Meta-analytical approach to explain variation in microbial phosphorus uptake rates in aquatic ecosystems

Despite the fact that microbial uptake represents an important transformation of nutrients in aquatic ecosystems, few comprehensive studies have identified key parameters and evaluated their relative importance in explaining variation in uptake rates. Therefore, we per- formed an assessment of peer-reviewed literature that reported aquatic microbial phosphorus (P) uptake rates. The search yielded 36 different papers which presented results of 102 uptake esti- mates. We then constructed a meta-analysis to examine the effects of key parameters on uptake. Microbial group (benthic, planktonic), source (culture, wild), and sample time (long, short) were significant parameters in explaining observed variation in published P uptake rates. Planktonic microbes had higher P uptake rates (65 µg P µg chl a -1 d �1 ), compared with benthic (9 µg P µg chl a -1 d �1 ). Lower affinity for P by benthic microbes could be attributed to adnate growth forms, which can create boundary layers separating cells from ambient P and promoting internal P cycling. Cultured microbes exhibited higher P uptake rates compared with wild samples, although this trend was not significant (F = 2.63, p = 0.108), suggesting that cultured microbes in these studies represented reasonable analogues. Shorter sampling times yielded over 3-fold higher P uptake rates (58 µg P µg chl a -1 d �1 ) and appear to represent more accurate estimates of gross uptake. Microbes subject to longer time regimes may be physiologically altered by experi- mental conditions; estimates might therefore not reflect instantaneous uptake. Our results high- light the influence of ecological variation on P assimilation and provide criteria for developing a general model to predict observed variation in microbial P uptake rates.