The environmental influences on the growth and grazing of marine protists

Marine protists are important components at the base of the marine food web. The growth and grazing of protistan organisms in response to the toxicity of petroleum hydrocarbons and elevated seawater temperature at the community and species levels were investigated. In exposure to 10 μL L⁻¹ of chemically dispersed crude oil in the mesocosms, the grazing impacts of microzooplankton (20-200 μm) on phytoplankton were reduced. While the microzooplankton grazing accounted for ~50% of the phytoplankton’ population growth in the control treatment, there was a de-coupling between these two parameters in the oil-loaded treatment. The de-coupling could potentially lead to algal blooms in the natural environment under certain conditions. In contrast, in exposure to chemically dispersed crude oil in the microcosms, the grazing impacts of nanoplankton (2-20 μm) on bacteria did not differ among the treatments of control and low (2 μL L⁻¹) and high (8 μL L⁻¹) concentrations. The tight couplings between the nanoplankton grazing and bacterial population growth in the control and oil-loaded treatments could have kept the abundance of bacterial cells steady. The community compositions of bacteria in the low and high dose crude oil treatments became increasing similar and different from those in the control treatment. It is believed to be related to the availability of carbon and inorganic nutrients. The relatively high abundance of hydrocarbon-degrading bacteria Cycloclasticus and Alcanivorax in the oil-loaded treatments indicated the presence of biodegradation. Exposure experiments were conducted to investigate the responses of marine protistan species to the toxicity of soluble petroleum hydrocarbon and elevated seawater temperature. In exposure to increasing concentrations of the water accommodated fraction (WAF) of crude oil, the heterotrophic dinoflagellates Oxyrrhis marina and Protoperidinium sp. and ciliates Euplotes sp. and Metacylis sp. showed species- specific vulnerabilities to oil toxicity, as reflected by their specific growth rates. When compared to the control treatment, their population grazing impacts and per capita ingestion rates were reduced with exposure to the WAF of crude oil alone and the mixture of crude oil and dispersant at the same concentration. In exposure to elevated seawater temperature, the Florida strain of mixotrophic dinoflagellate Fragilidium subglobosum obtained a specific growth rate of ~0.3 d⁻¹ at both 19°C and 23°C in mono-specific culture but zero or negative growth rates in cultures with added prey dinoflagellate Tripos tripos. F. subglobosum grown at 19°C showed higher maximum photosynthetic efficiency than at 23°C but did not differ in cellular chlorophyll-a content or cell size. This strain of F. subglobosum is believed to be non-mixotrophic and therefore the hypothesis that this dinoflagellate species becomes more heterotrophic at elevated temperature was not proved or disapproved