Heterotrophic and mixotrophic nanoflagellates in a mesotrophic lake: Abundance and grazing impacts across season and depth

Abstract

Nanoflagellates are recognized as fundamental members of the planktonic microbial food web due to their contribution to photosynthetic fixation of carbon and phagotrophic ingestion of bacteria. Although the presence, and often dominance, of mixotrophic nutrition among phytoflagellates has been well documented within a variety of aquatic ecosystems and in a range of taxa, few studies have assessed the abundance and grazing impact of mixotrophs in comparison to their photosynthetic and heterotrophic counterparts across seasons at multiple depths. Abundance and, as appropriate, bacterivory of phototrophic, heterotrophic, and mixotrophic nanoflagellates were quantified at three depths over a 15‐month period in Lake Lacawac (Pocono Mountains, Pennsylvania, U.S.A.) via microscopic analysis of ingested bacterial surrogates. The absolute and relative abundances of all nanoflagellate trophic groups varied with season and depth, likely as a result of differences in physiological tolerance. Whereas the abundance of phototrophic and heterotrophic nanoflagellates varied with depth in both the presence and absence of thermal stratification, mixotrophic protists were evenly distributed throughout the water column during mixis. The abundances of phototrophic and mixotrophic protists were significantly correlated, but only in surface waters during periods of thermal stratification. Grazing rate and consequent impact by bacterivorous nanoflagellates varied with depth across season, particularly during thermal stratification. Mixotrophic bacterivory exceeded that of heterotrophs in the epi‐ and metalimnion during stratification, removing an estimated maximum of 80% of the bacterial standing stock d−1. The relative contribution of heterotrophic nanoflagellates to community bacterivory increased with depth, even during mixis, and dominated the grazing impact in the hypolimnion.

Predator and prey biodiversity relationship and its consequences on marine ecosystem functioning—interplay between nanoflagellates and bacterioplankton

“…To obtain predator diversity estimates, we extracted the OTUs from the 18S rDNA pool that was affiliated to the common bacterivorous nanoflagellates according to the literature ( Supplementary Table S1), which contained mostly the heterotrophic nanoflagellates (HNF). Although a number of pigmented nanoflagellates are bacterivorous (known as mixotrophic nanoflagellates), we did not include them in the predator assemblages, given controversy surrounding the extent to which these groups contribute to the top-down control on bacterial communities [62][63][64][65].…”

Response of Growth and Grazing Rate of Nanoflagellates on Synechococcus spp. to Experimental Nutrient Enrichment

“…The important role of pigmented nanoflagellatesas important bacterial consumers has been demonstrated in previous studies [6,8], but their significance in regulating bacterial communities is still under discussion. PNFs are also important Synechococcus grazers in marine environments [15].…”

“…Although grazing by heterotrophic nanoflagellates is showed by top-down control in the bacterial communities, pigmented nanoflagellates (PNFs) have been considered as important bacterivores in planktonic food webs in recent studies [6][7][8]. Some field studies observed that in marine and freshwater systems, PNFs outnumber the heterotrophic nanoflagellates, and thus have a greater grazing impact on the bacterial community [6,[8][9][10]. PNFs can survive in different environments by relying on bacterivory during low-light conditions, darkness, dissolved nutrient concentration, and photosynthesis when prey concentration is low [11][12][13][14].…”

How Does Environmental Inter-annual Variability Shape Aquatic Microbial Communities? A 40-Year Annual Record of Sedimentary DNA From a Boreal Lake (Nylandssjön, Sweden)

“…Among them, the diversity and community composition of aquatic protistan communities (i.e., unicellular eukaryotes) have been well studied using DNA sequencing methods (Simon et al, 2015;Filker et al, 2016;Debroas et al, 2017). However, their temporal dynamics have been studied mostly on short timescales, i.e., from days to seasons (Debroas et al, 2015;Mukherjee et al, 2017;Princiotta and Sanders, 2017), and rarely over timescales relevant to investigate their responses to inter-annual variability (except Cram et al, 2015 for marine bacterial communities) or longerterm pressures. The longest DNA-based monitoring time serieswhich extend up to one decade (e.g., BATS, HOT, SPOT and BBMO; Fuhrman et al, 2015;Giner et al, 2018) -have been useful to identify day length (e.g., luminosity) and temperature as major drivers of community turnover.…”