dynamics, with changes in temperature amplifying or exacerbating the nutrient effect with implications for higher trophic levels and carbon flux.

 

Diatoms have well‐recognized roles in fixing and exporting carbon and supplying energy to marine ecosystems, but only recently have we begun to explore the diversity and importance of nano‐ and pico‐diatoms. Here, we describe a small (ca. 5 μm) diatom from the genusChaetocerosisolated from a wintertime temperate estuary (2°C, Narragansett Bay, Rhode Island), with a unique obligate specialization for low‐light environments (< 120 μmol photons m⁻² s⁻¹). This diatom exhibits a striking interaction between irradiance and thermal responses whereby as temperatures increase, so does its susceptibility to light stress. Historical 18S rRNA amplicon data from our study site show this isolate was abundant throughout a 6‐yr period, and its presence strongly correlates with winter and early spring months when light and temperature are low. Two amplicon sequence variants matching this isolate had a circumpolar distribution in Tara Polar Ocean Circle samples, indicating its unusual light and temperature requirements are adaptations to life in a cold, dark environment. We expect this isolate's low light, psychrophilic niche to shrink as future warming‐induced stratification increases both light and temperature levels experienced by high latitude marine phytoplankton.

 

Synechococcus, a genus of unicellular cyanobacteria found throughout the global surface ocean, is a large driver of Earth's carbon cycle. Developing a better understanding of its diversity and distributions is an ongoing effort in biological oceanography. Here, we introduce 12 new draft genomes of marineSynechococcusisolates spanning five clades and utilize ~100 environmental metagenomes largely sourced from the TARA Oceans project to assess the global distributions of the genomic lineages they and other reference genomes represent. We show that five newly provided clade‐II isolates are by far the most representative of the recoveredin situpopulations (most ‘abundant’) and have biogeographic distributions distinct from previously available clade‐II references. Additionally, these isolates form a subclade possessing the smallest genomes yet identified of the genus (2.14 ± 0.05Mbps; mean ± 1SD) while concurrently hosting some of the highest GC contents (60.67 ± 0.16%). This is in direct opposition to the pattern inSynechococcus’s nearest relative,Prochlorococcus– wherein decreasing genome size has coincided with a strongdecreasein GC content – suggesting this new subclade ofSynechococcusappears to have convergently undergone genomic reduction relative to the rest of the genus, but along a fundamentally different evolutionary trajectory.