The present study sought to characterize the synthesis pathways producing the essential polyunsaturated fatty acid (PUFA) 20:5n-3 (EPA). For this, the incorporation of ¹³C was experimentally monitored into 10 fatty acids (FA) during the growth of the diatom Chaetoceros muelleri for 24 h. Chaetoceros muelleri preferentially and quickly incorporated ¹³C into C₁₈ PUFAs such as 18:2n-6 and 18:3n-6 as well as 16:0 and 16:1n-7, which were thus highly ¹³C-enriched. During the experiment, 20:5n-3 and 16:3n-4 were among the least-enriched fatty acids. The calculation of the enrichment percentage ratio of a fatty acid B over its suspected precursor A allowed us to suggest that the diatom produced 20:5n-3 (EPA) by a combination between the n-3 (via 18:4n-3) and n-6 (via 18:3n-6 and 20:4n-6) synthesis pathways as well as the alternative ω-3 desaturase pathway (via 20:4n-6). In addition, as FA from polar lipids were generally more enriched in ¹³C than FA from neutral lipids, particularly for 18:1n-9, 18:2n-6 and 18:3n-6, the existence of acyl-editing mechanisms and connectivity between polar and neutral lipid fatty acid pools were also hypothesized. Because 16:3n-4 and 20:5n-3 presented the same concentration and enrichment dynamics, a structural and metabolic link was proposed between these two PUFAs in C. muelleri.

The synthetic pathways responsible for the production of the polyunsaturated fatty acids 22:6n‐3 and 20:5n‐3 were studied in the Dinophyte Alexandrium minutum. The purpose of this work was to follow the progressive incorporation of an isotopic label (13CO2) into 11 fatty acids to better understand the fatty acid synthesis pathways in A. minutum. The Dinophyte growth was monitored for 54 hours using high‐frequency sampling. A. minutum presented a growth in two phases. A lag phase was observed during the first 30 hours of development and had been associated with the probable temporary encystment of Dinophyte cells. An exponential growth phase was then observed after t30. A. minutum rapidly incorporated 13C into 22:6n‐3, which ended up being the most 13C‐enriched polyunsaturated fatty acid (PUFA) in this experiment, with a higher 13C atomic enrichment than 18:4n‐3, 18:5n‐3, 20:5n‐3, and 22:5n‐3. Overall, the 13C atomic enrichment (AE) was inversely proportional to number of carbons in n‐3 PUFA. C18 PUFAs, 18:4n‐3, and 18:5n‐3, were indeed among the least 13C‐enriched FAs during this experiment. They were assumed to be produced by the n‐3 PUFA pathway. However, they could not be further elongated or desaturated to produce n‐3 C20‐C22 PUFA, because the AEs of the n‐3 C18 PUFAs were lower than those of the n‐ 3 C20‐C22 PUFAs. Thus, the especially high atomic enrichment of 22:6n‐3 (55.8% and 54.9% in neutral lipids (NLs) and polar lipids (PLs), respectively) led us to hypothesize that this major PUFA was synthesized by an O2‐independent Polyketide Synthase (PKS) pathway. Another parallel PKS, independent of the one leading to 22:6n‐3, was also supposed to produce 20:5n‐3. The inverse order of the 13C atomic enrichment for n‐3 PUFAs was also suspected to be related to the possible β‐ oxidation of long‐chain n‐3 PUFAs occurring during A. minutum encystment.

