Cell Size Dependent Responses of Phytoplankton Assemblages to Nitrate and Phosphate Additions in Surface Waters of the Northern South China Sea

Abstract

Bioavailability of nitrogen (N) and phosphorus (P) is known to affect marine phytoplankton physiology, thus influencing their primary productivity; and it's of general interest to see how the N or/and P additions affect the differently cell-sized phytoplankton assemblages. Data from the northern South China Sea showed that P addition increased up to 6 times of total chl a content within 24 h in the estuarine water; and N+P addition increased more than 20 times of chl a within 144 h in the pelagic water. The P addition powered 18.0% and 149% increase in the carbon fixation of larger (>3 μm) and smaller (<3 μm) cell assemblies from the estuarine water, respectively; while the limited effects were observed between them in the pelagic water. Additions of N and P interactively increased the carbon fixation of both cell-sized assemblies in the pelagic water, but only small cell-sized fraction in the estuarine water. Moreover, the estuarine phytoplankton cells appeared to respond faster to the nutrient additions than the pelagic ones, and the smaller cells respond to a greater extent than their larger counterparts in the estuarine water but not in the pelagic water.

Differential Physiological Responses of Small Thalassiosira pseudonana and Large Thalassiosira punctigera to the Shifted-High Light and Nitrogen

“…In the field, differently cell-sized phytoplankton assemblies usually have differential responses to environmental variables, including nutrients [18,34], light [4,29], and their interactions [49]. Here, we showed that the upward-shifted light insignificantly affected the growth of the small T. pseudonana, but reduced that of the large T. punctigera, resulting from the differential effects upon the cellular compositions, photosynthesis, and antioxidant abilities (Figure 7).…”

“…Perhaps the culture time was still not long enough for the smaller T. pseudonana to adapt completely to such a harsh light shift, although the photosynthetic capacity was already steady (Figure S1). Yi et al [34] also recognized that the small cell-dominated phytoplankton assemblages respond much slower to nutrient addition than large cell-dominated phytoplankton assemblages.…”

“…Cell size of diatoms varies from less than 2 µm to over 200 µm in equivalent spherical diameters, with over nine orders of magnitude in biovolume [24,25]. The cell size often affects the diatom's physiological processes, including nutrients absorption and utilization efficiencies [13,26,27], photosynthetic capacity [28,29], growth response [30][31][32], and ability to adapt to the nutrients and light variables [33][34][35]. For example, small diatoms that have a high surface-to-volume ratio usually have faster growth rates and higher photosynthetic abilities than large ones, and usually dominate in oligotrophic waters [27,36], although some very large-celled diatoms like Ethmodiscus and Rhizosolenia also survive in the oligotrophic waters by undergoing a periodic vertical migration between surface waters, where they photosynthesize, and deeper waters, where they take up nutrients at the chemocline [37].…”

“…For example, small diatoms that have a high surface-to-volume ratio usually have faster growth rates and higher photosynthetic abilities than large ones, and usually dominate in oligotrophic waters [27,36], although some very large-celled diatoms like Ethmodiscus and Rhizosolenia also survive in the oligotrophic waters by undergoing a periodic vertical migration between surface waters, where they photosynthesize, and deeper waters, where they take up nutrients at the chemocline [37]. On the other hand, larger diatoms have heavier pigment packages, dropping their light-absorbing efficiency per unit of pigment-protein complex [28,33,38], outcompeting their smaller counterparts to endure variable lights, and thus dominating widely in coastal or estuarine areas [18,34]. Moreover, larger volumes, enabling them to reserve more nutrients inside [39], and lower metabolic expenditures, allowing them to endure or utilize fluctuating lights [28], also endow larger cells with more advantages over smaller ones.…”