Pedro Echeveste

Cell size dependent toxicity thresholds of polycyclic aromatic hydrocarbons to natural and cultured phytoplankton populations

The toxicity of pyrene and phenanthrene to phytoplankton was studied by analyzing the effect on the growth, abundance and cell viability of cultured species and natural communities of the Atlantic Ocean and the Mediterranean Sea. A decrease in cell abundance, and growth rate was observed as concentration of PAHs increased, with catastrophic cell mortality induced at the highest PAH concentration tested. A strong positive linear relationship was observed between the LC50 (the PAH concentration at which cell population will decline by a half), and the species cell volume, for both phenanthrene and pyrene. Natural communities were however significantly more sensitive to PAHs than cultured phytoplankton, as indicated by the lower slope (e.g. 0.23 and 0.65, respectively, for pyrene) of the relationship LC50 vs. cell volume. The results highlight the importance of cell size in determining the phytoplankton sensitivity to PAHs identifying the communities from the oligotrophic ocean to be more vulnerable.

Decrease in the abundance and viability of oceanic phytoplankton due to trace levels of complex mixtures of organic pollutants.

Long range atmospheric transport and deposition is a significant introduction pathway of organic pollutants to remote oceanic regions, leading to their subsequent accumulation in marine organisms. Persistent organic pollutants (POPs) bioconcentrate in planktonic food webs and these exert a biogeochemical control on the regional and global cycling of POPs. Therefore, an important issue is to determine whether the anthropogenic chemical perturbation of the biosphere introduced by the myriad of organic pollutants present in seawater influences phytoplankton abundance and productivity. The results reported here from five sets of experiments performed in the NE Atlantic Ocean show that there is a toxic effect induced by trace levels of complex mixtures of organic pollutants on phytoplankton oceanic communities. The levels of single pollutant, such as phenanthrene and pyrene, at which lethality of phytoplankton is observed are high in comparison to field levels. Complex mixtures of organic pollutants, however, have an important toxic effect on phytoplankton abundances, viability and concentrations of Chlorophyll a at pollutant concentrations 20-40 folds those found in the open ocean. The toxicity of these complex mixtures of organic pollutants exceeds by 10(3) times the toxicity expected for a single pollutant. Therefore, our results point out the need for a systematic investigation of the influence of complex mixtures of organic hydrophobic pollutants to oceanic phytoplankton communities, a perturbation not accounted for on previous assessments of anthropogenic pressures in the marine environment.

Toxic thresholds of cadmium and lead to oceanic phytoplankton: Cell size and ocean basin-dependent effects

Thresholds of cadmium (Cd) and lead (Pb) toxic to oceanic phytoplankton were examined in natural communities from the Mediterranean and Black Seas and the North East Atlantic Ocean. At concentrations of added Cd and Pb greater than 0.11 µg L(-1) , cell abundances and growth rates decreased with increasing addition of Cd and Pb, for all phytoplankton populations. The lethal concentrations at which populations decreased by half (LC50s), ranged from 0.23 to 498.7 µg L(-1) Cd for Atlantic Prochlorococcus and Black Sea picoeukaryotes, respectively, and from 20 to 465.2 µg L(-1) Pb for Mediterranean Synechococcus and Black Sea nanoplankton, respectively. These lethal concentrations were significantly lower than those previously reported for phytoplankton cultures. The LC50s were strongly related to population cell size, increasing as cell size increased, indicating that oceanic picocyanobacteria Prochlorococcus and Synechococcus populations were the most sensitive, and the largest phytoplankton cells the most resistant. Based on this relationship, differences in sensitivity to Cd across systems were detected, with Black Sea phytoplankton communities being more resistant (up to 100 times) than similar sized phytoplankton of the Mediterranean Sea and Atlantic Ocean.

Tolerance of polar phytoplankton communities to metals

Large amounts of pollutants reach polar regions, particularly the Arctic, impacting their communities. In this study we analyzed the toxic levels of Hg, Cd and Pb to natural phytoplankton communities of the Arctic and Southern Oceans, and compared their sensitivities with those observed on phytoplankton natural communities from temperate areas. Mercury was the most toxic metal for both Arctic and Antarctic communities, while both Cd and Pb were toxic only for the Antarctic phytoplankton. Total cell abundance of the populations forming the Arctic community increased under high Cd and Pb concentrations, probably due to a decrease of the grazing pressure or the increase of the most resistant species, although analysis of individual cells indicated that cell death was already induced at the highest levels. These results suggest that phytoplankton may have acquired adapting mechanisms to face high levels of Pb and Cd in the Arctic Ocean.

Size-dependence of volatile and semi-volatile organic carbon content in phytoplankton cells

The content of volatile and semivolatile organic compounds (VOC and SOC), measured as exchangeable dissolved organic carbon (EDOC), was quantified in 9 phytoplanktonic species that spanned 4 orders of magnitude in cell volume, by disrupting the cells and quantifying the gaseous organic carbon released. EDOC content varied 4 orders of magnitude, from 0.0015 to 14.12 pg C cell-1 in the species studied and increased linearly with increasing phytoplankton cell volume following the equation EDOC (pg C cell-1) = -2.35 x cellular volume (CV, µm3 cell-1) 0.90 (± 0.3), with a slope (0.90) not different from 1 indicating a constant increase in volatile carbon as the cell size of phytoplankton increased. The percentage of EDOC relative to total cellular carbon was small but varied 20 fold from 0.28 % to 5.17 %, and no obvious taxonomic pattern in the content of EDOC was appreciable for the species tested. The cell release rate of EDOC is small compared to the amount of carbon in the cell and difficult to capture. Nonetheless, the results point to a potential flux of volatile and semivolatile phytoplankton-derived organic carbon to the atmosphere that has been largely underestimated and deserves further attention in the future.