High photosynthetic plasticity may reinforce invasiveness of upside-down zooxanthellate jellyfish in Mediterranean coastal waters

Ecological profiling of non-native species is essential to predict their dispersal and invasiveness potential across different areas of the world. Cassiopea is a monophyletic taxonomic group of scyphozoan mixotrophic jellyfish including C. andromeda, a recent colonizer of sheltered, shallow-water habitats of the Mediterranean Sea, such as harbors and other light-limited, eutrophic coastal habitats. To assess the ecophysiological plasticity of Cassiopea jellyfish and their potential to spread across the Mare Nostrum by secondary introductions, we investigated rapid photosynthetic responses of jellyfish to irradiance transitions—from reduced to increased irradiance conditions (as paradigm of transition from harbors to coastal, meso/oligotrophic habitats). Laboratory incubation experiments were carried out to compare oxygen fluxes and photobiological variables in Cassiopea sp. immature specimens pre-acclimated to low irradiance (PAR = 200 μmol photons m−2 s−1) and specimens rapidly exposed to higher irradiance levels (PAR = 500 μmol photons m−2 s−1). Comparable photosynthetic potential and high photosynthetic rates were measured at both irradiance values, as also shown by the rapid light curves. No significant differences were observed in terms of symbiont abundance between control and treated specimens. However, jellyfish kept at the low irradiance showed a higher content in chlorophyll a and c (0.76±0.51SD vs 0.46±0.13SD mg g-1 AFDW) and a higher Ci (amount of chlorophyll per cell) compared to jellyfish exposed to higher irradiance levels. The ratio between gross photosynthesis and respiration (P:R) was >1, indicating a significant input from the autotrophic metabolism. Cassiopea sp. specimens showed high photosynthetic performances, at both low and high irradiance, demonstrating high potential to adapt to sudden changes in light exposure. Such photosynthetic plasticity, combined with Cassiopea eurythermal tolerance and mixotrophic behavior, jointly suggest the upside-down jellyfish as a potentially successful invader in the scenario of a warming Mediterranean Sea.

Photosynthetic plasticity of a tropical tree species, Tabebuia rosea, in response to elevated temperature and [CO2]

Atmospheric and climate change will expose tropical forests to conditions they have not experienced in millions of years. To better understand the consequences of this change we studied photosynthetic acclimation of the neotropical tree species Tabebuia rosea to combined 4°C warming and twice-ambient (800 ppm) CO. We measured temperature responses of the maximum rates of ribulose 1,5-bisphosphate carboxylation (V), photosynthetic electron transport (J), net photosynthesis (P), and stomatal conductance (gs), and fitted the data using a probabilistic Bayesian approach. To evaluate short-term acclimation plants were then switched between treatment and control conditions and re-measured after 1–2 weeks. Consistent with acclimation, the optimum temperatures (T) for V, J and P were 1–5°C higher in treatment than in control plants, while photosynthetic capacity (V, J, and P at T) was 8–25% lower. Likewise, moving control plants to treatment conditions moderately increased temperature optima and decreased photosynthetic capacity. Stomatal density and sensitivity to leaf-to-air vapor pressure deficit were not affected by growth conditions, and treatment plants did not exhibit stronger stomatal limitations. Collectively, these results illustrate the strong photosynthetic plasticity of this tropical tree species as even fully-developed leaves of saplings transferred to extreme conditions partially acclimated.

On the Relationship between a Novel Prorocentrum sp. and Colonial Phaeocystis antarctica under Iron and Vitamin B12 Limitation: Ecological Implications for Antarctic Waters

We collected live mixed natural samples from the northeastern Ross Sea during the austral summer of 2017 and isolated a novel Prorocentrum sp. (Dinophyceae) associated with mucilaginous Phaeocystis antarctica (Coccolithophyceae) colonies. The haptophyte P. antarctica is a key species of the phytoplankton community in the Ross Sea, where blooms are subjected to iron limitation and/or co-limitation with other micronutrients (e.g., vitamin B12) during the summer. We first performed preliminary genetic analyses to determine the specific identity of the novel Prorocentrum sp., which indicated that it represented a previously undescribed species. The formal description of this new species is in process. To further assess its relationship with P. antarctica, we obtained their monospecific and mixed cultures and evaluated their responses to different irradiance levels and iron and vitamin B12 limitation. Our results indicated differential susceptibility of the two species to iron limitation and differential photosynthetic plasticity under high irradiance. Iron limitation reduced colony formation in P. antarctica and decreased the chlorophyll-a content in Prorocentrum sp., whereas B12 limitation did not affect growth or photosynthetic efficiency in either species. In addition, P. antarctica could photosynthesize efficiently under different irradiance levels, due to its ability to modulate the light adsorption cross-section of PSII, whereas Prorocentrum sp. exhibited lower photosynthetic plasticity and an inability to modulate both the maximum photochemical efficiency and effective adsorption cross-section of PSII under high irradiance. The trophic interaction between Prorocentrum sp. and P. antarctica could present ecological implications for the food webs and biogeochemical cycles of the Antarctic ecosystem. Considering the predicted climate-driven shifts in global ocean surface light regimes and changes in iron or vitamin B12 transfer, which are most likely to impact changes in the phytoplankton community structure, our results present implications for carbon export to deeper waters, ecological functioning, and associated biogeochemical changes in the future.