Light Availability May Control Extracellular Phosphatase Production in Turbid Environments

Abstract

Extracellular phosphatase production by phytoplankton was investigated in the moderately eutrophic Lipno reservoir, Czech Republic during 2009 and 2010. We hypothesized that production of extracellular phosphatases is an additional mechanism of phosphorus acquisition enabling producers to survive rather than to dominate the phytoplankton. Hence, we examined the relationship between light availability and phosphatase production, as light plays an important role in polymictic environments. Bulk phosphatase activity was measured using a common fluorometric assay, and the production of phosphatases was studied using the Fluorescently Labelled Enzyme Activity technique, which enabled direct microscopic detection of phosphatase-positive cells. In total, 29 taxa of phytoplankton were identified during both years. Only 17 taxa from the total number of 29 showed production of extracellular phosphatases. Species dominating the phytoplankton rarely produced extracellular phosphatases. In contrast, taxa exhibiting phosphatase activity were present in low biomass in the phytoplankton assemblage. Moreover, there was a significant relationship between the proportion of phosphatase positive species in samples and the Z(eu):Z(mix) ratio (a proxy of light availability). A laboratory experiment with different light intensities confirmed the influence of light on production of phosphatases. Our seasonal study confirmed that extracellular phosphatase production is common in low-abundance populations but not in dominant taxa of the phytoplankton. It also suggested the importance of sufficient light conditions for the production of extracellular phosphatases.

An Experimental Insight into Extracellular Phosphatases – Differential Induction of Cell-Specific Activity in Green Algae Cultured under Various Phosphorus Conditions

“…An early application of the ELF ® 97 Endogenous Phosphatase Detection Kit successfully visualized phosphatase-positive cells in both algal cultures and natural phytoplankton, i.e., directly tagged the P-limited algal species ( González-Gil et al, 1998 ; Dyhrman and Palenik, 1999 ; Rengefors et al, 2001 ), although further studies revealed some uncertainties and/or potential misinterpretations (e.g., Rengefors et al, 2003 ; Dignum et al, 2004 ; Ou et al, 2010 ). The principle shortcoming of the ELF ® 97 method as it is applied in recent studies, including the original paper by González-Gil et al (1998) , is that only the occurrence of ELFA-labeling, i.e., merely qualitative estimates of phosphatase-positive algal cells and/or species, presence/absence of tagged phytoplankton, etc., could be reported ( Štrojsová et al, 2003 ; Cao et al, 2005 ; Rychtecký et al, 2015 ; Ren et al, 2017 ). The only quantification of phosphatase activity accessible within the limits of this original method is to score a percentage of ELFA-labeled cells (e.g., Rengefors et al, 2003 ; Dyhrman and Ruttenberg, 2006 ; Litchman and Nguyen, 2008 ; Young et al, 2010 ).…”

“…The paradigm of phosphatase expression only under P deficiency is overly simplistic as algae may constitutively express some phosphatase activity, but also may not efficiently regulate it in response to P availability ( Young et al, 2010 ). There is also increasing awareness that all phytoplankton species do not react uniformly to P depletion (e.g., Litchman and Nguyen, 2008 ) and many species indeed do not produce extracellular phosphatases at all under such circumstances, while other species can exhibit constitutive activity (e.g., Rengefors et al, 2001 , 2003 ; Štrojsová et al, 2003 , 2005 , 2008 ; Rychtecký et al, 2015 ). For instance, ELFA-labeled, i.e., phosphatase-positive phytoplankton species were reported from eutrophic lakes under high concentrations of soluble reactive P (SRP) (e.g., Cao et al, 2005 , 2009 ).…”