The Influence of Micronutrient Trace Metals on Microcystis aeruginosa Growth and Toxin Production

Microcystis aeruginosa is a widespread cyanobacteria capable of producing hepatotoxic microcystins. Understanding the environmental factors that influence its growth and toxin production is essential to managing the negative effects on freshwater systems. Some micronutrients are important cofactors in cyanobacterial proteins and can influence cyanobacterial growth when availability is limited. However, micronutrient requirements are often species specific, and can be influenced by substitution between metals or by luxury uptake. In this study, M. aeruginosa was grown in modified growth media that individually excluded some micronutrients (cobalt, copper, iron, manganese, molybdenum) to assess the effect on growth, toxin production, cell morphology and iron accumulation. M. aeruginosa growth was limited when iron, cobalt and manganese were excluded from the growth media, whereas the exclusion of copper and molybdenum had no effect on growth. Intracellular microcystin-LR concentrations were variable and were at times elevated in treatments undergoing growth limitation by cobalt. Intracellular iron was notably higher in treatments grown in cobalt-deplete media compared to other treatments possibly due to inhibition or competition for transporters, or due to irons role in detoxifying reactive oxygen species (ROS).

Assessing the importance of cobalt as a micronutrient for freshwater cyanobacteria

Micronutrients play key roles in numerous metabolic processes in cyanobacteria. However, our understanding of whether the micronutrient cobalt influences the productivity of freshwater systems or the occurrence of cyanobacterial blooms is limited. This study aimed to quantify the concentration of Co necessary for optimal cyanobacterial growth by exposing Microcystis aeruginosa to a range of Co concentrations under culture conditions. Extended exposure to concentrations below ˜0.06 μg · L^-1 resulted in notable inhibition of M. aeruginosa growth. A clear negative relationship was observed between Co concentration in solution and intracellular Fe quota of M. aeruginosa, possibly due to decreased transport of Fe at higher Co concentrations. Cyanocobalamin and any Co within the structure of cyanocobalamin appears to be non-bioavailable to M. aeruginosa, instead they likely rely on the synthesis of a structural variant - pseudocobalamin, which may have implications for the wider algal community as the variants of cobalamin are not necessarily functionally exchangeable. To evaluate the likelihood of Co limitation of cyanobacterial growth under field conditions, a survey of 10 freshwater reservoirs in South-Eastern Australia was conducted. Four of the ten sites had dissolved Co concentrations below the 0.06 μg · L^-1 threshold value. All four of these sites rarely undergo cyanobacterial blooms, strengthening evidence of the potential for Co to limit growth, perhaps either alone or in combination with phosphorus.

A Review of the Effect of Trace Metals on Freshwater Cyanobacterial Growth and Toxin Production

Cyanobacterial blooms are becoming more common in freshwater systems, causing ecological degradation and human health risks through exposure to cyanotoxins. The role of phosphorus and nitrogen in cyanobacterial bloom formation is well documented and these are regularly the focus of management plans. There is also strong evidence that trace metals are required for a wide range of cellular processes, however their importance as a limiting factor of cyanobacterial growth in ecological systems is unclear. Furthermore, some studies have suggested a direct link between cyanotoxin production and some trace metals. This review synthesises current knowledge on the following: (1) the biochemical role of trace metals (particularly iron, cobalt, copper, manganese, molybdenum and zinc), (2) the growth limitation of cyanobacteria by trace metals, (3) the trace metal regulation of the phytoplankton community structure and (4) the role of trace metals in cyanotoxin production. Iron dominated the literature and regularly influenced bloom formation, with 15 of 18 studies indicating limitation or colimitation of cyanobacterial growth. A range of other trace metals were found to have a demonstrated capacity to limit cyanobacterial growth, and these metals require further study. The effect of trace metals on cyanotoxin production is equivocal and highly variable. Better understanding the role of trace metals in cyanobacterial growth and bloom formation is an essential component of freshwater management and a direction for future research.

Prevalent reliance of bacterioplankton on exogenous vitamin B1 and precursor availability

