Eastern Baltic cod (Gadus morhua) has experienced a dramatic decline in biomass, distribution, size and condition over recent decades, related to a combination of fishing pressure and environmental stressors. Reduced parental size and condition are known to impact reproductive success, affecting egg quality, fertilization and hatching success and larval survival. However, how these reproductive traits vary throughout the spawning season at the population level remains poorly understood for this stock. To address this knowledge gap, this study investigated egg and larval characteristics of captive Eastern Baltic cod over an entire spawning season. Fish from three different size classes were kept in three separate tanks and allowed to spawn naturally. Eggs were collected daily from April to August, with peak spawning observed in early to mid-June. We assessed egg and larval size, deformities, thiamine concentration, daily survival and hatching success over time. Our findings revealed high variability in these reproductive traits both across the spawning season and between size groups, without consistent temporal trends. Notably, smaller fish produced the largest eggs with the highest total thiamine concentrations, which contrasts with previous studies linking larger egg size with larger females. At the same time, these eggs showed the lowest survival and hatching rates. The only consistent pattern across all tanks was a positive relationship between egg and larval size, though unrelated to survival. These results suggest higher than expected complexity of reproductive dynamics in Eastern Baltic cod and raise the question whether the stock has undergone a shift in reproductive strategy, possibly reflecting an increased allocation towards egg quality over quantity during the recent period of stock decline.

Several wild bird and fish species across the Northern Hemisphere have been shown to episodically be thiamine deficient. This may lead to mass-mortality events, especially in offspring. To understand the mechanisms underlying thiamine deficiency we need a better understanding of the dynamics and somatic allocation of the vitamin. Here we focus on a common, ecologically and economically important species, that is, Atlantic cod (Gadus morhua), which has been suggested to be sensitive to thiamine deficiency. We sampled cod of varying sizes and maturity stages in a system where thiamine deficiency regularly occurs (i.e., Baltic Sea) and compare these with cod from the North Atlantic, where this deficiency has not been recorded. Results show that thiamine concentrations were tissue-specific. Concentrations in muscle and liver generally declined during growth and maturation, whereas concentrations in gonads increased. Of the total thiamine in a female's body, approximately 70% of the total pool was allocated to the gonads at the onset of reproduction, suggesting that micronutrients constitute a major investment when spawning. Free thiamine was the dominating vitamer in gonads and increased in proportion of total thiamine as gonads developed, whereas the muscle and liver's relative composition of vitamers was constant with thiamine diphosphate dominating. Transketolase activity and latency suggest that livers were saturated with thiamine and there was no evidence of ongoing thiamine deficiency. Likewise, thiamine concentrations were similar between areas with different histories of thiamine deficiency when accounting for differences in size and reproductive state, suggesting that thiamine statuses were comparable. We show that life cycle and tissue-specific dynamics in thiamine concentrations should be considered when assessing the thiamine status of a species. Furthermore, we discuss how specific life history traits related to spawning may put species at higher risk of thiamine deficiency.

Many coregonine species have declined drastically across the Northern Hemisphere, including populations of Coregonus maraena (whitefish) in the Baltic Sea, and the mechanisms leading to these declines are not well investigated. An abrupt population crash occurred in the 1990s, coinciding with heavy declines in salmonid recruitment, also known as thiamine deficiency syndrome. Thiamine, i.e., vitamin B1, is an essential micronutrient needed for a functional metabolism. Offspring with thiamine deficiency have a high mortality posing significant negative impact on populations. Here, we aim to determine if whitefish, like other salmonids in the Baltic Sea, is affected by thiamine deficiency. Anadromous whitefish were therefore sampled during spawning in rivers of Southeastern Sweden, and we compared tissue concentrations and thiamine-dependent enzyme latencies to published thresholds. Further, we tested whether the variation in thiamine concentrations among individuals could be explained by physiological and morphological traits. Results showed that latency of thiamine-dependent enzymes along with egg thiamine concentrations suggest no evident thiamine deficiency. Concentrations were generally higher in the liver compared to muscle tissues. While females had lower liver thiamine concentrations compared to males, the opposite was found for muscle tissues, suggesting sex-specific patterns of allocation of the vitamin. Concentrations in eggs were positively related to the condition of the females and, similar to muscle and liver tissues, tended to be negatively related to standardized gill raker length. The latter is often used as a proxy for characterizing the feeding niche of coregonines. As has been observed in a number of other organisms (e.g., fish and molluscs), there was a reduction in thiamine concentration with length. Hence, the populations studied here showed no evidence of exhibiting thiamine deficiency. The variation in thiamine concentrations could largely be attributed to intrinsic physiological traits as well as traits associated with coregonine feeding niche.

