Phytoplankton size structure is key for the ecology and biogeochemistry of pelagic ecosystems, but the relationship between cell size and maximum growth rate (μ(max) ) is not yet well understood. We used cultures of 22 species of marine phytoplankton from five phyla, ranging from 0.1 to 10(6) μm(3) in cell volume (V(cell) ), to determine experimentally the size dependence of growth, metabolic rate, elemental stoichiometry and nutrient uptake. We show that both μ(max) and carbon-specific photosynthesis peak at intermediate cell sizes. Maximum nitrogen uptake rate (V(maxN) ) scales isometrically with V(cell) , whereas nitrogen minimum quota scales as V(cell) (0.84) . Large cells thus possess high ability to take up nitrogen, relative to their requirements, and large storage capacity, but their growth is limited by the conversion of nutrients into biomass. Small species show similar volume-specific V(maxN) compared to their larger counterparts, but have higher nitrogen requirements. We suggest that the unimodal size scaling of phytoplankton growth arises from taxon-independent, size-related constraints in nutrient uptake, requirement and assimilation.
We have determined the scaling relationship between photosynthesis rate and cell size in natural phytoplankton assemblages of contrasting marine environments. We found that phytoplankton photosynthesis in the ocean does not scale as the L-power of cell size, but scales approximately isometrically with cell size, indicating that a single model cannot predict the metabolism-size relationship in all photosynthetic organisms. The scaling relationship between cellular chlorophyll a content and cell size is also isometric. Taxonomical changes along the size spectrum may explain the deviation of phytoplankton photosynthesis from the general allometric rule. The size scaling exponent for photosynthesis is significantly higher (1.14) in coastal productive waters than in the oligotrophic open ocean (0.96), which provides a physiological basis to explain the dominance of larger cells in nutrient-rich environments. The size scaling exponent for phytoplankton abundance is significantly less negative in coastal productive waters (20.90) than in the oligotrophic open ocean (21.25). The observed size scaling relationships imply that carbon fixation per unit volume decreases with cell size in oligotrophic waters, whereas the opposite occurs in productive ones. By controlling the metabolism-size scaling relationship, nutrient supply plays a major role in determining community size structure and the energy flow through the pelagic ecosystem.The relative importance of small and large phytoplankton is a key feature of the planktonic community, which strongly affects the fate of recently synthesized organic matter in the pelagic ecosystem (Kiørboe 1993;Legendre and Rassoulzadegan 1996;Falkowski et al. 1998). Small cells account for the bulk of phytoplankton biomass in open-ocean oligotrophic waters, where most of the newly produced organic carbon is recycled within the photic layer through complex microbial food webs. By contrast, larger cells dominate in nutrient-rich productive waters, where a major fraction of primary production is channeled through short food chains and exported toward the ocean interior, thus contributing to CO 2 sequestration. According to the allometric theory in biology (Peters 1983;Brown et al. 2000;Niklas and Enquist 2001), individual metabolic rates (M) scale with body size (V) as M 3 V 3/4 (the Lpower rule or Kleiber's law). A compilation of plant biomass production data covering 20 orders of magnitude in body size, and including laboratory measurements for microalgae, gives support to the view that the L-scaling rule applies to all photosynthetic organisms (Niklas and Enquist 2001). If the L-power rule holds, it follows that mass-specific metabolism and growth rates must scale as V 21/4 . This means that smaller cells should dominate all types of pelagic environments, on account of their faster metabolism and growth rates. However, trophic and hydrodynamic mechanisms may also play a role in controlling phytoplankton loss rates and, therefore, their size structure. Thus, the dominance of larger ph...
In the size range from lO-4 to 10' pg (carbon) body weight, the biomass of plankton in the euphotic layer of the North Pacific Central Gyre decreases as an allometric function of body weight. Even in a steady state ecosystem such as that analyzed here, there is variability in space and time; this suggests that one must be careful in extrapolating the relation to less predictable marine areas. In obtaining dynamic information from biomass spectra, one must distinguish changes due to the flow of energy within the spectrum (growth, predation, reproduction) from changes due to emigration from or immigration into the spectrum of the particular area sampled, such as those due to the diel vertical migration of macrozooplankton in the largest size classes.
We have determined the seasonal (July 2001-July 2002 and vertical variability in the photosynthetic production of dissolved organic carbon (DOCp) and particulate organic carbon (POCp) in a coastal upwelling ecosystem (Ría de Vigo, Northwest Spain), together with the relationship between irradiance and DOCp and the time-course of DOCp over 24-h periods. Euphotic layer-integrated rates of DOCp and POCp ranged between 5 and 190 mg C m Ϫ2 h Ϫ1 and between 40 and 1,130 mg C m Ϫ2 h Ϫ1 , respectively. Irradiance was the most important variable affecting the vertical variability of the percentage of extracellular release [PER, defined as DOCp/(DOCp ϩ POCp)]. Whereas POCp decreased markedly below the surface, DOCp remained constant or even increased, thus causing a sharp increase in PER with depth. Biomass-specific rates of DOC production also increased with depth. These observations were confirmed by the results of photosynthesis-irradiance experiments, which consistently showed highest DOCp and PER values at subsaturating irradiances. Our results argue against the view that the release of DOC is an overflow mechanism occurring preferentially under conditions of high irradiance and low nutrient concentration. PER was uncorrelated with the size structure of phytoplankton biomass and productivity, and Ͼ80% of the variability in integrated DOCp was explained by POCp. These findings indicate that the relative importance of dissolved primary production was independent of the dominant type of planktonic trophic organization. Moreover, production of DOC stopped at night, which strongly indicates that trophic processes were not involved in the release of dissolved photosynthate. Our data support a purely physiological mechanism of passive DOC release by normally growing cells, which is enhanced under suboptimal irradiances but proceeds at a similar biomass-specific rate throughout the year. On an integrated basis, PER averaged 19 Ϯ 1%, thus indicating that even in eutrophic waters, total primary productivity can be significantly underestimated if the dissolved products of photosynthesis are not taken into account.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.