The production of polyunsaturated fatty acids (PUFA) in Tisochrysis lutea was studied using the gradual incorporation of a ¹³C-enriched isotopic marker, ¹³CO₂, for 24 h during the exponential growth of the algae. The ¹³C enrichment of eleven fatty acids was followed to understand the synthetic pathways the most likely to form the essential polyunsaturated fatty acids 20:5n-3 (EPA) and 22:6n-3 (DHA) in T. lutea. The fatty acids 16:0, 18:1n-9 + 18:3n-3, 18:2n-6, and 22:5n-6 were the most enriched in ¹³C. On the contrary, 18:4n-3 and 18:5n-3 were the least enriched in ¹³C after long chain polyunsaturated fatty acids such as 20:5n-3 or 22:5n-3. The algae appeared to use different routes in parallel to form its polyunsaturated fatty acids. The use of the PKS pathway was hypothesized for polyunsaturated fatty acids with n-6 configuration (such as 22:5n-6) but might also exist for n-3 PUFA (especially 20:5n-3). With regard to the conventional n-3 PUFA pathway, Δ6 desaturation of 18:3n-3 appeared to be the most limiting step for T. lutea, “stopping” at the synthesis of 18:4n-3 and 18:5n-3. These two fatty acids were hypothesized to not undergo any further reaction of elongation and desaturation after being formed and were therefore considered “end-products”. To circumvent this limiting synthetic route, Tisochrysis lutea seemed to have developed an alternative route via Δ8 desaturation to produce longer chain fatty acids such as 20:5n-3 and 22:5n-3. 22:6n-3 presented a lower enrichment and appeared to be produced by a combination of different pathways: the conventional n-3 PUFA pathway by desaturation of 22:5n-3, the alternative route of ω-3 desaturase using 22:5n-6 as precursor, and possibly the PKS pathway. In this study, PKS synthesis looked particularly effective for producing long chain polyunsaturated fatty acids. The rate of enrichment of these compounds hypothetically synthesized by PKS is remarkably fast, making undetectable the ¹³C incorporation into their precursors. Finally, we identified a protein cluster gathering PKS sequences of proteins that are hypothesized allowing n-3 PUFA synthesis.

Stable isotope composition of the eastern oyster Crassostrea virginica was characterized from three estuaries in the Ten Thousand Islands (Florida). Freshwater in flow from watershed management is affecting this region and has deeply modified the salinity gradient. Stable carbon and nitrogen isotopes were used to trace specific sources of organic matter and the influence of these sources of carbon on the diet of the eastern oyster in the Greater Everglades. Oysters, as well as particulate organic matter (POM) and benthicmicroalgae (BMA) were sampled in three bays at three different stations from upstream to downstream during both wet and dry seasons. Significant salinity differences were observed in all estuaries between summer (wet) and winter (dry) seasons and were linked to freshwater inputs into the bays. Temperature followed a typical seasonal trend. Oysters were enriched in δ¹⁵N compared with POM and BMA, and similarly or slightly enriched in δ¹³C for both seasons. Lighter δ¹³C values in the upper stations in the estuaries suggest input of organic matter from terrestrial sources. Stable isotopes showed that oysters fed more on POM than on BMA. Condition index of oysters varied between stations and seasons in the three estuaries. Because the quality of organic matter consumed by oysters ultimately impacts the health of the oyster, changes in water quality and quantity of freshwater entering estuaries will have implications for future management of the habitat for this ecologically and economically important species.

New Hanover County, North Carolina, has been experiencing rapid population growth and is expected to continue this growth, leading to increased land use and development in the area. The county is also threatened by sea level rise (SLR) and its effects because of its coastal location and frequent occurrences of major storms and hurricanes. This study used a land change modeler to map the land cover change throughout the county over a period of 20 years, and predicted land cover distribution in the area in the years 2030 and 2050. Statistics revealed that the developed land in the area increased by 85 km² between 2000 and 2010, and by 60 km² between 2010 and 2020. Such land is predicted to increase by another 73 km² by 2030, and 63 km² by 2050. This increase in development is expected to occur mainly in the central area of the county and along the barrier islands. Modeling of SLR illustrated that the northwestern part of New Hanover County along the Cape Fear River, as well as the beach towns located on the barrier islands, are estimated be the most affected locations. Results indicate that sections of major highways throughout the county, including I-140 near downtown Wilmington and US-421 in Carolina Beach, may be inundated by SLR, which might delay residents during mandatory evacuations for emergency situations such as hurricanes. Some routes may be unusable, leading to traffic congestion on other routes, which may impede some residents from reaching safety before the emergency. Wrightsville Beach and Carolina Beach are estimated to have the highest levels of inundation, with 71.17% and 40.58% of their land being inundated under the most extreme SLR scenario of 3 m, respectively. The use of the present research approach may provide a practical, quick, and low-cost method in modeling rapidly growing urban areas along the eastern United States coastline and locating areas at potential risk of future SLR inundation.