Long-term ocean warming impacts the marine environment, and these effects will be exacerbated by future climate change affecting, e.g., biogeochemical processes and microbial communities. However, how the sediment microbial cell abundance and live/dead ratio respond to warming is poorly understood. In this study, sediment core samples were collected from a Baltic Sea bay artificially heated on average 5°C for > 50 years above a nearby (control) bay unaffected by the heating. Contrary to the expected increased productivity in the heated bay, qPCR-based sediment cell abundances showed decreased cell numbers along the sediment depth gradient in the heated bay compared to the control bay. This could reflect that a portion of the cells' metabolic energy was diverted to a heat related stress response rather than being used for replication. In addition, live/dead cell ratios showed no clear differences in either bay suggesting the majority of the cells were alive. Finally, sediment depth gradient 16S rRNA gene sequencing confirmed previous studies, showing that prolonged warming shallows sediment biogeochemical zones and related microbial communities. In conclusion, future climate change related warming will likely decrease microbial cell abundances that form part of the food web base, potentially impacting the entire ecosystem.

Several aquatic top predators suffer from deficiency in vitamin B1 (thiamine), sometimes combined with low levels of carotenoid pigments, e.g., astaxanthin. The mechanisms leading to correlations between carotenoid pigmentation and thiamine status are not known. These substances and their precursors are produced by single-celled organisms and transferred to higher trophic levels via zooplankton. However, little is known about the factors regulating this transfer process and how it is affected by environmental stressors and zooplankton diet. We therefore exposed a common copepod, Temora longicornis, to ultraviolet radiation (UVR), which is an important environmental stressor, and to food items of different quality in terms of carotenoid profile. Astaxanthin was the most abundant carotenoid found in copepods. Its concentrations were negatively affected by UVR regardless of diet type, and the availability of an astaxanthin precursor (beta-carotene) in the diet did not affect the response. Thiamine, on the other hand, showed a varying response, with elevated levels in copepods exposed to UVR at low beta-carotene diet and lower levels in copepods exposed to UVR and high beta-carotene diet. Altogether, this indicates that astaxanthin was consumed for photoprotection in the zooplankton and that thiamine dynamics might be modulated by UVR under certain dietary conditions. Hence, the concentrations of astaxanthin and thiamine in copepods are dynamic and to some extent regulated by exposure to UVR. Thus, the ability of zooplankton to transfer these substances to higher trophic levels depends, to some extent, on the exposure to environmental stressors.

Zooplankton are crucial for food webs and biogeochemical cycles. However, warming associated with climatechange may alter their seasonal timing and reproductive strategies. This study investigated how long-termwarming impacted zooplankton (mainly copepods) phenology and overwintering strategies by comparing a Bal-tic Sea bay, heated by warm water discharge for more than 50 yr, with an unaffected control bay. Field observa-tions showed that copepod and phytoplankton population growth began earlier in the heated bay than in thecontrol bay, suggesting that copepod abundance was driven by both temperature and food availability in theheated bay and by a stronger temperature dependence in the control bay. Resting eggs are normally producedas a life-history strategy to survive unfavorable environmental conditions. Our laboratory incubation experi-ment showed fewer dormant resting eggs hatched from the heated bay sediment compared with the controlbay, supporting an evolutionary change in overwintering strategy. In conclusion, the results seemed to suggestthat copepods adjusted their life-history in elevated temperatures by relying less on the strategy of usingsediment-stored dormant eggs and instead started their spring development earlier, when phytoplankton foodwas available. Hence, this study suggests that climate change can shift copepod overwintering strategies, leadingto potential cascading effects in the food web and affecting overall biodiversity and productivity.

This Assessment Update by the Environmental Effects Assessment Panel (EEAP) of the United Nations Environment Programme (UNEP) addresses the interacting effects of changes in stratospheric ozone, solar ultraviolet (UV) radiation, and climate on the environment and human health. These include new modelling studies that confirm the benefits of the Montreal Protocol in protecting the stratospheric ozone layer and its role in maintaining a stable climate, both at low and high latitudes. We also provide an update on projected levels of solar UV-radiation during the twenty-first century. Potential environmental consequences of climate intervention scenarios are also briefly discussed, illustrating the large uncertainties of, for example, Stratospheric Aerosol Injection (SAI). Modelling studies predict that, although SAI would cool the Earth's surface, other climate factors would be affected, including stratospheric ozone depletion and precipitation patterns. The contribution to global warming of replacements for ozone-depleting substances (ODS) are assessed. With respect to the breakdown products of chemicals under the purview of the Montreal Protocol, the risks to ecosystem and human health from the formation of trifluoroacetic acid (TFA) as a degradation product of ODS replacements are currently de minimis. UV-radiation and climate change continue to have complex interactive effects on the environment due largely to human activities. UV-radiation, other weathering factors, and microbial action contribute significantly to the breakdown of plastic waste in the environment, and in affecting transport, fate, and toxicity of the plastics in terrestrial and aquatic ecosystems, and the atmosphere. Sustainability demands continue to drive industry innovations to mitigate environmental consequences of the use and disposal of plastic and plastic-containing materials. Terrestrial ecosystems in alpine and polar environments are increasingly being exposed to enhanced UV-radiation due to earlier seasonal snow and ice melt because of climate warming and extended periods of ozone depletion. Solar radiation, including UV-radiation, also contributes to the decomposition of dead plant material, which affects nutrient cycling, carbon storage, emission of greenhouse gases, and soil fertility. In aquatic ecosystems, loss of ice cover is increasing the area of polar oceans exposed to UV-radiation with possible negative effects on phytoplankton productivity. However, modelling studies of Arctic Ocean circulation suggests that phytoplankton are circulating to progressively deeper ocean layers with less UV irradiation. Human health is also modified by climate change and behaviour patterns, resulting in changes in exposure to UV-radiation with harmful or beneficial effects depending on conditions and skin type. For example, incidence of melanoma has been associated with increased air temperature, which affects time spent outdoors and thus exposure to UV-radiation. Overall, implementation of the Montreal Protocol and its Amendments has mitigated the deleterious effects of high levels of UV-radiation and global warming for both environmental and human health.

