Meteorological conditions induce strong shifts in mixotrophic and heterotrophic flagellate bacterivory over small spatio‐temporal scales

Abstract

Mixotrophic bacterivory by small phytoflagellates (SP) can equal that of heterotrophic flagellates (HF) in marine euphotic waters. Due to their differing nutritional requirements from prey, SP and HF bacterivory are thought to differentially impact nutrient and carbon cycling through microbial food webs. However, little is known on how the relative dominance of SP and HF feeding shifts along environmental gradients, especially over short spatio‐temporal scales. Here, SP and HF bacterivory rates were compared over 5 d at three depths in the upper water column of a dynamic coastal zone, through the use of surrogate prey. SP and HF bacterivory strongly fluctuated over both spatial (< 5 m) and temporal (day to day) scales despite relatively stable abundance, with a respective seven‐ and ten‐fold difference between minimum and maximum bacterivory rates. Irradiance regulated SP community bacterivory, leading to a grouping into high rates under light limitation, mid‐level rates under optimal irradiance levels for growth and low rates at very high irradiance levels. SP bacterivory only dominated over that of HF at irradiance ranges generally considered optimal for photosynthetic growth (∼ 40–300 μmol photons m−2 s−1). SP bacterivory, but not that of HF, was positively correlated to total bacterial abundance, with HF potentially selectively targeting actively growing bacteria. Overall, the present study indicates that meteorological conditions affecting irradiance levels could strongly impact microbial food web dynamics and the cycling of carbon and nutrients through coastal marine systems.

Relative importance of bacterivorous mixotrophs in an estuary-coast environment

“…Our results suggested that mixotrophs were suppressed at the highly turbid estuary where strict autotrophs and heterotrophs were superior competitors, but became dominant outside of the estuary plume. These niche shifts are consistent with many studies that have suggested mixotrophs benefit from low-nutrients and high-irradiance environments (Tittel et al, 2003; Stukel et al, 2011; Anderson et al, 2019; Edwards et al, 2019; 2022). One explanation for this is that mixotrophs are penalized for possessing dual apparatus of phototrophy and phagotrophy within the same cells, i.e.…”

Relative importance of bacterivorous mixotrophs in an estuary‐coast environment

“…Our results suggested that mixotrophs were suppressed at the highly turbid estuary, where strict autotrophs and heterotrophs were superior competitors, but became dominant outside of the estuary plume. These niche shifts are consistent with many studies that have suggested mixotrophs benefit from low-nutrients and high-irradiance environments (Tittel et al 2003;Stukel et al 2011;Anderson and Hansen 2019;Edwards 2019;Edwards et al 2022). One explanation for this is that mixotrophs are penalized for possessing dual apparatus of phototrophy and phagotrophy within the same cells, that is, they have poorer photosynthetic performance compared to autotrophs and lower ingestion rates compared to heterotrophs (Raven 1997).…”

An under-ice bloom of mixotrophic haptophytes in low nutrient and freshwater-influenced Arctic waters

“…Species of Prymnesium and Chrysochromulina are mixotrophs, combining photosynthesis and prey uptake. This provides them with a competitive advantage in light-and nutrient-limited environments [23][24][25][26][27][28] . Predation/ grazing provides mixotrophic Chrysochromulina species with organic carbon as an energy source as well as additional nitrogen, allowing them to grow when light is limiting photosynthesis or dissolved inorganic nutrients in the sea ice and seawater are low [25][26][27][28] .…”

“…This provides them with a competitive advantage in light-and nutrient-limited environments [23][24][25][26][27][28] . Predation/ grazing provides mixotrophic Chrysochromulina species with organic carbon as an energy source as well as additional nitrogen, allowing them to grow when light is limiting photosynthesis or dissolved inorganic nutrients in the sea ice and seawater are low [25][26][27][28] . The concentration of haptophytes in the present bloom (~ 20 × 10 5 cells l −1 ) was similar to those cell concentrations found during the peak of harmful Chrysochromulina blooms in the Skagerrak in southern Norway 18,19 .…”