Vitamin B1 (B1 herein) is a vital enzyme cofactor required by virtually all cells, including bacterioplankton, which strongly influence aquatic biogeochemistry and productivity and modulate climate on Earth. Intriguingly, bacterioplankton can be de novo B1 synthesizers or B1 auxotrophs, which cannot synthesize B1 de novo and require exogenous B1 or B1 precursors to survive. Recent isolate-based work suggests select abundant bacterioplankton are B1 auxotrophs, but direct evidence of B1 auxotrophy among natural communities is scant. In addition, it is entirely unknown if bulk bacterioplankton growth is ever B1-limited. We show by surveying for B1-related genes in estuarine, marine, and freshwater metagenomes and metagenome-assembled genomes (MAGs) that most naturally occurring bacterioplankton are B1 auxotrophs. Pyrimidine B1-auxotrophic bacterioplankton numerically dominated metagenomes, but multiple other B1-auxotrophic types and distinct uptake and B1-salvaging strategies were also identified, including dual (pyrimidine and thiazole) and intact B1 auxotrophs that have received little prior consideration. Time-series metagenomes from the Baltic Sea revealed pronounced shifts in the prevalence of multiple B1-auxotrophic types and in the B1-uptake and B1-salvaging strategies over time. Complementarily, we documented B1/precursor limitation of bacterioplankton production in three of five nutrient-amendment experiments at the same time-series station, specifically when intact B1 concentrations were ≤3.7 pM, based on bioassays with a genetically engineered Vibrio anguillarum B1-auxotrophic strain. Collectively, the data presented highlight the prevalent reliance of bacterioplankton on exogenous B1/precursors and on the bioavailability of the micronutrients as an overlooked factor that could influence bacterioplankton growth and succession and thereby the cycling of nutrients and energy in aquatic systems.

Linking nonstructural carbohydrate dynamics to gas exchange and leaf hydraulic behavior in Pinus edulis and Juniperus monosperma

Leaf hydraulics, gas exchange and carbon storage in Pinus edulis and Juniperus monosperma, two tree species on opposite ends of the isohydry-anisohydry spectrum, were analyzed to examine relationships between hydraulic function and carbohydrate dynamics. Leaf hydraulic vulnerability, leaf water potential (Ψl ), leaf hydraulic conductance (Kleaf ), photosynthesis (A), stomatal conductance (gs) and nonstructural carbohydrate (NSC) content were analyzed throughout the growing season. Leaf hydraulic vulnerability was significantly lower in the relatively anisohydric J. monosperma than in the more isohydric P. edulis. In P. edulis, Ψl dropped and stayed below 50% loss of leaf hydraulic conductance (P₅₀) early in the day during May, August and around midday in September, leading to sustained reductions in Kleaf . In J. monosperma, Ψl dropped below P₅₀ only during August, resulting in the maintenance of Kleaf during much of the growing season. Mean A and gs during September were significantly lower in P. edulis than in J. monosperma. Foliar total NSC was two to three times greater in J. monosperma than in P. edulis in June, August and September. Consistently lower levels of total NSC in P. edulis suggest that its isohydric strategy pushes it towards the exhaustion of carbon reserves during much of the growing season.

Temporal variations of mobile carbohydrates in Abies fargesii at the upper tree limits

Low temperatures are associated high-altitude treelines, but the functional mechanism of treeline formation remains controversial. The relative contributions of carbon limitation (source activity) and growth limitation (sink activity) require more tests across taxa and regions. We examined temporal variations of mobile carbon supply in different tissues of Abies fargesii across treeline ecotones on north- and south-facing slopes of the Qinling Mountains, China. Non-structural carbohydrate (NSC) concentrations in tissues along the altitudinal gradient on both slopes changed significantly in the early and late growing season, but not in the mid-growing season, indicating the season-dependent carbon supply status. Late in the growing season on both slopes, trees at the upper limits had the highest NSC concentrations and total soluble sugars and lowest starch concentrations compared to trees at the lower elevations. NSC concentrations tended to increase in needles and branches throughout the growing season with increasing elevation on both slopes, but declined in roots and stems. NSC concentrations across sampling dates also indicated increases in needles and branches, and decreases in roots and stem with increasing elevation. Overall altitudinal trends of NSC in A. fargesii revealed no depletion of mobile carbon reserves at upper elevation limits, suggesting limitation of sink activity dominates tree life across treeline ecotones in both north- and south-facing slopes. Carbon reserves in storage tissues (especially roots) in the late growing season might also play an important role in winter survival and early growth in spring at upper elevations on both slopes, which define the uppermost limit of A. fargesii.

A cool experimental approach to explain elevational treelines, but can it explain them?

At alpine treeline, trees give way to low-stature alpine vegetation. The main reason may be that tree canopies warm up less in the sun and experience lower average temperatures than alpine vegetation. Low growth temperatures limit tissue formation more than carbon gain, but whether this mechanism universally determines potential treeline elevations is the subject of debate. To study low-temperature limitation in two contrasting treeline tree species, Fajardo and Piper (American Journal of Botany 101: 788-795) grew potted seedlings at ground level or suspended at tree-canopy height (2 m), introducing a promising experimental method for studying the effects of alpine-vegetation and tree-canopy microclimates on tree growth. On the basis of this experiment, the authors concluded that lower temperatures at 2 m caused carbon limitation in one of the species and that treeline-forming mechanisms may thus be taxon-dependent. Here we contest that this important conclusion can be drawn based on the presented experiment, because of confounding effects of extreme root-zone temperature fluctuations and potential drought conditions. To interpret the results of this elegant experiment without logistically challenging technical modifications and to better understand how low temperature leads to treeline formation, studies on effects of fluctuating vs. stable temperatures are badly needed. Other treeline research priorities are interactions between temperature and other climatic factors and differences in microclimate between tree canopies with contrasting morphology and physiology. In spite of our criticism of this particular study, we agree that the development of a universal treeline theory should include continuing explorations of taxon-specific treeline-forming mechanisms.

