Interactions of anthropogenic stress factors on marine phytoplankton

Abstract

Phytoplankton are the main primary producers in aquatic ecosystems. Their biomass production and CO 2 sequestration equals that of all terrestrial plants taken together. Phytoplankton productivity is controlled by a number of environmental factors, many of which currently undergo substantial changes due to anthropogenic global climate change. Most of these factors interact either additively or synergistically. Light availability is an absolute requirement for photosynthesis, but excessive visible and UV radiation impair productivity. Increasing temperatures enhance stratification and decrease the depth of the upper mixing layer exposing the cells to higher solar radiation and reduce nutrient upward transport from upwelling deeper water. At the same time, stratospheric ozone depletion exposes phytoplankton to higher solar UV-B radiation especially in polar and mid-latitudes. Terrestrial runoff carrying sediments and dissolved organic matter into coastal waters leads to eutrophication while reducing UV penetration. All these environmental forcings are known to affect physiological and ecological processes. Ocean acidification due to increased atmospheric CO 2 concentrations changes the seawater chemistry; it reduces calcification in phytoplankton, macroalgae and many zoological taxa. Ocean warming results in changing species composition and favors blooms of toxic prokaryotic and eukaryotic phytoplankton. Increasing pollution from crude oil spills, persistent organic pollutants, heavy metal as well as industrial and household wastewaters affect phytoplankton which is augmented by solar UV radiation. Extensive analyses of the impacts of multiple stressors are scarce. Here, we review reported findings on the impacts of anthropogenic stressors on phytoplankton with an emphasis on their interactive effects and make an effort to provide a prospect for future studies.

Phytoplankton as Key Mediators of the Biological Carbon Pump: Their Responses to a Changing Climate

“…Overview of the various environmental factors that affect phytoplankton productivity (adapted from ref. [46]). …”

“…Increased CO 2 availability may benefit phytoplankton species, or they may be harmed by the pH decrease depending on the species, other environmental conditions or other stressors [46]. Most phytoplankton species possess an active uptake mechanism for inorganic carbon, utilizing CO 2 and/or bicarbonate.…”

“…In the presence of a CCM, the cell may remain carbon saturated, even when ambient CO 2 levels are low. Thus, it is difficult to predict the changes in photosynthetic rates of cells, which might increase (due to increased availability of the substrate and/or less energy expenditure needed to operate the CCM), decrease (due to adverse effects of low pH), or remain the same under ocean acidification scenario [46]. The shift in CO 2 :HCO 3 − ratio may benefit species able to utilize only CO 2 by diffusive uptake.…”

“…Aquatic primary producers like marine phytoplankton dwell in the surface layers in order to harvest sufficient solar radiation for photosynthesis [46]. Different species have different optimal irradiances at which photosynthetic efficiency is optimal; however, photosynthetic plasticity allows cells to grow over a broader range of light levels.…”

“…UV-B radiation can have several deleterious effects on marine primary producers: UV-B breaks proteins and cell membranes, interferes with enzymatic reactions, impairs motility and orientation and induces lesions in DNA (such as the formation of cyclobutane pyrimidine dimers) [46]. UV-B radiation can also impair accessory pigments, which funnel solar energy to the reaction centers during photosynthesis, thus inhibiting photosynthesis and growth [46,78].…”