Relative Phytoplankton growth responses to physically and chemically dispersed South Louisiana sweet crude oil

Abstract

We conducted controlled laboratory exposure experiments to assess the toxic effects of water-accommodated fractions (WAFs) of South Louisiana sweet crude oil on five phytoplankton species isolated from the Gulf of Mexico. Experiments were conducted with individual and combinations of the five phytoplankton species to determine growth inhibitions to eight total petroleum hydrocarbon (TPH) equivalent concentrations ranging from 461 to 7,205 ppb. The composition and concentration of crude oil were altered by physical and chemical processes and used to help evaluate crude oil toxicity. The impact of crude oil exposure on phytoplankton growth varied with the concentration of crude oil, species of microalgae, and their community composition. At a concentration of TPH < 1,200 ppb, dinoflagellate species showed significantly better tolerance, while diatom species showed a higher tolerance to crude oil at higher concentrations of TPH. For both groups, the larger species were more tolerant to crude oil than smaller ones. The toxicity potential of crude oil seems to be strongly influenced by the concentration of polycyclic aromatic hydrocarbons (PAHs). The addition of the dispersant, Corexit® EC9500A, increased the amount of crude oil up to 50-fold in the water column, while the physical enhancement (vigorous mixing of water column) did not significantly increase the amount of TPH concentration in the water column. The species response to crude oil was also examined in the five-species community. Each phytoplankton species showed considerably less tolerance to crude oil in the five-species community compared to their individual responses. This study provides baseline information about individual phytoplankton responses to crude oil and dispersed crude oil for subsequent research efforts seeking to understand the impacts of oil on the phytoplankton in the bigger picture.

Effects of Macondo Oil on Phytoplankton from Grand Isle, Louisiana

“…The mechanisms by which low concentrations of oil enhance phytoplankton growth are unclear, but enhancements of growth or photosynthetic rates similar to the effects we observed (Figure 1) have been reported in several other studies [13,15,19,23,29,31,57]. The positive effects may be indirect and involve interactions with into lipids or other components of the cell in sufficient concentrations to upset normal metabolic functions.…”

“…A number of previous investigators have reported stimulatory effects of oil on phytoplankton growth or photosynthesis at low oil concentrations, typically less than 1 mg L -1 , and inhibition at high oil concentrations [13,15,19,23,[29][30][31]. The toxicity of crude oil to phytoplankton results from a variety of effects that reflect the complex composition of crude oil.…”

“…Results of numerous studies have indicated that the toxicity of lipophilic compounds reflects their effects on the cytoplasmic membrane and/or membrane-embedded enzymes [54]. Polycyclic aromatic hydrocarbons appear to be particularly toxic [15]. Effects include cell lysis, loss of membrane integrity, changes in lipid-protein interactions, modifications of membrane fluidity, decreases in intracellular pH, and changes in membrane structure that affect the functionality of the membrane as a selective barrier and matrix for enzymes [54].…”

“…Several studies, for example, have indicated that diatoms are more tolerant to oil than other classes of phytoplankton [28,30,34,59], although diatoms are sometimes outcompeted by other classes of phytoplankton in the presence of oil [60,61]. The impact of oil on diatoms varies between species and as a function of cell size [15,30,31,34,62]. The mechanisms that underlie the cell size effects are unclear and may involve surfaceto-volume considerations [33,63] as well as changes in predation pressure [30].…”

“…Several previous studies have examined the effects on individual phytoplankton species of crude oil [11][12][13][14][15][16][17], various components of crude oil [18][19][20][21][22][23], and refined oil products [23][24][25][26][27]. The general implication of these studies has been that low concentrations of crude oil or components thereof stimulate photosynthesis by some species [13,19,23] but have no positive effect on other species.…”