Evaluating the Effects of Solar Radiation on Biodiversity of Rhizosphere Bacteria in Shrub-Tree Communities in Granite Outcrop Areas

Evaluating the Effects of Solar Radiation on Biodiversity of Rhizosphere Bacteria in Shrub-Tree Communities in Granite Outcrop Areas

By Yitao Dai

Course: Biology 141

Lab Section: Tuesday 1:40 pm

Instructor: Elizabeth Gleim

Due Date: 04/15/2016

Abstract

Granite outcrops on Arabia Mountain, Georgia, create a unique plant community: a shrub-tree community, with extremely inconstant temperatures, high insolation, nutrient-poor substrate and frequent drought. But rhizosphere bacteria still exist in soil. Most of them have positive effects on plant growth. Four species of plants were chosen from both shaded and sunny areas in two shrub tree communities to study the effects of sunlight on the biodiversity (species richness and species diversity) of rhizosphere bacteria. The results showed that the solar radiation only increased the species richness of type 1 bacteria, type 2 bacteria, and type 6 bacteria. However, solar radiation decreased the species diversity of rhizosphere bacteria in shrub-tree community. Those three types of rhizosphere bacteria which still had higher species richness in arid areas caused by intensive insolation could be further studied and applied in agriculture as fertilizers in arid zones.

Introduction

Granite Outcrops, which are widely distributed in the Piedmont region from North Carolina to Alabama, exist extensively in Georgia, especially around the Atlanta area (Murdy, 1968). Due to the shallow depressions, crevices, and vegetation mats on the rock surface, the plant communities on granite outcrops are restricted as islands surrounded by granitic rock, and thus are classified in to four main types of communities: diamorpha communities, lichen-annual herb communities, annual-perennial herb communities, and herb-shrub communities (Burbanck and Platt, 1964). However, some annual-perennial herb and herb-shrub communities are dominated by trees like loblolly pine, creating the fifth outcrop vegetation community: a shrub-tree community (Philips, 1981). As one of the pioneer ecosystems, a granite outcrop ecosystem has such severe environmental conditions: extremely inconstant temperatures, high insolation, nutrient-poor substrate and frequent drought (Snyder and Wullstein, 1973). Although shrub-tree communities have relatively greater soil depth (water-holding capacity) than other herb communities, and most plants on outcrops have C4 and CAM photosynthetic pathways to adapt high temperatures and frequent drought, the question is how shrub-tree communities have enough nutrients for those shrubby understory and trees (Houle, 1987).

Because of the rapid growth rate and the ability to utilize a wide range of substances, bacteria are the most common type among soil microorganisms (Glick, 1995). Besides some phytopathogenic bacteria that affect plant health, there are two beneficial types of bacteria: symbiotic bacteria, like rhizobia, and beneficial free-living soil bacteria that are near, or even within the roots of plants (Glick, 1995). These rhizosphere bacteria promote plant growth by providing growth factors and nutrients, and thus can be used as bio-fertilizers or biological disease control agents in agricultural application (Davison, 1988). According to a previous study, a plant beneficial bacteria, Azotobacter, was found in association with lichens to provide nitrogen by N2 fixation in a granite outcrop ecosystem (Synder and Wullstein, 1973). There are probably other plant beneficial rhizosphere bacteria in shrub-tree communities.

Therefore, the objective of the present paper was to determine whether the sunlight intensity increases the level of biodiversity of rhizosphere bacteria in the shrub-tree community or not. Different from other four communities, trees in shrub-tree communities create shaded areas for other plants under them. Intensive solar radiation, an important character of granite outcrops, doesn’t exist in the shaded area of shrub-tree communities. However, the effect of sun exposure on rhizosphere bacteria growth in shrub-tree communities is still a blank area. According to a previous study about the effect of sun exposure on the bacterial growth in humic water, there was a positive relation between the number of viable bacteria and the irradiation time (Corin et al., 1998).  Although rhizoshphere bacteria are different from the bacteria in humic water, we hypothesized that the sun exposure increases the level of biodiversity of rhizosphere bacteria in the shrub tree community. According to Simpson’s index, biodiversity should be measured in two parts: species diversity and species richness (He and Hu, 2005). So if the sun exposure increases the level of diversity and growth of rhizosphere bacteria in the shrub tree community, a greater number of different types of bacteria and amount of bacteria will be observed in the areas with sun exposure.

Materials and Methods

There were two parts in the materials and methods: collection of rhizosphere bacteria and investigation of bacterial type and growth. On Arabia Mountain, a granite monadnock located at 25 km east of Atlanta, Georgia, four species of plants (four replicates) were chosen in two shrub-tree communities, and each species had samples located in both shaded and sunny areas (Houle, 1987). They were Spiderwort (Tradescantia), Reindeer Lichen (Cladonia portentosa), Yellow Jessamine (Gelsemium sempervirens), and Hairy Cap Moss (Polytrichum). For all shaded and sunny areas, a light meter was used to measure the sunlight intensity in the unit of footcandle (Table 1). Then the sterile technique was used eight times to collect rhizosphere microorganisms from the same species of plants in shaded and sunny areas respectively. The roots (or rhizoids) of plants was pulled out by a small shovel carefully, and sterile swabs were used to swab roots to collect microorganisms. Next, the swabs were swabbed in a zigzag way in the agar plates to conserve the rhizosphere microorganisms. Finally, the eight samples were labeled and incubated in agar plates under room temperature for 2 days.

A stereoscopic microscope was used to identify the colonies of bacteria upon color, shape, margin and surface characteristics. After the colonies of bacteria were distinguished and recorded, the grid method was used to estimate and calculate the percentage growth of each colony in each plate. A circular transparency grid with 163 squares was put on the plate, and Sharpie markers with different colors were used to draw an “X” or a small “x” in each square to represent the different areas of growth of each colony. Then the total number of squares with bacterial growth was counted by digital counting pens for each colony. An “X” was counted as a square, and a small “x” was counted as a quarter of a square. Finally, the percentage of each colony growth was calculated by dividing the number of squares with bacterial growth with the total number of squares (shown below), and was recorded.