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The extent and ecological significance of intraspecific functional diversity within marine microbial populations is still poorly understood, and it remains unclear if such strain-level microdiversity will affect fitness and persistence in a rapidly changing ocean environment. In this study, we cultured 11 sympatric strains of the ubiquitous marine picocyanobacteriumSynechococcusisolated from a Narragansett Bay (RI) phytoplankton community thermal selection experiment. Thermal performance curves revealed selection at cool and warm temperatures had subdivided the initial population into thermotypes with pronounced differences in maximum growth temperatures. Curiously, the genomes of all 11 isolates were almost identical (average nucleotide identities of >99.99%, with >99% of the genome aligning) and no differences in gene content or single nucleotide variants were associated with either cool or warm temperature phenotypes. Despite a very high level of genomic similarity, sequenced epigenomes for two strains showed differences in methylation on genes associated with photosynthesis. These corresponded to measured differences in photophysiology, suggesting a potential pathway for future mechanistic research into thermal microdiversity. Our study demonstrates that present-day marine microbial populations can harbor cryptic but environmentally relevant thermotypes which may increase their resilience to future rising temperatures. 

Abstract Organismal distributions are largely mediated by temperature, suggesting thermal trait variability plays a key role in defining species' niches. We employed a trait‐based approach to better understand how inter‐ and intraspecific thermal trait variability could explain diatom community dynamics using 24 strains from 5 species in the diatom genusSkeletonema, isolated from Narragansett Bay (NBay), where this genus can comprise up to 99% of the microplankton. Strain‐specific thermal reaction norms were generated using growth rates obtained at temperatures ranging from −2°C to 36°C. Comparison of thermal reaction norms revealed inter‐ and intraspecific similarities in the thermal optima, but significant differences approaching the thermal limits. Cellular elemental composition was determined for two thermally differentiated species and again, the most variation occurred approaching the thermal limits. To determine the potential impact of interspecific variability on community composition, a species succession model was formulated utilizing each species' empirically determined thermal reaction norm and historical temperature data from NBay. Seasonal succession in the modeled community resembled the timing of species occurrence in the field, but not species' relative abundance. The model correctly predicted the timing of the dominant winter–spring species,Skeletonema marinoi, within 0–14 d of its observed peak occurrence in the field. Interspecific variability approaching the thermal limits provides an alternative mechanism for temporal diatom succession, leads to altered cellular elemental composition, and thus has the potential to influence carbon flux and nutrient cycling, suggesting that growth approaching the thermal limits be incorporated into both empirical and modeling efforts in the future. 

Abstract 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. 

Abstract 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.