Evolutionary conservation and modulation of a juvenile growth-regulating genetic program

Body size varies enormously among mammalian species. In small mammals, body growth is typically suppressed rapidly, within weeks, whereas in large mammals, growth is suppressed slowly, over years, allowing for a greater adult size. We recently reported evidence that body growth suppression in rodents is caused in part by a juvenile genetic program that occurs in multiple tissues simultaneously and involves the downregulation of a large set of growth-promoting genes. We hypothesized that this genetic program is conserved in large mammals but that its time course is evolutionarily modulated such that it plays out more slowly, allowing for more prolonged growth. Consistent with this hypothesis, using expression microarray analysis, we identified a set of genes that are downregulated with age in both juvenile sheep kidney and lung. This overlapping gene set was enriched for genes involved in cell proliferation and growth and showed striking similarity to a set of genes downregulated with age in multiple organs of the juvenile mouse and rat, indicating that the multiorgan juvenile genetic program previously described in rodents has been conserved in the 80 million years since sheep and rodents diverged in evolution. Using microarray and real-time PCR, we found that the pace of this program was most rapid in mice, more gradual in rats, and most gradual in sheep. These findings support the hypothesis that a growth-regulating genetic program is conserved among mammalian species but that its pace is modulated to allow more prolonged growth and therefore greater adult body size in larger mammals.

Seasonal carbohydrate dynamics and growth in Douglas-fir trees experiencing chronic, fungal-mediated reduction in functional leaf area

Stored non-structural carbohydrates (NSCs) could play an important role in tree survival in the face of a changing climate and associated stress-related mortality. We explored the effects of the stomata-blocking and defoliating fungal disease called Swiss needle cast on Douglas-fir carbohydrate reserves and growth to evaluate the extent to which NSCs can be mobilized under natural conditions of low water stress and restricted carbon supply in relation to potential demands for growth. We analyzed the concentrations of starch, sucrose, glucose and fructose in foliage, twig wood and trunk sapwood of 15 co-occurring Douglas-fir trees expressing a gradient of Swiss needle cast symptom severity quantified as previous-year functional foliage mass. Growth (mean basal area increment, BAI) decreased by ∼80% and trunk NSC concentration decreased by 60% with decreasing functional foliage mass. The ratio of relative changes in NSC concentration and BAI, an index of the relative priority of storage versus growth, more than doubled with increasing disease severity. In contrast, twig and foliage NSC concentrations remained nearly constant with decreasing functional foliage mass. These results suggest that under disease-induced reductions in carbon supply, Douglas-fir trees retain NSCs (either actively or due to sequestration) at the expense of trunk radial growth. The crown retains the highest concentrations of NSC, presumably to maintain foliage growth and shoot extension in the spring, partially compensating for rapid foliage loss in the summer and fall.

The impacts of water stress on phloem transport in Douglas-fir trees

Despite the critical role that phloem plays in a number of plant functional processes and the potential impact of water stress on phloem structural and phloem sap compositional characteristics, little research has been done to examine how water stress influences phloem transport. The objectives of this study were to develop a more accurate understanding of how water stress affects phloem transport in trees, both in terms of the short-term impacts of water stress on phloem sap composition and the longer-term impacts on sieve cell anatomical characteristics. Phloem sieve cell conductivity (kp) was evaluated along a gradient of tree height and xylem water potential in Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees in order to evaluate the influence of water stress on phloem transport capacity. The Hagen-Poiseuille equation was used with measurements of sieve cell anatomical characteristics, water content of phloem sap, non-structural carbohydrate content of phloem sap and shoot water potential (Ψl) to evaluate impacts of water stress on kp. Based on regression analysis, for each 1 MPa decrease in mean midday Ψl, sieve cell lumen radius decreased by 2.63 µm MPa(-1). Although there was no significant trend in sucrose content with decreasing Ψl, glucose and fructose content increased significantly with water stress and sieve cell relative water content decreased by 13.5% MPa(-1), leading to a significant increase in sugar molar concentration of 0.46 mol l(-1) MPa(-1) and a significant increase in viscosity of 0.27 mPa s MPa(-1). Modeled kp was significantly influenced both by trends in viscosity as well as by water stress-related trends in sieve cell anatomy.