Climate change driven ocean warming is a worldwide environmental issue that can impact cycling of greenhouse gases. However, how methane production in marine sediments as a potential contributor to atmospheric greenhouse gases versus its consumption at the sulfate–methane transition zone will be affected by climate change related warming is still not well constrained. In this study, sediments from two Baltic Sea bays with long-term temperature differences were collected during summer and winter. The primary difference between the two bays was that one had been heated by a nearby power plant for 50 years, resulting in a 5.1 °C increase in annual average temperature compared to an unheated control bay. The results showed that near-seafloor sediment methane concentrations were 50 times higher compared to present-day conditions. Furthermore, the sediment fluxes along with microbial community composition changes suggested that long-term warming may thin the sulfate reduction zone, such that methanotrophic archaea and sulfate reducing bacteria peaked at shallower sediment depths in the heated bay. Overall, the results from long-term warming in natural sediment environment indicated that future climate change warming may increase the risk of methane release to the water and eventually the atmosphere.

The lack of a fitness-based theory of micronutrient allocation to specific tissues hinders understanding of the ultimate causes of mass juvenile mortality due to thiamine (vitamin B1) deficiency, which is an emerging threat to marine and coastal ecosystems worldwide. We modeled the optimal allocation of thiamine in salmon to somatic and reproductive tissues to investigate correlations between tissue thiamine levels, adult mortality, juvenile recruitment, and excretion rates that change with thiamine concentration. The model showed a positive correlation between thiamine levels in gonads and muscles, with a slope that increased with time. This was driven by a constrained thiamine input in salmon, but a negative or no correlation was found in scenarios with high thiamine input. These predictions were confirmed by analysis of empirical data from Atlantic salmon (Salmo salar) populations that differ in the occurrence of episodic thiamine deficiency. A positive correlation was indicative of low thiamine input, regardless of how juvenile recruitment and adult survival increased with thiamine concentration. The model output suggests that renal (i.e., kidney) reuptake is fundamental to understanding micronutrient allocation strategies. Measuring correlations between micronutrient concentrations in reproductive and somatic tissues of adults may help to detect early signs of thiamine deficiency before mass mortality of juveniles occurs. This can complement the previously suggested tissue concentrations and food web indicators. Future studies should try to distinguish and quantify the factors that alter the net thiamine input in salmonids and the subsequent allocation to offspring. Particular attention should be given to changes in thiamine uptake from the diet, including intestinal uptake mechanisms and effects of thiaminase activity. Additionally, more information is needed on internal factors that reduce thiamine availability, such as thiamine degradation as an antioxidant during lipid metabolism, and other physiological factors that can potentially increase thiamine loss, including allocation mechanisms and renal processes.

Background: Vitamin B1 (thiamin) is essential to life; yet little is known of the regulation of its availability in marine environments or how it varies seasonally. Since microbes are the key synthesizers of the vitamin in marine environments, we here used metatranscriptomics to examine the seasonal dynamics of B1 acquisition strategies (including both uptake and synthesis pathways) in Baltic Sea bacterioplankton.

Results: Elevated B1-related gene expression was observed in summer, coinciding with increased temperatures and bacterial activity and decreased nutrient availability. Different bacterial taxa exhibited distinct B1 acquisition strategies. We identified filamentous Cyanobacteria of the order Nostocales as critical to sustaining B1 production during summer, potentially compensating for limited synthesis in heterotrophic bacteria, especially for 4-amino-5-hydroxymethylpyrimidine (HMP) synthesis. Also, Pelagibacterales accounted for major portions of the community transcription, primarily taking up and salvaging the B1 precursor HMP during summer. This study highlights the partitioning of B1 synthesis, salvage, and uptake among microbial taxa, underscoring that transcriptional activity was more dynamic over time than changes in the genomic potential.

Conclusions: We emphasize the influence of environmental conditions on microbial community dynamics and B1 cycling in general, and the potential implications of global change-induced increases in filamentous Cyanobacteria blooms on vitamin food web transfer in particular.